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binutils-gdb/gdb/linux-tdep.h
Simon Marchi 82d23ca811 gdb: fix auxv caching 
There's a flaw in the interaction of the auxv caching and the fact that
target_auxv_search allows reading auxv from an arbitrary target_ops
(passed in as a parameter).  This has consequences as explained in this
thread:

  https://inbox.sourceware.org/gdb-patches/20220719144542.1478037-1-luis.machado@arm.com/

In summary, when loading an AArch64 core file with MTE support by
passing the executable and core file names directly to GDB, we see the
MTE info:

    $ ./gdb -nx --data-directory=data-directory -q aarch64-mte-gcore aarch64-mte-gcore.core
    ...
    Program terminated with signal SIGSEGV, Segmentation fault
    Memory tag violation while accessing address 0x0000ffff8ef5e000
    Allocation tag 0x1
    Logical tag 0x0.
    #0  0x0000aaaade3d0b4c in ?? ()
    (gdb)

But if we do it as two separate commands (file and core) we don't:

    $ ./gdb -nx --data-directory=data-directory -q -ex "file aarch64-mte-gcore" -ex "core aarch64-mte-gcore.core"
    ...
    Program terminated with signal SIGSEGV, Segmentation fault.
    #0  0x0000aaaade3d0b4c in ?? ()
    (gdb)

The problem with the latter is that auxv data gets improperly cached
between the two commands.  When executing the file command, auxv gets
first queried here, when loading the executable:

    #0  target_auxv_search (ops=0x55555b842400 <exec_ops>, match=0x9, valp=0x7fffffffc5d0) at /home/simark/src/binutils-gdb/gdb/auxv.c:383
    #1  0x0000555557e576f2 in svr4_exec_displacement (displacementp=0x7fffffffc8c0) at /home/simark/src/binutils-gdb/gdb/solib-svr4.c:2482
    #2  0x0000555557e594d1 in svr4_relocate_main_executable () at /home/simark/src/binutils-gdb/gdb/solib-svr4.c:2878
    #3  0x0000555557e5989e in svr4_solib_create_inferior_hook (from_tty=1) at /home/simark/src/binutils-gdb/gdb/solib-svr4.c:2933
    #4  0x0000555557e6e49f in solib_create_inferior_hook (from_tty=1) at /home/simark/src/binutils-gdb/gdb/solib.c:1253
    #5  0x0000555557f33e29 in symbol_file_command (args=0x7fffffffe01c "aarch64-mte-gcore", from_tty=1) at /home/simark/src/binutils-gdb/gdb/symfile.c:1655
    #6  0x00005555573319c3 in file_command (arg=0x7fffffffe01c "aarch64-mte-gcore", from_tty=1) at /home/simark/src/binutils-gdb/gdb/exec.c:555
    #7  0x0000555556e47185 in do_simple_func (args=0x7fffffffe01c "aarch64-mte-gcore", from_tty=1, c=0x612000047740) at /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:95
    #8  0x0000555556e551c9 in cmd_func (cmd=0x612000047740, args=0x7fffffffe01c "aarch64-mte-gcore", from_tty=1) at /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:2543
    #9  0x00005555580e63fd in execute_command (p=0x7fffffffe02c "e", from_tty=1) at /home/simark/src/binutils-gdb/gdb/top.c:692
    #10 0x0000555557771913 in catch_command_errors (command=0x5555580e55ad <execute_command(char const*, int)>, arg=0x7fffffffe017 "file aarch64-mte-gcore", from_tty=1, do_bp_actions=true) at /home/simark/src/binutils-gdb/gdb/main.c:513
    #11 0x0000555557771fba in execute_cmdargs (cmdarg_vec=0x7fffffffd570, file_type=CMDARG_FILE, cmd_type=CMDARG_COMMAND, ret=0x7fffffffd230) at /home/simark/src/binutils-gdb/gdb/main.c:608
    #12 0x00005555577755ac in captured_main_1 (context=0x7fffffffda10) at /home/simark/src/binutils-gdb/gdb/main.c:1299
    #13 0x0000555557775c2d in captured_main (data=0x7fffffffda10) at /home/simark/src/binutils-gdb/gdb/main.c:1320
    #14 0x0000555557775cc2 in gdb_main (args=0x7fffffffda10) at /home/simark/src/binutils-gdb/gdb/main.c:1345
    #15 0x00005555568bdcbe in main (argc=10, argv=0x7fffffffdba8) at /home/simark/src/binutils-gdb/gdb/gdb.c:32

Here, target_auxv_search is called on the inferior's target stack.  The
target stack only contains the exec target, so the query returns empty
auxv data.  This gets cached for that inferior in `auxv_inferior_data`.

In its constructor (before it is pushed to the inferior's target stack),
the core_target needs to identify the right target description from the
core, and for that asks the gdbarch to read a target description from
the core file.  Because some implementations of
gdbarch_core_read_description (such as AArch64's) need to read auxv data
from the core in order to determine the right target description, the
core_target passes a pointer to itself, allowing implementations to call
target_auxv_search it.  However, because we have previously cached
(empty) auxv data for that inferior, target_auxv_search searched that
cached (empty) auxv data, not auxv data read from the core.  Remember
that this data was obtained by reading auxv on the inferior's target
stack, which only contained an exec target.

The problem I see is that while target_auxv_search offers the
flexibility of reading from an arbitrary (passed as an argument) target,
the caching doesn't do the distinction of which target is being queried,
and where the cached data came from.  So, you could read auxv from a
target A, it gets cached, then you try to read auxv from a target B, and
it returns the cached data from target A.  That sounds wrong.  In our
case, we expect to read different auxv data from the core target than
what we have read from the target stack earlier, so it doesn't make
sense to hit the cache in this case.

To fix this, I propose splitting the code paths that read auxv data from
an inferior's target stack and those that read from a passed-in target.
The code path that reads from the target stack will keep caching,
whereas the one that reads from a passed-in target won't.  And since,
searching in auxv data is independent from where this data came from,
split the "read" part from the "search" part.

From what I understand, auxv caching was introduced mostly to reduce
latency on remote connections, when doing many queries.  With the change
I propose, only the queries done while constructing the core_target
end up not using cached auxv data.  This is fine, because there are just
a handful of queries max, done at this point, and reading core files is
local.

The changes to auxv functions are:

 - Introduce 2 target_read_auxv functions.  One reads from an explicit
   target_ops and doesn't do caching (to be used in
   gdbarch_core_read_description context).  The other takes no argument,
   reads from the current inferior's target stack (it looks just like a
   standard target function wrapper) and does caching.

   The first target_read_auxv actually replaces get_auxv_inferior_data,
   since it became a trivial wrapper around it.

 - Change the existing target_auxv_search to not read auxv data from the
   target, but to accept it as a parameter (a gdb::byte_vector).  This
   function doesn't care where the data came from, it just searches in
   it.  It still needs to take a target_ops and gdbarch to know how to
   parse auxv entries.

 - Add a convenience target_auxv_search overload that reads auxv
   data from the inferior's target stack and searches in it.  This
   overload is useful to replace the exist target_auxv_search calls that
   passed the `current_inferior ()->top_target ()` target and keep the
   call sites short.

 - Modify parse_auxv to accept a target_ops and gdbarch to use for
   parsing entries.  Not strictly related to the rest of this change,
   but it seems like a good change in the context.

Changes in architecture-specific files (tdep and nat):

 - In linux-tdep, linux_get_hwcap and linux_get_hwcap2 get split in two,
   similar to target_auxv_search.  One version receives auxv data,
   target and arch as parameters.  The other gets everything from the
   current inferior.  The latter is for convenience, to avoid making
   call sites too ugly.

 - Call sites of linux_get_hwcap and linux_get_hwcap2 are adjusted to
   use either of the new versions.  The call sites in
   gdbarch_core_read_description context explicitly read auxv data from
   the passed-in target and call the linux_get_hwcap{,2} function with
   parameters.  Other call sites use the versions without parameters.

 - Same idea for arm_fbsd_read_description_auxv.

 - Call sites of target_auxv_search that passed
   `current_inferior ()->top_target ()` are changed to use the
   target_auxv_search overload that works in the current inferior.

Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Reviewed-By: Luis Machado <luis.machado@arm.com>
Change-Id: Ib775a220cf1e76443fb7da2fdff8fc631128fe66
2022-10-11 13:52:18 -04:00

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/* Target-dependent code for GNU/Linux, architecture independent.
Copyright (C) 2009-2022 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef LINUX_TDEP_H
#define LINUX_TDEP_H
#include "bfd.h"
#include "displaced-stepping.h"
struct inferior;
struct regcache;
/* Enum used to define the extra fields of the siginfo type used by an
architecture. */
enum linux_siginfo_extra_field_values
{
/* Add bound fields into the segmentation fault field. */
LINUX_SIGINFO_FIELD_ADDR_BND = 1
};
/* Defines a type for the values defined in linux_siginfo_extra_field_values. */
DEF_ENUM_FLAGS_TYPE (enum linux_siginfo_extra_field_values,
linux_siginfo_extra_fields);
/* This function is suitable for architectures that
extend/override the standard siginfo in a specific way. */
struct type *linux_get_siginfo_type_with_fields (struct gdbarch *gdbarch,
linux_siginfo_extra_fields);
/* Return true if ADDRESS is within the boundaries of a page mapped with
memory tagging protection. */
bool linux_address_in_memtag_page (CORE_ADDR address);
typedef char *(*linux_collect_thread_registers_ftype) (const struct regcache *,
ptid_t,
bfd *, char *, int *,
enum gdb_signal);
extern enum gdb_signal linux_gdb_signal_from_target (struct gdbarch *gdbarch,
int signal);
extern int linux_gdb_signal_to_target (struct gdbarch *gdbarch,
enum gdb_signal signal);
/* Default GNU/Linux implementation of `displaced_step_location', as
defined in gdbarch.h. Determines the entry point from AT_ENTRY in
the target auxiliary vector. */
extern CORE_ADDR linux_displaced_step_location (struct gdbarch *gdbarch);
/* Implementation of gdbarch_displaced_step_prepare. */
extern displaced_step_prepare_status linux_displaced_step_prepare
(gdbarch *arch, thread_info *thread, CORE_ADDR &displaced_pc);
/* Implementation of gdbarch_displaced_step_finish. */
extern displaced_step_finish_status linux_displaced_step_finish
(gdbarch *arch, thread_info *thread, gdb_signal sig);
/* Implementation of gdbarch_displaced_step_copy_insn_closure_by_addr. */
extern const displaced_step_copy_insn_closure *
linux_displaced_step_copy_insn_closure_by_addr
(inferior *inf, CORE_ADDR addr);
/* Implementation of gdbarch_displaced_step_restore_all_in_ptid. */
extern void linux_displaced_step_restore_all_in_ptid (inferior *parent_inf,
ptid_t ptid);
extern void linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch,
int num_disp_step_buffers);
extern int linux_is_uclinux (void);
/* Fetch the AT_HWCAP entry from auxv data AUXV. Use TARGET and GDBARCH to
parse auxv entries.
On error, 0 is returned. */
extern CORE_ADDR linux_get_hwcap (const gdb::optional<gdb::byte_vector> &auxv,
struct target_ops *target, gdbarch *gdbarch);
/* Same as the above, but obtain all the inputs from the current inferior. */
extern CORE_ADDR linux_get_hwcap ();
/* Fetch the AT_HWCAP2 entry from auxv data AUXV. Use TARGET and GDBARCH to
parse auxv entries.
On error, 0 is returned. */
extern CORE_ADDR linux_get_hwcap2 (const gdb::optional<gdb::byte_vector> &auxv,
struct target_ops *target, gdbarch *gdbarch);
/* Same as the above, but obtain all the inputs from the current inferior. */
extern CORE_ADDR linux_get_hwcap2 ();
/* Fetch (and possibly build) an appropriate `struct link_map_offsets'
for ILP32 and LP64 Linux systems. */
extern struct link_map_offsets *linux_ilp32_fetch_link_map_offsets ();
extern struct link_map_offsets *linux_lp64_fetch_link_map_offsets ();
#endif /* linux-tdep.h */
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