[Bug c/66661] New: incorrect memory access in optimization with flexible array member

[Bug c/66661] New: incorrect memory access in optimization with flexible array member

[P at draigBrady dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

            Bug ID: 66661
           Summary: incorrect memory access in optimization with flexible
                    array member
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: P at draigBrady dot com
  Target Milestone: ---

On a heap allocated structure, direct access to flexible array members with
optimization at -O2 can result in reads to memory beyond the heap object.
I.E. gcc assumes alignment/padding is allocated when accessing flexible array
members. The attached file is a summary of the code involved though does _not_
reproduce the issue.

To reproduce one can:

  git clone --depth=1 git://git.sv.gnu.org/coreutils.git
  cd coreutils/
  git checkout 53883af0
  export LSAN_OPTIONS=exitcode=0
  ./bootstrap && ./configure --quiet && \
  make -j8 AM_CFLAGS='-fsanitize=address -fsanitize=undefined'
  src/chmod a+rx ..

Also attached is the disassembly of the problematic code,
and for comparison good code achieved by using a (char*) cast
on the flexi array to force byte at a time access.

[Bug c/66661] incorrect memory access in optimization with flexible array member

[P at draigBrady dot com]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #1 from Pádraig Brady <P at draigBrady dot com> ---
Created attachment 35849
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35849&action=edit
summary code (does not reproduce issue)
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #3 from Pádraig Brady <P at draigBrady dot com> ---
Created attachment 35851
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35851&action=edit
disassembly of forced good mem access
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #2 from Pádraig Brady <P at draigBrady dot com> ---
Created attachment 35850
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35850&action=edit
disassembly of problematic mem access
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Subject: [Bug middle-end/66661] incorrect memory access in optimization with flexible array member
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #4 from Pádraig Brady <P at draigBrady dot com> ---
I should note that I worked around the issue by increasing the allocation for
the structure on the heap up to a multiple of alignof(the_struct). See:
http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=49078a78
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Subject: [Bug middle-end/66661] incorrect memory access in optimization with flexible array member
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Pádraig Brady from comment #4)
> I should note that I worked around the issue by increasing the allocation
> for the structure on the heap up to a multiple of alignof(the_struct). See:
> http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=49078a78

That is not just a workaround but a correct patch.  The size of the memory
allocated needs to be a multiply of the alignment of the struct to be valid.

The reason why it works for you with your reduced testcase is because the
alignment of the struct is 1.
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Subject: [Bug middle-end/66661] incorrect memory access in optimization with flexible array member
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf661

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
More to the point this might actually be a -fsanitize­dress bug so reopening
for that issue.  By the way we know the alignment of the struct is 4 which
means loading a 4 byte would be valid.

[Bug middle-end/66661] incorrect memory access in optimization with flexible array member

[P at draigBrady dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #7 from Pádraig Brady <P at draigBrady dot com> ---
Created attachment 35852
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35852&action=edit
reproducer
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf658

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
See also pr64639 (which this bug may be a duplicate of).

[Bug middle-end/66661] incorrect memory access in optimization with flexible array member

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

Richard Biener <rguenth at gcc dot gnu.org> changed:

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----------------------------------------------------------------------------
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         Resolution|---                         |INVALID

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yes, consider

struct X { int n; char x[1]; };

which has sizeof(X) == 8 unless you use __attribute__((packed)) (in which case
alignment also gets dropped down to 1).

[Bug middle-end/66661] incorrect memory access in optimization with flexible array member

[P at draigBrady dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #9 from Pádraig Brady <P at draigBrady dot com> ---
I'm not understanding completely TBH. Are flexible array members not special?
Should the optimizations be restricted on access through the flexible array,
because I presume most/all existing allocation code is not considering this
alignment/padding issue. I certainly didn't notice any examples when looking
into a workaround which I came up with independently. For my reference there
are some notes RE GCC's divergence from C99 re padding and flexi arrays at:
https://sites.google.com/site/embeddedmonologue/home/c-programming/data-alig
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Subject: [Bug target/64783] -march=armv8.1-a should be supported
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd783

--- Comment #3 from mwahab at gcc dot gnu.org ---
I've just noticed this has been assigned to me. Support for -march=armv8.1-a
has been added to the Aarch64 backend, the ARM backend is still to be done.

Author: mwahab
Date: Tue Jun 16 13:38:37 2015
New Revision: 224519

URL: https://gcc.gnu.org/viewcvs?rev"4519&root=gcc&view=rev
Log:
2015-06-16  Matthew Wahab  <matthew.wahab@arm.com>

        * config/aarch64/aarch64-arches.def: Add "armv8.1-a".
        * config/aarch64/aarch64-options-extensions.def: Update "fP",
        "simd" and "crypto".  Add "lse", "pan", "lor" and "rdma".
        * gcc/config/aarch64/aarch64.h (AARCH64_FL_LSE): New.
        (AARCH64_FL_PAN): New.
        (AARCH64_FL_LOR): New.
        (AARCH64_FL_RDMA): New.
        (AARCH64_FL_FOR_ARCH8_1): New.
        * doc/invoke.texi (AArch64 Options): Add "armv8.1-a" to
        -march. Add "lse", "pan", "lor", "rdma" to feature modifiers.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/aarch64/aarch64-arches.def
    trunk/gcc/config/aarch64/aarch64-option-extensions.def
    trunk/gcc/config/aarch64/aarch64.h
    trunk/gcc/doc/invoke.texi

[Bug middle-end/66661] incorrect memory access in optimization with flexible array member

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

--- Comment #10 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Thu, 25 Jun 2015, P at draigBrady dot com wrote:

> I'm not understanding completely TBH. Are flexible array members not special?
> Should the optimizations be restricted on access through the flexible array,
> because I presume most/all existing allocation code is not considering this
> alignment/padding issue. I certainly didn't notice any examples when looking
> into a workaround which I came up with independently. For my reference there
> are some notes RE GCC's divergence from C99 re padding and flexi arrays at:
> https://sites.google.com/site/embeddedmonologue/home/c-programming/data-alig

That page doesn't exist - I assume you mean: 
https://sites.google.com/site/embeddedmonologue/home/c-programming/data-alignment-structure-padding-and-flexible-array-member

That page is over ten years out of date - it's quoting the wording in C99 
as it was before it was corrected by TC2 (published 2004-11-15).  You 
should look at the post-TC2 wording rather than the old wording with 
various defects in it.