Re: musl segfaulting when compiled with -O2

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2017-03-07 16:45:40 -0500]:
> On Tue, Mar 07, 2017 at 05:55:29PM -0300, Breno Leitao wrote:
> >   $ wget http://cdn-fastly.deb.debian.org/debian/pool/main/m/musl/musl_1.1.16-2_ppc64el.deb
> > 
> >   $ dpkg-deb -x musl_1.1.16-2_ppc64el.deb .
> > 
> >   $ lib/powerpc64le-linux-musl/libc.so 
> > [2]    25672 segmentation fault (core dumped) lib/powerpc64le-linux-musl/libc.so
> 
> Can you do a register dump and a dissassembly dump and around the
> point that crashes? The Debian package is fully stripped (lacks debug
> symbols) so it's hard to follow what it's doing, but I didn't see
> anything obviously wrong.
> 
> Rich

i used qemu-ppc64le -singlestep -d in_asm,cpu to debug this

the relevant code is the end of _dlstart_c:
...
   8c574:       09 00 00 48     bl      8c57c <writev+0x28c>
   8c578:       28 98 fa ff     fsub    f31,f26,f19
   8c57c:       a6 02 48 7d     mflr    r10
   8c580:       00 00 2a 81     lwz     r9,0(r10)
   8c584:       14 52 29 7d     add     r9,r9,r10
   8c588:       a6 03 29 7d     mtctr   r9
   8c58c:       18 00 41 f8     std     r2,24(r1)
   8c590:       78 3b e4 7c     mr      r4,r7
   8c594:       78 4b 2c 7d     mr      r12,r9
   8c598:       21 04 80 4e     bctrl

it's a pc relative address load and jump (to call __dls2):

   bl 1f
   data          // data == 0xfffa9828
1: mflr r10      // r10 = &data == 0x8c57c
   lwz r9,0(r10) // r9 = *r10 (== data)
   add r9,r9,r10 // r9 += r10 (== 0xfffa9828+0x8c57c = 0x100035da0)
   mtctr r9      // ctr = r9 (== 0x100035da0)
   std r2,24(r1)
   mr r4,r7
   mr r12,r9
   bctrl         // ctr(r12,r4)

it seems __dls2 is at 0x35da0 so the pc relative data
should have been signextended.