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Date: Mon, 18 May 2015 16:14:23 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: ppc soft-float regression
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.
Rich
View attachment "reprocess_rela.diff" of type "text/plain" (474 bytes)
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