RE: [musl] sigsetjmp

[<sidneym@codeaurora.org>]

> -----Original Message-----
> From: Rich Felker <dalias@libc.org>
> Sent: Monday, June 8, 2020 12:11 PM
> To: sidneym@codeaurora.org
> Cc: musl@lists.openwall.com
> Subject: Re: [musl] sigsetjmp
> 
> On Sun, Jun 07, 2020 at 08:45:11PM -0500, sidneym@codeaurora.org wrote:
> > Wanting to make sure I'm reading the requirements correctly.
> >
> > Looks like this routine only needs to save the link register and env,
> > call setjmp then restore the link register and env followed by the tail
call.
> 
> Yes, that's correct. This is an unconventional design but necessary so
that the
> stack frame has already been restored when signals are unmasked by
> siglongjmp. See the message for commit
> 583e55122e767b1586286a0d9c35e2a4027998ab for a description of how this
> works.
> 
> > Hexagon was out of date so I did this:
> >
> >
> >
> > ..balign 4
> >
> > ..type sigsetjmp,@function
> >
> > sigsetjmp:
> >
> >         // if savemask is 0 sigsetjmp behaves like setjmp
> >
> >         {
> >
> >                 p0 = cmp.eq(r1, #0)
> >
> >                 if (p0.new) jump:t ##setjmp
> >
> >         }
> >
> >         {
> >
> >                 memw(r0+#64+4) = r16  // save r16 in __ss[0]
> >
> >                 memw(r0+#64)   = r31  // save linkregister in __fl
> >
> >                 r16 = r0
> >
> >         }
> 
> This is not correct. __ss[0] is occupied by the saved signal mask, and
will be
> clobbered when it's saved in the tail call. Instead you need to use unused
space
> in struct __jmp_buf_tag. The canonical place is
> (char*)__ss+8 (the "HURD ABI area" :) assuming _NSIG==65.

I was not able to find a description of the HURD ABI area is it documented
someplace?

So upon entry to sigsetjmp __ss[0] is holding the saved mask and using
__ss[0] to buffer r16 will clobber it, correct?  As it is coded __ss[0] is
just used over the call to setjmp to preserve the value of r16.

If __ss is r0+#64+4, then you are suggesting that I use r0+#64+4+8 or
__ss[2]. I need to look into this some more because with that I'm have some
testcase issues.

Thanks,


> 
> Rich