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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