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Message-ID: <20150518220731.GA31132@euler>
Date: Tue, 19 May 2015 00:07:31 +0200
From: Felix Janda <felix.janda@...teo.de>
To: musl@...ts.openwall.com
Subject: Re: ppc soft-float regression
Rich Felker wrote:
> On Mon, May 18, 2015 at 04:10:43PM -0400, Rich Felker wrote:
> > OK I've looked at this and I understand what's happening. PowerPC does
> > not have a separate relocation type for GOT entries; instead it uses
> > the same relocation type used for address constants global data. These
> > do not get re-processed after the main program and libraries are
> > added, because unlike GOT slots, they have addends, and if the addend
> > is inline (using REL rather than RELA) then it's already been
> > clobbered by the early relocation phase and can't easily be recovered.
> >
> > I see three possible solutions:
> >
> > 1. Treat R_PPC_ADDR32 as a GOT relocation instead of a regular
> > symbolic relocation in data. This would suppress the addend (giving
> > wrong address) if inline addends (REL) were used, but in practice
> > powerpc aways uses RELA. I consider this a hack, and perhaps risky,
> > since in principle someone could make powerpc binaries with REL.
> >
> > 2. Re-process not just GOT type relocs, but also any RELA
> > (non-inline-addend) relocs again on the second pass. This would
> > work as long as powerpc only uses RELA, and if REL is ever used,
> > the worst that would happen is the current bug (losing environ,
> > etc.) rather than silently wrong relocations in global data. This
> > approach is not a hack, but I consider it something of an
> > incomplete fix.
> >
> > 3. Re-process all symbolic relocations. For REL-type (inline addend),
> > we have to recover the original addend, which can be done by
> > calling find_sym again, but using ldso instead of the current
> > library chain head as the context to search for the symbol in, then
> > subtracting the resulting address to get back the original addend.
> >
> > I like the third solution best, even though it incurs a small code
> > size cost and a performance cost for archs using REL, because it's
> > completely robust against any weird ways some archs might end up using
> > relocations. The expected number of such relocations is tiny anyway;
> > on my i386 builds it's 14.
> >
> > If option 3 proves to be difficult or costly, however, we could
> > consider option 2 as a temporary measure to get powerpc working. It
> > wouldn't even need to be reverted, because option 3 includes/subsumes
> > the work that would be done for option 2.
>
> Attached is a patch to implement option 2. I'll probably commit it
> soon anyway but here is it in case you want to test sooner. I verified
> it fixes the test program on powerpc for me.
Thanks for the quick fix! The new commit fixes also the other segfaults
I've seen.
Felix
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