Date: Wed, 2 Aug 2017 08:38:25 -0500 From: Bobby Bingham <koorogi@...rogi.info> To: musl@...ts.openwall.com Subject: Re: possible bug in setjmp implementation for ppc64 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 > > 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. > > Rich
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