libffmpeg chromium crashes due to unaligned SSE accesses

[Dimitry Andric]

I just tried building multimedia/ffmpeg on i386-freebsd11, with the default port settings, and it seems to work just fine.  I tried transcoding a few files, and there were no stack alignment problems or SIGBUSes.

Looking at the build logs, I see 

C compiler                cc
ARCH                      x86 (generic)
big-endian                no
runtime cpu detection     yes
yasm                      yes
MMX enabled               yes
MMXEXT enabled            yes
3DNow! enabled            yes
3DNow! extended enabled   yes
SSE enabled               yes
SSSE3 enabled             yes
AVX enabled               yes
FMA4 enabled              yes
i686 features enabled     yes
CMOV is fast              no
EBX available             yes
EBP available             yes
...

The command line flags used for compilation (wrapped for clarity) don't seem to include specific ones that change stack alignment behavior:

cc \
-I. \
-I./ \
-DLIBICONV_PLUG \
-D_ISOC99_SOURCE \
-D_FILE_OFFSET_BITS=64 \
-D_LARGEFILE_SOURCE \
-DHAVE_AV_CONFIG_H \
-O2 \
-pipe \
-march=corei7 \
-DLIBICONV_PLUG \
-fno-strict-aliasing \
-msse \
-I/usr/local/include/vorbis \
-I/usr/local/include \
-std=c99 \
-fomit-frame-pointer \
-I/usr/local/include \
-I/usr/local/include/freetype2 \
-I/usr/local/include/libpng15 \
-I/usr/local/include \
-I/usr/local/include/p11-kit-1 \
-I/usr/local/include/freetype2 \
-I/usr/local/include/libpng15 \
-I/usr/local/include/opencv \
-I/usr/local/include \
-I/usr/local/include/schroedinger-1.0 \
-I/usr/local/include/orc-0.4 \
-Wdeclaration-after-statement \
-Wall \
-Wno-parentheses \
-Wno-switch \
-Wno-format-zero-length \
-Wdisabled-optimization \
-Wpointer-arith \
-Wredundant-decls \
-Wno-pointer-sign \
-Wwrite-strings \
-Wtype-limits \
-Wundef \
-Wmissing-prototypes \
-Wno-pointer-to-int-cast \
-Wstrict-prototypes \
-O3 \
-fno-math-errno \
-fno-signed-zeros \
-Qunused-arguments \
-Werror=implicit-function-declaration \
-Werror=missing-prototypes \
-Werror=return-type \
-MMD \
-c \

I'll build chromium with the default options, and see what happens.

-Dimitry

On 09 May 2014, at 19:09, Adrian Chadd <adrian.chadd at gmail.com> wrote:

> What's the magic to get the normal ffmpeg port to work right?
> 
> 
> -a
> 
> 
> On 9 May 2014 10:05, Dimitry Andric <dim at freebsd.org> wrote:
>> On 09 May 2014, at 18:42, Adrian Chadd <adrian.chadd at gmail.com> wrote:
>>> On 9 May 2014 06:50, Pedro Giffuni <pfg at freebsd.org> wrote:
>>>> Hello;
>>>> 
>>>> El 5/9/2014 5:56 AM, Adrian Chadd escribió:
>>>> 
>>>>> Hi guys,
>>>>> 
>>>>> I filed a PR recently with chromium crashes in its internal libffmpeg:
>>>>> 
>>>>> http://www.freebsd.org/cgi/query-pr.cgi?pr=189317
>>>>> 
>>>>> What do you two think? It's that Linux 16 byte alignment on i386 issue
>>>>> that has been creeping up every few years.
>>>>> 
>>>> 
>>>> Ouch, that's clang, right?
>>> 
>>> I gather so? It's whatever the binary package building cluster is
>>> using. I think it's clang for i386.
>> 
>> For 10.x and 11.x, that should indeed be clang.
>> 
>> 
>>> 
>>>> I recently brought this from OpenBSD, no idea if it's related:
>>>> 
>>>> http://svnweb.freebsd.org/base?view=revision&revision=265231
>>>> 
>>>> For now I guess we should just patch the libffmpeg port like the NetBSD guys
>>>> did.
>>> 
>>> Kind of? The x86-64 ABI requires 16 byte alignment for a lot of stuff.
>>> The i386 32 bit ABI doesn't require 16 byte alignment as per
>>> everything pre-Linux-in-2005ish. Linux / gcc flipped the "i386 == 16
>>> byte alignment now" switch. I vaguely recall that they made
>>> _everything_ 16 byte aligned but I can't be sure.
>> 
>> Yes, actually the gcc guys just flipped the switch somewhere in 2008,
>> without any consideration for backwards compatibility, and this lead to
>> quite a bit of wailing, but they WONTFIXed it anyway:
>> 
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38496
>> 
>> So the problem is that there are quite a lot of projects that simply
>> assume everything on x86 has 16-byte aligned stacks, and you can use SSE
>> instructions that require strict alignment (e.g. movaps) on any random
>> stack-allocated variable.  Obviously, on i386-freebsd, that is not the
>> case, as we still maintain the old SysV 4-byte alignment.
>> 
>> FFmpeg is one of those projects that assumes 16-byte alignment, and also
>> has a lot of hand-written SSE assembly, either inline or in separate
>> yasm sources.  The brute-force way of fixing trouble with alignment is
>> to add -mstackrealign to CFLAGS, but I'm not sure if that is the correct
>> solution here.
>> 
>> As far as I know, the current FFmpeg port seems to work OK on
>> i386-freebsd, so maybe it could be enough to fix up the Chromium version
>> of FFmpeg in a similar manner as the regular FFmpeg port?  I'm not sure
>> I will have enough time to have look at it soon, though...
>> 
>> -Dimitry
>> 

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