[Mesa-dev] [Mesa-stable] [PATCH] mapi: avoid text relocation in x86 tsd stubs

[Jonathan Gray]

On Thu, Nov 01, 2018 at 12:26:34PM -0700, Ian Romanick wrote:
> On 10/31/2018 09:08 PM, Jonathan Gray wrote:
> > Make similiar changes to libglvnd to avoid a text relocation in
> > x86 tsd stubs fixing the build with lld.
> > 
> > Signed-off-by: Jonathan Gray <jsg at jsg.id.au>
> > Cc: mesa-stable at lists.freedesktop.org
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108541
> > ---
> >  src/mapi/entry_x86_tsd.h | 14 +++++++++-----
> >  1 file changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/src/mapi/entry_x86_tsd.h b/src/mapi/entry_x86_tsd.h
> > index 0c28c8ff068..e08a02f3db2 100644
> > --- a/src/mapi/entry_x86_tsd.h
> > +++ b/src/mapi/entry_x86_tsd.h
> > @@ -31,7 +31,7 @@
> >  #define HIDDEN
> >  #endif
> >  
> > -#define X86_ENTRY_SIZE 32
> > +#define X86_ENTRY_SIZE 64
> >  
> >  __asm__(".text\n"
> >          ".balign 32\n"
> > @@ -44,12 +44,16 @@ __asm__(".text\n"
> >     func ":"
> >  
> >  #define STUB_ASM_CODE(slot)         \
> > -   "movl " ENTRY_CURRENT_TABLE ", %eax\n\t" \
> > +   "call 1f\n\t"                    \
> > +   "1:\n\t"                         \
> > +   "popl %eax\n\t"                  \
> > +   "addl $_GLOBAL_OFFSET_TABLE_+[.-1b], %eax\n\t" \
> > +   "movl " ENTRY_CURRENT_TABLE "@GOT(%eax), %eax\n\t" \
> > +   "mov (%eax), %eax\n\t"           \
> >     "testl %eax, %eax\n\t"           \
> > -   "je 1f\n\t"                      \
> > -   "jmp *(4 * " slot ")(%eax)\n"    \
> > +   "jne 1f\n\t"                     \
> > +   "call " ENTRY_CURRENT_TABLE_GET "@PLT\n\t" \
> >     "1:\n\t"                         \
> > -   "call " ENTRY_CURRENT_TABLE_GET "\n\t" \
> >     "jmp *(4 * " slot ")(%eax)"
> 
> After this change, the code is:
> 
> #define STUB_ASM_CODE(slot)         \
>    "call 1f\n\t"                    \
>    "1:\n\t"                         \
>    "popl %eax\n\t"                  \
>    "addl $_GLOBAL_OFFSET_TABLE_+[.-1b], %eax\n\t" \
>    "movl " ENTRY_CURRENT_TABLE "@GOT(%eax), %eax\n\t" \
>    "mov (%eax), %eax\n\t"           \
>    "testl %eax, %eax\n\t"           \
>    "jne 1f\n\t"                     \
>    "call " ENTRY_CURRENT_TABLE_GET "@PLT\n\t" \
>    "1:\n\t"                         \
>    "jmp *(4 * " slot ")(%eax)"
> 
> So there's going to be two labels "1:".  Does that even assemble?

Yes, the call/jmp is always forward as it is '1f'.
This also runs glxinfo, glxgears etc on a pentium m running OpenBSD/i386.

https://github.com/NVIDIA/libglvnd/blob/master/src/GLdispatch/vnd-glapi/entry_x86_tsd.c#L58

libglvnd has two labels like this as well, the ebx use there isn't needed.