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Date: Wed, 14 Sep 2016 10:40:53 -0400
From: Rich Felker <>
Subject: Re: incompatibility between libtheora/mmx and musl ?

On Wed, Sep 14, 2016 at 10:28:42AM -0400, Rich Felker wrote:
> > > (or you can try some hack in _ogg_malloc/free if you are
> > > sure that's what they are using)
> > 
> > Yes it is present/used for this very purpose, to enable easy "hijacking".
> > 
> > OTOH when I checked the arguments in gdb they looked always sane, up to
> > the last and crashing realloc() call. That's why I do not expect seeing
> > anything unusual there.
> > 
> > Valgrind did not see any bad free()s either.
> > 
> > > there can be some call abi issue (register clobbering,
> > > stack alignment,..) because of the asm, but that's hard
> > > to check.
> > 
> > Is musl in any way special compared to glibc/uclibc in its register usage?
> Not in principle; this is mandated by the ABI. But it's possible that
> their violation of ABI contracts is visible with some implementations
> but not others. For example if they're calling malloc from code that's
> using asm it's possible that they assume the floating point registers
> (or mmx state) are call-saved rather than call-clobbered. This is an
> invalid assumption that might happen to actively break on musl but not
> glibc. IIRC you need some special instructions to switch between x87
> and (original) mmx usage; perhaps they're missing this somewhere.

Another possibility: they may be changing the x87 control word to
something that yields non-conforming behavior. musl does not support
this (unless of course you change it back before any musl code could
get invoked).


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