Re: [PATCH] fork in libpthread cannot use IFUNC resolver [BZ #19861]

[Florian Weimer <fweimer at redhat dot com>]

On 05/27/2016 09:51 PM, Roland McGrath wrote:
> > 	[BZ #19861]
> > 	Do not use IFUNC resolver with potentially unrelcated symbol.
>
> typo: "unrelocated"
>
> > --- a/nptl/pt-fork.c
> > +++ b/nptl/pt-fork.c
> > @@ -25,48 +25,20 @@
> >     the historical ABI requires it.  For static linking, there is no need to
> >     provide anything here--the libc version will be linked in.  For shared
> >     library ABI compatibility, there must be __fork and fork symbols in
> > -   libpthread.so; so we define them using IFUNC to redirect to the libc
> > -   function.  */
> > +   libpthread.so.  */
>
> I think it's worthwhile for this comment to address the IFUNC issue.
> e.g., "It's tempting to avoid the tail-call overhead in the shared library
> case by using IFUNC here, but ..."
>
> > -DEFINE_FORK (fork_ifunc)
> > +strong_alias (fork_compat, fork_ifunc)
> >  compat_symbol (libpthread, fork_ifunc, fork, GLIBC_2_0);
> >
> > -DEFINE_FORK (__fork_ifunc)
> > +strong_alias (fork_compat, __fork_ifunc)
> >  compat_symbol (libpthread, __fork_ifunc, __fork, GLIBC_2_0);
>
> Nothing that remains should have "ifunc" in the name.

Thanks.  I believe I have addressed your comments.

Florian

fork in libpthread cannot use IFUNC resolver [BZ #19861]

This commit only addresses the fork case, the vfork case has to be a
tail call, which is why the generic code needs an IFUNC resolver
there.

2016-05-30  Florian Weimer  <fweimer@redhat.com>

	[BZ #19861]
	Do not use IFUNC resolver with potentially unrelocated symbol.
	* nptl/pt-fork.c [HAVE_IFUNC]: Remove.
	(DEFINE_FORK): Remove macro and inline definition.
	(fork_alias): Renamed from fork_ifunc.
	(__fork_alias): Renamed from __fork_ifunc.

diff --git a/nptl/pt-fork.c b/nptl/pt-fork.c
index b65d6b4..db9b61d 100644
--- a/nptl/pt-fork.c
+++ b/nptl/pt-fork.c
@@ -25,48 +25,25 @@
    the historical ABI requires it.  For static linking, there is no need to
    provide anything here--the libc version will be linked in.  For shared
    library ABI compatibility, there must be __fork and fork symbols in
-   libpthread.so; so we define them using IFUNC to redirect to the libc
-   function.  */
+   libpthread.so.
+
+   With an IFUNC resolver, it would be possible to avoid the
+   indirection, but the IFUNC resolver might run before the
+   __libc_fork symbol has been relocated, in which case the IFUNC
+   resolver would not be able to provide the correct address.  */
 
 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_22)
 
-# if HAVE_IFUNC
-
-static __typeof (fork) *
-__attribute__ ((used))
-fork_resolve (void)
-{
-  return &__libc_fork;
-}
-
-#  ifdef HAVE_ASM_SET_DIRECTIVE
-#   define DEFINE_FORK(name) \
-  asm (".set " #name ", fork_resolve\n" \
-       ".globl " #name "\n" \
-       ".type " #name ", %gnu_indirect_function");
-#  else
-#   define DEFINE_FORK(name) \
-  asm (#name " = fork_resolve\n" \
-       ".globl " #name "\n" \
-       ".type " #name ", %gnu_indirect_function");
-#  endif
-
-# else  /* !HAVE_IFUNC */
-
 static pid_t __attribute__ ((used))
 fork_compat (void)
 {
   return __libc_fork ();
 }
 
-# define DEFINE_FORK(name) strong_alias (fork_compat, name)
+strong_alias (fork_compat, fork_alias)
+compat_symbol (libpthread, fork_alias, fork, GLIBC_2_0);
 
-# endif  /* HAVE_IFUNC */
-
-DEFINE_FORK (fork_ifunc)
-compat_symbol (libpthread, fork_ifunc, fork, GLIBC_2_0);
-
-DEFINE_FORK (__fork_ifunc)
-compat_symbol (libpthread, __fork_ifunc, __fork, GLIBC_2_0);
+strong_alias (fork_compat, __fork_alias)
+compat_symbol (libpthread, __fork_alias, __fork, GLIBC_2_0);
 
 #endif