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Subject: Re: [edk2-devel] [PATCH 04/10] MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE
To: Andrew Fish <afish@apple.com>, devel@edk2.groups.io,
 Mike Kinney <michael.d.kinney@intel.com>
Cc: "Gao, Liming" <liming.gao@intel.com>
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From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <fdfb259c-9293-2bcd-300b-92b5741ba8c0@redhat.com>
Date: Thu, 18 Apr 2019 10:45:40 +0200
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On 04/17/19 20:31, Andrew Fish wrote:

> I was actually writing a mail to some people that sit on the C/C++
> standards committee that are UB experts to get some clarification when
> you sent this mail. [...]
>
> I'm basically asking if this code pedantic conforms to C99 and C11:
>
> EFI_COMMON_SECTION_HEADER gSec = { { 0x01, 0x02, 0x3 }, 0x10 };
>
> return *(UINT32 *)gSec.Size & 0x00ffffff;

I'm really interested as to how they will respond!

(Because I think (see up-thread) that the code doesn't conform.)

> I ran the clang static analyzer and runtime ubsan on the above code
> and it did not complain (I force strict aliasing via
> -fstrict-aliasing, and I'm using the Sys V ABI since this is just the
> command line compiler on my Mac).

Yesterday I tried something similar with both gcc and clang (not recent
versions of them, unfortunately), and they were all too happy with the
code as well, when I thought they should have flagged it.

The options I used were:

  -fsyntax-only -Wall -Wextra -std=c99 -pedantic \
  -fstrict-aliasing -Wstrict-aliasing

Beyond that, I checked the documentation of "-fstrict-aliasing" in the
gcc manual at
<https://gcc.gnu.org/onlinedocs/gcc-8.3.0/gcc/Optimize-Options.html>. It
provides several examples; one of them is:

> Similarly, access by taking the address, casting the resulting pointer
> and dereferencing the result has undefined behavior, even if the cast
> uses a union type, e.g.:
>
> union a_union {
>   int i;
>   double d;
> };
>
> int f() {
>   double d = 3.0;
>   return ((union a_union *) &d)->i;
> }

I think the access in this example is well defined (I seem able to
deduce it using the effective type rules), and so I consider this a bug
in the gcc docs. I reached out to someone in the toolchain team at Red
Hat to confirm or disprove.

Thanks
Laszlo