Date: Wed, 2 Aug 2017 00:58:16 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: possible bug in setjmp implementation for ppc64 On Tue, Aug 01, 2017 at 11:31:55PM -0500, Bobby Bingham wrote: > On Tue, Aug 01, 2017 at 11:55:56PM -0400, Rich Felker wrote: > > On Tue, Aug 01, 2017 at 07:28:45PM -0500, Bobby Bingham wrote: > > > On Tue, Aug 01, 2017 at 07:07:59PM -0400, Rich Felker wrote: > > > > On Tue, Aug 01, 2017 at 06:45:33PM -0400, Rich Felker wrote: > > > > > On Tue, Aug 01, 2017 at 08:28:27AM +0300, Alexander Monakov wrote: > > > > > > On Tue, 1 Aug 2017, Bobby Bingham wrote: > > > > > > > I think this either requires having different versions of setjmp/longjmp > > > > > > > for static and dynamic libc, > > > > > > > > > > > > Do you mean for non-pic vs pic objects? As I understand, when libc.a is > > > > > > built with -fpic (so it's suitable for static-pie), setjmp-longjmp need > > > > > > to preserve saved TOC at (r1+24). So presumably source code would need > > > > > > to test #ifdef __PIC__? > > > > > > > > > > > > > or to increase the size of jmpbuf so we can always save/restore both > > > > > > > r2 and the value on the stack, but this would be an ABI change. > > > > > > > > > > > > Would that work for non-pic, i.e. is (r1+24) a reserved location even in > > > > > > non-pic mode? If not, you can't overwrite it from longjmp. > > > > > > > > > > Pretty much certainly so; there is no separate "non-PIC ABI". PIC code > > > > > is just code that doesn't happen to do certain things not permissible > > > > > in PIC. It doesn't have additional permissions to do things that > > > > > otherwise wouldn't be permitted in "non-PIC code". > > > > > > > > > > In any case just saving and restoring both is not an ABI change, since > > > > > there's plenty of free space (896 bits worth of non-existant signals) > > > > > in the jmp_buf due to the "Hurd sigset_t" mess. > > > > > > > > It might also be possible to manually create both the entry points for > > > > setjmp, rather than letting the assembler auto-generate them, in which > > > > case I think the choice of which value to save just depends on which > > > > entry point was used. Thoughts? > > > > > > I like this idea. It's slightly more complicated than that because of > > > the call to setjmp from sigsetjmp, but should still be ok. I'll work on > > > a patch. > > > > Hmm, can you elaborate on the situation with sigsetjmp? > > > > sigsetjmp calls setjmp, but I believe this will always use the intra-dso > entry point. Same for the call siglongjmp makes to longjmp. So calls > via sigsetjmp/siglongjmp will always be detected as local calls, even > when the originally caller of jig*jmp is in a different dso. > > My plan right now is create a __setjmp_toc function which is identical > to the normal setjmp except that the TOC pointer to save is passed in as > another parameter. setjmp will detect which entry point is used, pull > the TOC pointer from the right place, and call __setjmp_toc. sigsetjmp > will be updated similarly to detect which entry point is used and to > call __setjmp_toc directly instead of going through setjmp. I've been thinking about it and at first thought it sounded overly fragile and hard to understand, but now I think it makes sense and should work. It would just involve copying r2 to a call-clobbered argument register before loading the new value, right? I was considering whether you could just avoid loading the TOC pointer at all (leaving the correct value in r2 for setjmp to save), and this might work, but I think it would make calling __sigsetjmp_tail difficult and error-prone. > siglongjmp is current written in C by just calling longjmp. I'm tempted > to just add a "siglongjmp:" label in the asm for longjmp and add an > empty powerpc64/siglongjmp.c file to suppress the default > implementation. I want to ask if there's any reason it wouldn't be > valid for these two functions to have the same address. I don't see any reason to make this change (it won't make any functional difference -- call frames and such don't matter at this point), and at least the siglongjmp symbol would have to be weak to respect namespace if you did it that way. Rich
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