Hiding global symbols declared in assembly files

[Andrew Scherkus]

While attempting to link libvpx.a directly instead of loading it via libffmpegsumo.so, src/chrome/tools/build/mac/verify_order alerted me that additional symbols were being exported.

A quick scan reveals the global symbols declared inside of libvpx .asm files are now getting exported by Chrome.

Typically, -fvisibility=hidden takes care of this for us but it appears that yasm has no such global option but rather an ELF-specific option you can tack on to a global symbol [1].

FFmpeg actually uses said option via its cglobal() macro [2] but libvpx's sym() macro does no such thing [3]. Even if we were to patch libvpx it wouldn't solve the issue for mac's machoness.

I had something really hacky that uses objcopy to hide the offending symbols on a resulting binary, but that feels pretty dirty.

Andrew

[1] http://www.tortall.net/projects/yasm/manual/html/objfmt-elf-dir-global.html

[2] http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavutil/x86/x86inc.asm;hb=HEAD

[3] http://review.webmproject.org/gitweb?p=libvpx.git;a=blob;f=vpx_ports/x86_abi_support.asm;hb=HEAD

[Hironori Bono (坊野 博典)]

Greetings Andrew,

Our local copy of yasm has a local patch that allows us to set
private_extern bits of a Mach-O object, which is the same option as
the hidden option of ELF. (This change is r73761
<http://crrev.com/73761>.) Therefore, even though this works only on
Chromium, we can create hidden symbols if we add private_extern flags
to them as my 73971 <http://crrev.com/73971> did for libjpeg-turbo.
(Even though I have filed a bug to yasm
<http://www.tortall.net/projects/yasm/ticket/236> and sent my patch, I
cannot receive any responses.)

Regards,

Hironori Bono
E-mail: hb...@google.com

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[Andrew Scherkus]

Hironori! You're a life saver!

Much cleaner than anything I was planning on doing with objcopy haha

Shame that your yasm patch hasn't gained any traction :\

Andrew