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623 lines
19 KiB
Plaintext
623 lines
19 KiB
Plaintext
\input texinfo @c -*- Texinfo -*-
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@setfilename sframe-spec.info
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@settitle The SFrame Format
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@copying
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Copyright @copyright{} 2021-2023 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU General Public License, Version 3 or any
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later version published by the Free Software Foundation. A copy of the
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license is included in the section entitled ``GNU General Public
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License''.
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@end copying
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@dircategory Software development
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@direntry
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* SFrame: (sframe-spec). The Simple Frame format.
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@end direntry
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@titlepage
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@title The SFrame Format
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@subtitle Version 1
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@author Indu Bhagat
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@page
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@vskip 0pt plus 1filll
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@insertcopying
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@end titlepage
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@contents
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@ifnottex
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@node Top
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@top The SFrame format
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This manual describes version 1 of the SFrame file format. SFrame stands for
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Simple Frame format. SFrame format keeps track of the minimal necessary
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information needed for stack unwinding:
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@itemize @minus
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@item
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Canonical Frame Address (CFA).
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@item
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Frame Pointer (FP).
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@item
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Return Address (RA).
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@end itemize
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The reason for existence of the SFrame format is to support fast, online
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backtracing using a simple unwinder.
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@menu
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* Overview::
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* SFrame section::
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* Index::
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@end menu
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@end ifnottex
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@node Overview
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@unnumbered Overview
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@cindex Overview
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@tindex PT_GNU_SFRAME
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The SFrame unwind information is provided in a loaded section, known as the
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@code{.sframe} section. When available, the @code{.sframe} section appears in
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a new segment of its own, PT_GNU_SFRAME.
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The SFrame format is currently supported only for select ABIs, namely, AMD64
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and AAPCS64.
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The contents of the SFrame section are stored in the target endianness, i.e.,
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in the endianness of the system on which the section is targetted to be used.
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An SFrame section reader may use the magic number in the SFrame header to
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identify the endianness of the SFrame section.
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Addresses in this specification are expressed in bytes.
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The associated API to decode, probe and encode the SFrame section, provided via
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@code{libsframe}, is not accompanied here at this time. This will be added
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later.
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This document is intended to be in sync with the C code in @file{sframe.h}.
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Please report descrepancies between the two, if any.
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@node SFrame section
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@chapter SFrame section
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@cindex SFrame section
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The SFrame section consists of an SFrame header, starting with a preamble, and
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two other sub-sections, namely the SFrame Function Descriptor Entry (SFrame
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FDE) sub-section, and the SFrame Frame Row Entry (SFrame FRE) sub-section.
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@menu
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* SFrame Preamble::
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* SFrame Header::
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* SFrame Function Descriptor Entries::
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* SFrame Frame Row Entries::
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@end menu
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@node SFrame Preamble
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@section SFrame Preamble
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@cindex SFrame preamble
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The preamble is a 32-bit packed structure; the only part of the SFrame whose
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format cannot vary between versions.
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@example
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typedef struct sframe_preamble
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@{
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uint16_t sfp_magic;
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uint8_t sfp_version;
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uint8_t sfp_flags;
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@} ATTRIBUTE_PACKED sframe_preamble;
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@end example
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All values are stored in the endianness of the target system for which the
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SFrame section is intended. Further details:
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@multitable {Offset} {@code{uint8_t sfp_version}} {The magic number for SFrame section: 0xdee2. Defined}
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@headitem Offset @tab Name @tab Description
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@item 0x00
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@tab @code{uint16_t sfp_magic}
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@tab The magic number for SFrame section: 0xdee2. Defined as a macro @code{SFRAME_MAGIC}.
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@tindex SFRAME_MAGIC
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@item 0x02
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@tab @code{uint8_t sfp_version}
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@tab The version number of this SFrame section. @xref{SFrame version}, for the
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set of valid values. Current version is
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@code{SFRAME_VERSION_1}.
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@item 0x03
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@tab @code{uint8_t sfp_flags}
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@tab Flags (section-wide) for this SFrame section. @xref{SFrame flags}, for the
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set of valid values.
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@end multitable
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@menu
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* SFrame endianness::
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* SFrame version::
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* SFrame flags::
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@end menu
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@node SFrame endianness
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@subsection SFrame endianness
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@cindex endianness
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SFrame sections are stored in the target endianness of the system that consumes
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them. The SFrame library (@code{libsframe}) can, however, detect whether to
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endian-flip an SFrame section at decode time, by inspecting the
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@code{sfp_magic} field in the SFrame header (If it appears as 0xe2de,
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endian-flipping is needed).
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@node SFrame version
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@subsection SFrame version
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The version of the SFrame format can be determined by inspecting
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@code{sfp_version}. The following versions are currently valid:
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@tindex SFRAME_VERSION_1
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@cindex SFrame versions
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@multitable {SFRAME_VERSION_1} {Number} {First version, under development.}
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@headitem Version @tab Number @tab Description
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@item @code{SFRAME_VERSION_1}
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@tab 1 @tab First version, under development.
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@end multitable
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This section documents @code{SFRAME_VERSION_1}.
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@node SFrame flags
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@subsection SFrame flags
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@cindex SFrame flags
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@comment @vindex sfp_flags
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@comment @vindex SFrame section-wide flags
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@comment @subsection SFrame section-wide flags
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The preamble contains bitflags in its @code{sfp_flags} field that
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describe various section-wide properties.
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The following flags are currently defined.
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@multitable {@code{SFRAME_F_FRAME_POINTER}} {Versions} {Value} {Function Descriptor Entries}
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@headitem Flag @tab Versions @tab Value @tab Meaning
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@tindex SFRAME_F_FDE_SORTED
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@item @code{SFRAME_F_FDE_SORTED} @tab All @tab 0x1 @tab Function Descriptor
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Entries are sorted on PC.
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@tindex SFRAME_F_FRAME_POINTER
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@item @code{SFRAME_F_FRAME_POINTER} @tab All @tab 0x2
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@tab Functions preserve frame-pointer.
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@end multitable
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Further flags may be added in future.
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@node SFrame Header
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@section SFrame Header
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@cindex SFrame header
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The SFrame header is the first part of an SFrame section. It begins with the
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SFrame preamble. All parts of it other than the preamble
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(@pxref{SFrame Preamble}) can vary between SFrame file versions. It contains
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things that apply to the section as a whole, and offsets to the various other
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sub-sections defined in the format. As with the rest of the SFrame section,
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all values are stored in the endianness of the target system.
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The two sub-sections tile the SFrame section: each section runs from the offset
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given until the start of the next section. An explicit length is given for the
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last sub-section, the SFrame Frame Row Entry (SFrame FRE) sub-section.
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@example
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typedef struct sframe_header
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@{
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sframe_preamble sfh_preamble;
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uint8_t sfh_abi_arch;
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int8_t sfh_cfa_fixed_fp_offset;
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int8_t sfh_cfa_fixed_ra_offset;
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uint8_t sfh_auxhdr_len;
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uint32_t sfh_num_fdes;
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uint32_t sfh_num_fres;
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uint32_t sfh_fre_len;
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uint32_t sfh_fdeoff;
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uint32_t sfh_freoff;
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@} ATTRIBUTE_PACKED sframe_header;
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@end example
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The sub-section offsets, namely @code{sfh_fdeoff} and @code{sfh_freoff}, in the
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SFrame header are relative to the @emph{end} of the SFrame header; they are
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each an offset in bytes into the SFrame section where the SFrame FDE
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sub-section and the SFrame FRE sub-section respectively start.
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SFrame header allows specifying explicitly the fixed offsets from CFA, if any,
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from which FP or RA may be recovered. For example, in AMD64, the stack offset
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of the return address is @code{CFA - 8}. Since this offset is in close
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vicinity with the CFA in most ABIs, @code{sfh_cfa_fixed_fp_offset} and
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@code{sfh_cfa_fixed_ra_offset} are limited to signed 8-bit integers.
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SFrame format has provisioned for future ABIs/architectures that it may
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support. The @code{sframe_header} structure provides an unsigned 8-bit
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integral field to denote the size of an auxilliary SFrame header. The
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auxilliary SFrame header follows right after the @code{sframe_header}
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structure. As for the offset calculations, the @emph{end} of SFrame header
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must be the end of the auxilliary SFrame header, if the latter is present.
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Tieing it all together:
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@multitable {Offset} {@code{int8_t sfh_cfa_fixed_fp_offset}} {The ABI/arch identifier. See above}
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@headitem Offset @tab Name @tab Description
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@item 0x00
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@tab @code{sframe_preamble sfh_preamble}
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@tab The SFrame preamble. @xref{SFrame Preamble}.
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@item 0x04
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@tab @code{uint8_t sfh_abi_arch}
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@tab The ABI/arch identifier. @xref{SFrame ABI/arch identifier}.
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@item 0x05
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@tab @code{int8_t sfh_cfa_fixed_fp_offset}
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@tab The CFA fixed FP offset, if any.
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@item 0x06
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@tab @code{int8_t sfh_cfa_fixed_ra_offset}
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@tab The CFA fixed RA offset, if any.
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@item 0x07
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@tab @code{uint8_t sfh_auxhdr_len}
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@tab Size in bytes of the auxilliary header that follows the
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@code{sframe_header} structure.
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@item 0x08
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@tab @code{uint32_t sfh_num_fdes}
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@tab The number of SFrame FDEs in the section.
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@item 0xc
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@tab @code{uint32_t sfh_num_fres}
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@tab The number of SFrame FREs in the section.
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@item 0x10
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@tab @code{uint32_t sfh_fre_len}
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@tab The length in bytes of the SFrame FRE sub-section.
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@item 0x14
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@tab @code{uint32_t sfh_fdeoff}
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@tab The offset in bytes of the SFrame FDE sub-section. This sub-section
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contains @code{sfh_num_fdes} number of fixed-length array elements. The array
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element is of type SFrame function desciptor entry, each providing a
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high-level function description for backtracing.
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@xref{SFrame Function Descriptor Entries}.
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@item 0x18
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@tab @code{uint32_t sfh_freoff}
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@tab The offset in bytes of the SFrame FRE sub-section, the core of the SFrame
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section, which describes the unwind information using variable-length array
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elements. @xref{SFrame Frame Row Entries}.
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@end multitable
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@menu
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* SFrame ABI/arch identifier::
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@end menu
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@node SFrame ABI/arch identifier
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@subsection SFrame ABI/arch identifier
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@cindex SFrame ABI/arch identifier
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SFrame header identifies the ABI/arch of the target system for which the
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executable and it's unwind information is intended. There are currently three
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identifiable ABI/arch values in the format.
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@multitable {SFRAME_ABI_AARCH64_ENDIAN_LITTLE} {Value} {@code{AARCH64 little-endian}}
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@headitem ABI/arch Identifier @tab Value @tab Description
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@tindex SFRAME_ABI_AARCH64_ENDIAN_BIG
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@item @code{SFRAME_ABI_AARCH64_ENDIAN_BIG}
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@tab 1 @tab AARCH64 big-endian
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@tindex SFRAME_ABI_AARCH64_ENDIAN_LITTLE
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@item @code{SFRAME_ABI_AARCH64_ENDIAN_LITTLE}
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@tab 2 @tab AARCH64 little-endian
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@tindex SFRAME_ABI_AMD64_ENDIAN_LITTLE
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@item @code{SFRAME_ABI_AMD64_ENDIAN_LITTLE}
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@tab 3 @tab AMD64 little-endian
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@end multitable
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The presence of an explicit identification of ABI/arch in SFrame may allow
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unwinders to make certain ABI-specific decisions.
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@node SFrame Function Descriptor Entries
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@section SFrame FDE
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@cindex SFrame FDE
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The SFrame Function Descriptor Entry sub-section is a sorted array of
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fixed-length SFrame function descriptor entries (SFrame FDEs). Each SFrame FDE
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is a packed structure which contains information to describe a function's unwind
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information at a high-level.
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@example
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typedef struct sframe_func_desc_entry
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@{
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int32_t sfde_func_start_address;
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uint32_t sfde_func_size;
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uint32_t sfde_func_start_fre_off;
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uint32_t sfde_func_num_fres;
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uint8_t sfde_func_info;
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@} ATTRIBUTE_PACKED sframe_func_desc_entry;
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@end example
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@code{sfde_func_start_fre_off} is the offset to the first SFrame FRE for the
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function. This offset is relative to the @emph{end of the SFrame FDE}
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sub-section (unlike the offsets in the SFrame header, which are relative to the
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@emph{end} of the SFrame header).
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@code{sfde_func_info} is the "info word", containing information on the FRE
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type and the FDE type for the function @xref{The SFrame FDE info word}.
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Following table describes each component of the SFrame FDE structure:
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@multitable {Offset} {@code{uint32_t sfde_func_start_fre_off}} {The ABI/arch identifier. See above}
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@headitem Offset @tab Name @tab Description
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@item 0x00
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@tab @code{int32_t sfde_func_start_address}
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@tab Signed 32-bit integral field denoting the virtual memory address of the
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described function.
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@item 0x04
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@tab @code{uint32_t sfde_func_size}
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@tab Unsigned 32-bit integral field specifying the size of the function in
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bytes.
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@item 0x08
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@tab @code{uint32_t sfde_func_start_fre_off}
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@tab Unsigned 32-bit integral field specifying the offset in bytes of the
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function's first SFrame FRE in the SFrame section.
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@item 0x0c
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@tab @code{uint32_t sfde_func_num_fres}
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@tab Unsigned 32-bit integral field specifying the total number of SFrame FREs
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used for the function.
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@item 0x10
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@tab @code{uint8_t sfde_func_info}
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@tab The SFrame FDE info word. @xref{The SFrame FDE info word}.
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@end multitable
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@menu
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* The SFrame FDE info word::
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* The SFrame FDE types::
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* The SFrame FRE types::
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@end menu
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@cindex The SFrame FDE info word
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@node The SFrame FDE info word
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@subsection The SFrame FDE info word
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The info word is a bitfield split into three parts. From MSB to LSB:
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@multitable {Bit offset} {@code{isroot}} {Length of variable-length data for this type (some kinds only).}
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@headitem Bit offset @tab Name @tab Description
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@item 7--6
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@tab @code{unused}
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@tab Unused bits.
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@item 5
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@tab @code{pauth_key}
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@tab Specify which key is used for signing the return addresses in the SFrame
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FDE. Two possible values: SFRAME_AARCH64_PAUTH_KEY_A (0) or
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SFRAME_AARCH64_PAUTH_KEY_B (1).
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@item 4
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@tab @code{fdetype}
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@tab SFRAME_FDE_TYPE_PCMASK (1) or SFRAME_FDE_TYPE_PCINC (0). @xref{The SFrame FDE types}.
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@item 0--3
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@tab @code{fretype}
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@tab Choice of three SFrame FRE types. @xref{The SFrame FRE types}.
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@end multitable
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@node The SFrame FDE types
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@subsection The SFrame FDE types
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@tindex SFRAME_FDE_TYPE_PCMASK
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@tindex SFRAME_FDE_TYPE_PCINC
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SFrame format defines two types of FDE entries. The choice of which SFrame FDE
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type to use is made based on the instruction patterns in the relevant program
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stub.
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An SFrame FDE of type @code{SFRAME_FDE_TYPE_PCINC} is an indication that the PCs in the
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FREs should be treated as increments in bytes. This is used fo the the bulk of
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the executable code of a program, which contains instructions with no specific
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pattern.
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In contrast, an SFrame FDE of type @code{SFRAME_FDE_TYPE_PCMASK} is an
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indication that the PCs in the FREs should be treated as masks. This type is
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useful for the cases where a small pattern of instructions in a program stub is
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used repeatedly for a specific functionality. Typical usecases are pltN
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entries and trampolines.
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@multitable {Name of SFrame FDE type} {Value} {Unwinders perform a (PC >= FRE_START_ADDR)}
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@headitem Name of SFrame FDE type @tab Value @tab Description
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@item SFRAME_FDE_TYPE_PCINC
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@tab 0 @tab Unwinders perform a (PC >= FRE_START_ADDR) to look up a matching FRE.
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@item SFRAME_FDE_TYPE_PCMASK
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@tab 1 @tab Unwinders perform a (PC & FRE_START_ADDR_AS_MASK >= FRE_START_ADDR_AS_MASK)
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to look up a matching FRE.
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@end multitable
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@node The SFrame FRE types
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@subsection The SFrame FRE types
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A real world application can have functions of size big and small. SFrame
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format defines three types of SFrame FRE entries to represent the unwind
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information for such a variety of function sizes. These representations vary
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in the number of bits needed to encode the start address offset in the SFrame
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FRE.
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The following constants are defined and used to identify the SFrame FRE types:
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@multitable {SFRAME_FRE_TYPE_ADDR1} {@code{Value}} {The start address offset of FRE is an}
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@headitem Name @tab Value @tab Description
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@tindex SFRAME_FRE_TYPE_ADDR1
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@item @code{SFRAME_FRE_TYPE_ADDR1}
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@tab 0
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@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
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8-bit value.
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@tindex SFRAME_FRE_TYPE_ADDR2
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@item @code{SFRAME_FRE_TYPE_ADDR2}
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@tab 1
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@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
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16-bit value.
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@tindex SFRAME_FRE_TYPE_ADDR4
|
|
@item @code{SFRAME_FRE_TYPE_ADDR4}
|
|
@tab 2
|
|
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
|
|
32-bit value.
|
|
@end multitable
|
|
|
|
A single function must use the same type of FRE throughout. The choice of
|
|
which SFrame FRE is used to encode the unwind information of a function, is
|
|
stored in the @xref{The SFrame FDE info word}.
|
|
|
|
@node SFrame Frame Row Entries
|
|
@section SFrame FRE
|
|
@cindex SFrame FRE
|
|
|
|
The SFrame Frame Row Entry sub-section contains the core of the unwind
|
|
information.
|
|
|
|
An SFrame Frame Row Entry is a self-sufficient record containing SFrame unwind
|
|
info for a range of contiguous addresses, starting at the specified offset from
|
|
the start of the function. Each SFrame Frame Row Entry is followed by S*N
|
|
bytes, where:
|
|
|
|
@itemize @minus
|
|
@item
|
|
@code{S} is the size of the stack frame offset for the FRE, and
|
|
@item
|
|
@code{N} is the number of stack frame offsets in the FRE
|
|
@end itemize
|
|
|
|
The stack offsets, following the FRE, are interpreted in order as follows:
|
|
|
|
@itemize @minus
|
|
@item
|
|
The first offset is always used to locate the CFA, by interpreting it as:
|
|
CFA = @code{BASE_REG} + offset1.
|
|
@item
|
|
If RA is being tracked, the second offset is always used to locate the RA, by
|
|
interpreting it as: RA = CFA + offset2. If RA is @emph{not} being tracked
|
|
@emph{and} FP is being tracked, the second offset will be used to locate the
|
|
FP, by interpreting it as: FP = CFA + offset2.
|
|
@item
|
|
If both RA and FP are being tracked, the third offset will be used to locate
|
|
the FP, by interpreting it as FP = CFA + offset3.
|
|
@end itemize
|
|
|
|
The entities @code{S}, @code{N} and @code{BASE_REG} are identified using the
|
|
SFrame FRE info word, a.k.a. the @code{sframe_fre_info}
|
|
@xref{The SFrame FRE info word}.
|
|
|
|
Following are the definitions of the allowed SFrame FRE:
|
|
|
|
@example
|
|
typedef struct sframe_frame_row_entry_addr1
|
|
@{
|
|
uint8_t sfre_start_address;
|
|
sframe_fre_info sfre_info;
|
|
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr1;
|
|
@end example
|
|
|
|
@example
|
|
typedef struct sframe_frame_row_entry_addr2
|
|
@{
|
|
uint16_t sfre_start_address;
|
|
sframe_fre_info sfre_info;
|
|
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr2;
|
|
@end example
|
|
|
|
@example
|
|
typedef struct sframe_frame_row_entry_addr4
|
|
@{
|
|
uint32_t sfre_start_address;
|
|
sframe_fre_info sfre_info;
|
|
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr4;
|
|
@end example
|
|
|
|
@code{sfre_start_address} is an unsigned 8-bit/16-bit/32-bit integral field
|
|
identifies the start address of the range of program counters, for which the
|
|
SFrame FRE applies. The value encoded in the @code{sfre_start_address} field
|
|
is the offset in bytes of the start address of the SFrame FRE, from the start
|
|
address of the function.
|
|
|
|
Further FRE types may be added in future.
|
|
|
|
@menu
|
|
* The SFrame FRE info word::
|
|
@end menu
|
|
|
|
@cindex The SFrame FRE info word
|
|
@node The SFrame FRE info word
|
|
@subsection The SFrame FRE info word
|
|
|
|
The SFrame FRE info word is a bitfield split into four parts. From MSB to LSB:
|
|
|
|
@multitable {Bit offset} {@code{fre_cfa_base_reg_id}} {Size of stack offsets in bytes. Valid values}
|
|
@headitem Bit offset @tab Name @tab Description
|
|
@item 7
|
|
@tab @code{fre_mangled_ra_p}
|
|
@tab Indicate whether the return address is mangled with any authorization bits (signed RA).
|
|
|
|
@item 5-6
|
|
@tab @code{fre_offset_size}
|
|
@tab Size of stack offsets in bytes. Valid values are SFRAME_FRE_OFFSET_1B,
|
|
SFRAME_FRE_OFFSET_2B, and SFRAME_FRE_OFFSET_4B.
|
|
|
|
@item 1-4
|
|
@tab @code{fre_offset_count}
|
|
@tab A value of upto 3 is allowed to track all three of CFA, FP and RA.
|
|
|
|
@item 0
|
|
@tab @code{fre_cfa_base_reg_id}
|
|
@tab Distinguish between SP or FP based CFA recovery.
|
|
|
|
@end multitable
|
|
|
|
@multitable {SFRAME_FRE_OFFSET_4B} {@code{Value}} {All stack offsets following the fixed-length}
|
|
@headitem Name @tab Value @tab Description
|
|
|
|
@tindex SFRAME_FRE_OFFSET_1B
|
|
@item @code{SFRAME_FRE_OFFSET_1B}
|
|
@tab 0
|
|
@tab All stack offsets following the fixed-length FRE structure are 1 byte
|
|
long.
|
|
|
|
@tindex SFRAME_FRE_OFFSET_2B
|
|
@item @code{SFRAME_FRE_OFFSET_2B}
|
|
@tab 1
|
|
@tab All stack offsets following the fixed-length FRE structure are 2 bytes
|
|
long.
|
|
|
|
@tindex SFRAME_FRE_OFFSET_4B
|
|
@item @code{SFRAME_FRE_OFFSET_4B}
|
|
@tab 2
|
|
@tab All stack offsets following the fixed-length FRE structure are 4 bytes
|
|
long.
|
|
|
|
@end multitable
|
|
|
|
@node Index
|
|
@unnumbered Index
|
|
|
|
@syncodeindex tp cp
|
|
@printindex cp
|
|
|
|
@bye
|