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Date: Wed, 14 Sep 2016 10:39:42 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: incompatibility between libtheora/mmx and musl ?
On Wed, Sep 14, 2016 at 05:31:37PM +0300, Timo Teras wrote:
> On Wed, 14 Sep 2016 10:28:42 -0400
> Rich Felker <dalias@...c.org> wrote:
>
> > > > 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.
>
> Also, since it's Alpine (mentioned in first there with the bug report),
> we compile everything PIE. So on x86 ebx is reserved and should not be
> clobbered.
ebx is always a call-saved register in the x86 ABI, though. The
difference is that, if they make any cross-library calls that may go
through the PLT, ebx must point to the caller's GOT at the time of the
call. But this difference only applies in the main-program (for
static-linked libtheora, for instance); with dynamic linking the code
is already required to be PIC and thus already has to be taking this
into account.
Rich
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