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Date: Fri, 22 May 2015 02:23:46 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: ppc soft-float regression

On Tue, May 19, 2015 at 12:07:31AM +0200, Felix Janda wrote:
> 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.

Attached is a patch that finishes the job by completing option 3. I
haven't tested it much yet so I'll hold off on committing it for a
while but it seems to work fine (not break anything) on i386.

Rich

View attachment "reprocess_all_syms.diff" of type "text/plain" (888 bytes)

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