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5b56203a7c
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Simon Marchi 5b56203a7c gdb: fix value_subscript when array upper bound is not known 
Since commit 7c6f271296 ("gdb: make get_discrete_bounds check for
non-constant range bounds"), subscripting  flexible array member fails:

    struct no_size
    {
      int n;
      int items[];
    };

    (gdb) p *ns
    $1 = {n = 3, items = 0x5555555592a4}
    (gdb) p ns->items[0]
    Cannot access memory at address 0xfffe555b733a0164
    (gdb) p *((int *) 0x5555555592a4)
    $2 = 101  <--- we would expect that
    (gdb) p &ns->items[0]
    $3 = (int *) 0xfffe5559ee829a24  <--- wrong address

Since the flexible array member (items) has an unspecified size, the array type
created for it in the DWARF doesn't have dimensions (this is with gcc 9.3.0,
Ubuntu 20.04):

    0x000000a4:   DW_TAG_array_type
                    DW_AT_type [DW_FORM_ref4]       (0x00000038 "int")
                    DW_AT_sibling [DW_FORM_ref4]    (0x000000b3)

    0x000000ad:     DW_TAG_subrange_type
                      DW_AT_type [DW_FORM_ref4]     (0x00000031 "long unsigned int")

This causes GDB to create a range type (TYPE_CODE_RANGE) with a defined
constant low bound (dynamic_prop with kind PROP_CONST) and an undefined
high bound (dynamic_prop with kind PROP_UNDEFINED).

value_subscript gets both bounds of that range using
get_discrete_bounds.  Before commit 7c6f271296, get_discrete_bounds
didn't check the kind of the dynamic_props and would just blindly read
them as if they were PROP_CONST.  It would return 0 for the high bound,
because we zero-initialize the range_bounds structure.  And it didn't
really matter in this case, because the returned high bound wasn't used
in the end.

Commit 7c6f271296 changed get_discrete_bounds to return a failure if
either the low or high bound is not a constant, to make sure we don't
read a dynamic prop that isn't a PROP_CONST as a PROP_CONST.  This
change made get_discrete_bounds start to return a failure for that
range, and as a result would not set *lowp and *highp.  And since
value_subscript doesn't check get_discrete_bounds' return value, it just
carries on an uses an uninitialized value for the low bound.  If
value_subscript did check the return value of get_discrete_bounds, we
would get an error message instead of a bogus value.  But it would still
be a bug, as we wouldn't be able to print the flexible array member's
elements.

Looking at value_subscript, we see that the low bound is always needed,
but the high bound is only needed if !c_style.  So, change
value_subscript to use get_discrete_low_bound and
get_discrete_high_bound separately.  This fixes the case described
above, where the low bound is known but the high bound isn't (and is not
needed).  This restores the original behavior without accessing a
dynamic_prop in a wrong way.

A test is added.  In addition to the case described above, a case with
an array member of size 0 is added, which is a GNU C extension that
existed before flexible array members were introduced.  That case
currently fails when compiled with gcc <= 8.  gcc <= 8 produces DWARF
similar to the one shown above, while gcc 9 adds a DW_AT_count of 0 in
there, which makes the high bound known.  A case where an array member
of size 0 is the only member of the struct is also added, as that was
how PR 28675 was originally reported, and it's an interesting corner
case that I think could trigger other funny bugs.

Question about the implementation: in value_subscript, I made it such
that if the low or high bound is unknown, we fall back to zero.  That
effectively makes it the same as it was before 7c6f271296.  But should
we instead error() out?

gdb/ChangeLog:

	PR 26875, PR 26901
	* gdbtypes.c (get_discrete_low_bound): Make non-static.
	(get_discrete_high_bound): Make non-static.
	* gdbtypes.h (get_discrete_low_bound): New declaration.
	(get_discrete_high_bound): New declaration.
	* valarith.c (value_subscript): Only fetch high bound if
	necessary.

gdb/testsuite/ChangeLog:

	PR 26875, PR 26901
	* gdb.base/flexible-array-member.c: New test.
	* gdb.base/flexible-array-member.exp: New test.

Change-Id: I832056f80e6c56f621f398b4780d55a3a1e299d7
2020-12-09 13:52:12 -05:00
bfd Automatic date update in version.in 2020-12-09 00:00:17 +00:00
binutils Remove references to the unofficial SHF_GNU_BUILD_NOTE section flag. 2020-12-07 16:17:40 +00:00
config sync libiberty from gcc 2020-09-08 20:12:57 +09:30
contrib Import mklog.py from gcc repo 2020-09-25 10:24:44 -04:00
cpu Fix spelling mistakes 2020-10-05 14:20:15 +01:00
elfcpp gold: Update GNU_PROPERTY_X86_XXX macros 2020-10-13 05:21:05 -07:00
etc texi2pod.pl: import support for @t{...} from gcc 2020-01-15 12:58:09 -05:00
gas gas: Generate a new section for SHF_GNU_RETAIN 2020-12-08 16:41:32 -08:00
gdb gdb: fix value_subscript when array upper bound is not known 2020-12-09 13:52:12 -05:00
gdbserver gdbserver: add missing --disable-packet options to help text 2020-11-11 09:08:31 +00:00
gdbsupport gdbsupport: Use LOCALAPPDATA to determine cache dir 2020-12-08 09:50:12 -05:00
gnulib gnulib: Ensure all libraries are used when building gdb/gdbserver 2020-10-09 09:31:43 +01:00
gold [GOLD] gcc-11 stringop-overflow warning 2020-12-07 22:36:22 +10:30
gprof Unify Solaris procfs and largefile handling 2020-07-30 15:41:50 +02:00
include Remove references to the unofficial SHF_GNU_BUILD_NOTE section flag. 2020-12-07 16:17:40 +00:00
intl Regen with blessed automake-1.15.1 2020-02-20 13:02:24 +10:30
ld VAX/BFD: Do not warn about GOT addend mismatches if no GOT entry is made 2020-12-05 11:18:14 +00:00
libctf libctf: Pass format argument to asprintf 2020-11-25 19:11:36 +00:00
libdecnumber Run autoreconf -vf throughout 2020-07-29 16:03:55 -04:00
libiberty Sync libiberty and include with GCC for get_DW_UT_name. 2020-09-24 22:55:24 +02:00
opcodes IBM Z: Add risbgz and risbgnz extended mnemonics 2020-12-04 09:14:02 +01:00
readline Update readline/README to mention patchlevel 2020-06-30 15:17:07 -06:00
sim sim: pru: Add support for LMBD instruction 2020-11-12 22:41:10 +02:00
texinfo
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ChangeLog Add gnu global outputs to .gitignore 2020-12-02 10:00:27 -05:00
compile
config-ml.in Update top level configure files by synchronizing them with gcc. 2018-01-10 15:29:21 +00:00
config.guess Update top level config files with copies from the official repository. 2020-01-18 13:43:19 +00:00
config.rpath
config.sub Update top level config files with copies from the official repository. 2020-01-18 13:43:19 +00:00
configure config/debuginfod.m4: Use PKG_CHECK_MODULES 2020-07-24 15:16:20 -04:00
configure.ac config/debuginfod.m4: Use PKG_CHECK_MODULES 2020-07-24 15:16:20 -04:00
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install-sh
libtool.m4 Bump to autoconf 2.69 and automake 1.15.1 2018-06-19 16:55:06 -04:00
lt~obsolete.m4
ltgcc.m4
ltmain.sh
ltoptions.m4
ltsugar.m4
ltversion.m4
MAINTAINERS Move gdbserver to top level 2020-02-07 08:42:25 -07:00
Makefile.def gdb/gdbserver: add dependencies for distclean-gnulib 2020-10-14 15:05:14 +01:00
Makefile.in gdb/gdbserver: add dependencies for distclean-gnulib 2020-10-14 15:05:14 +01:00
Makefile.tpl Revert "Sync top level files with versions from gcc." 2019-05-30 11:17:19 +01:00
makefile.vms
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README
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setup.com
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ylwrap

README 

		   README for GNU development tools

This directory contains various GNU compilers, assemblers, linkers, 
debuggers, etc., plus their support routines, definitions, and documentation.

If you are receiving this as part of a GDB release, see the file gdb/README.
If with a binutils release, see binutils/README;  if with a libg++ release,
see libg++/README, etc.  That'll give you info about this
package -- supported targets, how to use it, how to report bugs, etc.

It is now possible to automatically configure and build a variety of
tools with one command.  To build all of the tools contained herein,
run the ``configure'' script here, e.g.:

	./configure 
	make

To install them (by default in /usr/local/bin, /usr/local/lib, etc),
then do:
	make install

(If the configure script can't determine your type of computer, give it
the name as an argument, for instance ``./configure sun4''.  You can
use the script ``config.sub'' to test whether a name is recognized; if
it is, config.sub translates it to a triplet specifying CPU, vendor,
and OS.)

If you have more than one compiler on your system, it is often best to
explicitly set CC in the environment before running configure, and to
also set CC when running make.  For example (assuming sh/bash/ksh):

	CC=gcc ./configure
	make

A similar example using csh:

	setenv CC gcc
	./configure
	make

Much of the code and documentation enclosed is copyright by
the Free Software Foundation, Inc.  See the file COPYING or
COPYING.LIB in the various directories, for a description of the
GNU General Public License terms under which you can copy the files.

REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info
on where and how to report problems.