Date: Wed, 2 Aug 2017 10:46:12 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: possible bug in setjmp implementation for ppc64 On Wed, Aug 02, 2017 at 08:38:25AM -0500, Bobby Bingham wrote: > On Wed, Aug 02, 2017 at 12:58:16AM -0400, Rich Felker wrote: > > > 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'm not sure what "new value" you're referring to here. > > The idea is basically: > > setjmp: # non-local entry point > r5 = r1 > goto __setjmp_toc > > .localentry # local entry point > r5 = r2 > > __setjmp_toc: > # all the existing code from setjmp, but save r5 instead of r2 For sigsetjmp, I think in this case you also need to duplicate the assembler-generated code that would load a new r2; otherwise you can't subsequently call __sigsetjmp_tail. For setjmp itself the above should suffice since setjmp does not need a TOC itself. Otherwise the pseudo-code above looks like what I expected after thinking about it for a bit. > > 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. > > I'm not sure why a change like this wouldn't be required. > > The requirements on longjmp here are: > * when called through the local entry point, restore the TOC pointer > into r2 > * when called via the PLT stub, restore the TOC pointer to the stack > > And siglongjmp needs to have the same behavior. If the main program > makes a cross-dso call to siglongjmp, it needs to restore the TOC > pointer to the stack. But siglongjmp works by making a local call to > longjmp, meaning without this change, it will only ever restore the TOC > pointer to r2. Whether the call to longjmp/siglongjmp was local or not is irrelevant. It's only whether the original call to setjmp/sigsetjmp was local or not that's relevant. And in either case I'm pretty sure it suffices to restore the saved value to both *(r1+24) and r2. Per the ABI, *(r1+24) can't be used for any purpose except saving the TOC, so upon return from setjmp, the caller's only options are to treat the value at *(r1+24) as indeterminate or assume it contains the TOC pointer. Likewise for r2, if the call was non-local, r2 is call-clobbered so it doesn't matter what it contains after return, and if the call was local, r2 is expected to contain the caller's TOC pointer. Rich
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