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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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