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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