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From: "Laszlo Ersek" <lersek@redhat.com>
To: Andrew Fish <afish@apple.com>,
	devel@edk2.groups.io, Mike Kinney <michael.d.kinney@intel.com>
Cc: "Gao, Liming" <liming.gao@intel.com>
Subject: Re: [edk2-devel] [PATCH 04/10] MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE
Date: Thu, 18 Apr 2019 10:45:40 +0200	[thread overview]
Message-ID: <fdfb259c-9293-2bcd-300b-92b5741ba8c0@redhat.com> (raw)
In-Reply-To: <6FA88780-622D-4788-BDF8-6C807D85D9D1@apple.com>

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

  parent reply	other threads:[~2019-04-18  8:45 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-12 23:31 [PATCH 00/10] patches for some warnings raised by "RH covscan" Laszlo Ersek
2019-04-12 23:31 ` [PATCH 01/10] MdePkg/PiFirmwareFile: express IS_SECTION2 in terms of SECTION_SIZE Laszlo Ersek
2019-04-15 17:01   ` [edk2-devel] " Philippe Mathieu-Daudé
2019-04-12 23:31 ` [PATCH 02/10] MdePkg/PiFirmwareFile: fix undefined behavior in SECTION_SIZE Laszlo Ersek
2019-04-14  7:19   ` [edk2-devel] " Jordan Justen
2019-04-15 16:15     ` Laszlo Ersek
2019-04-16  8:28       ` Liming Gao
2019-04-16  9:04       ` Jordan Justen
2019-04-16 10:59         ` Laszlo Ersek
2019-04-16 16:50           ` Philippe Mathieu-Daudé
2019-04-17 10:08             ` Laszlo Ersek
2019-04-16 18:48           ` Jordan Justen
2019-04-16 23:25             ` Andrew Fish
2019-04-17 10:29               ` Laszlo Ersek
2019-04-17 11:44                 ` Andrew Fish
2019-04-17 14:59                   ` Laszlo Ersek
2019-04-17 19:35                     ` Jordan Justen
2019-04-18  9:38                       ` Laszlo Ersek
2019-04-18 15:18                         ` Liming Gao
2019-04-17 10:01             ` Laszlo Ersek
2019-04-12 23:31 ` [PATCH 03/10] BaseTools/PiFirmwareFile: " Laszlo Ersek
2019-04-12 23:31 ` [PATCH 04/10] MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE Laszlo Ersek
2019-04-15 17:23   ` [edk2-devel] " Philippe Mathieu-Daudé
2019-04-17 17:52   ` Michael D Kinney
2019-04-17 18:31     ` Michael D Kinney
2019-04-18  9:06       ` Laszlo Ersek
2019-04-17 18:31     ` Andrew Fish
2019-04-17 18:36       ` Michael D Kinney
2019-04-18  8:48         ` Laszlo Ersek
2019-04-18  8:45       ` Laszlo Ersek [this message]
2019-04-18 23:12         ` Laszlo Ersek
2019-04-18 17:20     ` Philippe Mathieu-Daudé
2019-04-18 17:59       ` Michael D Kinney
2019-04-18 18:12         ` Philippe Mathieu-Daudé
2019-04-12 23:31 ` [PATCH 05/10] OvmfPkg/Sec: fix out-of-bounds reads Laszlo Ersek
2019-04-15 17:24   ` [edk2-devel] " Philippe Mathieu-Daudé
2019-04-12 23:31 ` [PATCH 06/10] OvmfPkg/QemuVideoDxe: avoid arithmetic on null pointer Laszlo Ersek
2019-04-12 23:31 ` [PATCH 07/10] OvmfPkg/AcpiPlatformDxe: suppress invalid "deref of undef pointer" warning Laszlo Ersek
2019-04-15 17:26   ` [edk2-devel] " Philippe Mathieu-Daudé
2019-04-12 23:31 ` [PATCH 08/10] OvmfPkg: suppress "Value stored to ... is never read" analyzer warnings Laszlo Ersek
2019-04-14  8:03   ` [edk2-devel] " Jordan Justen
2019-04-15 16:25     ` Laszlo Ersek
2019-04-16  9:26       ` Jordan Justen
2019-04-16 11:44         ` Laszlo Ersek
2019-04-12 23:31 ` [PATCH 09/10] OvmfPkg/AcpiPlatformDxe: catch theoretical nullptr deref in Xen code Laszlo Ersek
2019-04-15 17:28   ` [edk2-devel] " Philippe Mathieu-Daudé
2019-04-12 23:31 ` [PATCH 10/10] OvmfPkg/BasePciCapLib: suppress invalid "nullptr deref" warning Laszlo Ersek
2019-04-15 17:31   ` [edk2-devel] " Philippe Mathieu-Daudé
2019-04-16 11:01     ` Laszlo Ersek
2019-04-12 23:36 ` [PATCH 00/10] patches for some warnings raised by "RH covscan" Ard Biesheuvel
2019-04-15 16:16   ` Laszlo Ersek
2019-04-18 14:20 ` [edk2-devel] " Laszlo Ersek

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