Fix PowerPC64 ELFv1-specific problem in __elf_phdr_match_addr() leading to crash

in threaded programs that unload libraries.

Summary:
The GNOME update to 3.28 exposed a bug in __elf_phdr_match_addr(), which leads
to a crash when building devel/libsoup on powerpc64.

Due to __elf_phdr_match_addr() limiting its search to PF_X sections, on the
PPC64 ELFv1 ABI, it was never matching function pointers properly.

This meant that libthr was never cleaning up its atfork list in
__pthread_cxa_finalize(), so if a library with an atfork handler was unloaded,
libthr would crash on the next fork.

Normally, the null pointer check it does before calling the handler would avoid
this crash, but, due to PPC64 ELFv1 using function descriptors instead of raw
function pointers, a null check against the pointer itself is insufficient, as
the pointer itself was not null, it was just pointing at a function descriptor
that had been zeroed. (Which is an ABI violation.)

Calling a zeroed function descriptor on PPC64 ELFv1 causes a jump to address 0
with a zeroed r2 and r11.

Submitted by:	git_bdragon.rtk0.net
Reviewed By:	kib
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D18364

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/libc/gen/elf_utils.c b/lib/libc/gen/elf_utils.c
index 06c91e057487..417f1842aadd 100644
--- a/lib/libc/gen/elf_utils.c
+++ b/lib/libc/gen/elf_utils.c
@@ -47,8 +47,21 @@ __elf_phdr_match_addr(struct dl_phdr_info *phdr_info, void *addr)
 
 	for (i = 0; i < phdr_info->dlpi_phnum; i++) {
 		ph = &phdr_info->dlpi_phdr[i];
-		if (ph->p_type != PT_LOAD || (ph->p_flags & PF_X) == 0)
+		if (ph->p_type != PT_LOAD)
 			continue;
+
+		/* ELFv1 ABI for powerpc64 passes function descriptor
+		 * pointers around, not function pointers.  The function
+		 * descriptors live in .opd, which is a non-executable segment.
+		 * The PF_X check would therefore make all address checks fail,
+		 * causing a crash in some instances.  Don't skip over
+		 * non-executable segments in the ELFv1 powerpc64 case.
+		 */
+#if !defined(__powerpc64__) || (defined(_CALL_ELF) && _CALL_ELF == 2)
+		if ((ph->p_flags & PF_X) == 0)
+			continue;
+#endif
+
 		if (phdr_info->dlpi_addr + ph->p_vaddr <= (uintptr_t)addr &&
 		    (uintptr_t)addr + sizeof(addr) < phdr_info->dlpi_addr +
 		    ph->p_vaddr + ph->p_memsz)