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Re: [PATCH] PowerPC: Use correct PLT relocations for elf_machine_type_class.
On Wed, Feb 20, 2008 at 03:41:47PM -0600, Ryan S. Arnold wrote:
> On Wed, 2008-02-20 at 12:08 -0800, Ulrich Drepper wrote:
> > And what is it supposed to fix?
It was posted too soon, sorry. It doesn't fix anything. On the face
of it, the patch is obviously correct in that R_PPC64_ADDR24 has
nothing to do with PLT relocations while R_PPC64_JMP_SLOT does.
However, when you look at what is going on in detail, PowerPC64
doesn't need to distinguish between PLT relocs and other relocs.
See the comment in the following patch. This should speed ld.so by
simplifying evaluation of elf_machine_type_class, save symbol lookup
cache by not storing two classes of symbol values, and considerably
speed lookup of protected symbols.
Regression testing has *not* finished yet, so please don't apply the
patch just yet. Yes, posting too soon again, but I wanted to get this
out in case Ryan forwards the private email I sent him last night
which wasn't quite as clear. Comments welcome. Feel free to take my
description of ELF_RTYPE_CLASS_PLT's purpose and put it in the comment
in ldsodefs.h.
2008-02-21 Alan Modra <amodra@bigpond.net.au>
* sysdeps/powerpc/powerpc64/dl-machine.h (elf_machine_type_class):
Always set ELF_RTYPE_CLASS_PLT.
(elf_machine_lookup_noexec_p, elf_machine_lookup_noplt_p): Delete.
Index: sysdeps/powerpc/powerpc64/dl-machine.h
===================================================================
RCS file: /cvs/glibc/libc/sysdeps/powerpc/powerpc64/dl-machine.h,v
retrieving revision 1.25
diff -u -p -r1.25 dl-machine.h
--- sysdeps/powerpc/powerpc64/dl-machine.h 27 Oct 2006 23:11:46 -0000 1.25
+++ sysdeps/powerpc/powerpc64/dl-machine.h 20 Feb 2008 23:52:09 -0000
@@ -246,30 +246,35 @@ BODY_PREFIX "_dl_start_user:\n" \
" " END_2(_dl_start_user) "\n" \
" .popsection");
-/* Nonzero iff TYPE should not be allowed to resolve to one of
- the main executable's symbols, as for a COPY reloc. */
-#define elf_machine_lookup_noexec_p(type) ((type) == R_PPC64_COPY)
-
-/* Nonzero iff TYPE describes relocation of a PLT entry, so
- PLT entries should not be allowed to define the value. */
-#define elf_machine_lookup_noplt_p(type) ((type) == R_PPC64_JMP_SLOT)
-
-/* ELF_RTYPE_CLASS_PLT iff TYPE describes relocation of a PLT entry, so
- PLT entries should not be allowed to define the value.
- ELF_RTYPE_CLASS_NOCOPY iff TYPE should not be allowed to resolve to one
- of the main executable's symbols, as for a COPY reloc. */
+/* ELF_RTYPE_CLASS_NOCOPY iff TYPE should not be allowed to resolve to
+ one of the main executable's symbols, as for a COPY reloc.
+
+ To make function pointer comparisons work on most targets, the
+ relevant ABI states that the address of a non-local function in a
+ dynamically linked executable is the address of the PLT entry for
+ that function. This is quite reasonable since using the real
+ function address in a non-PIC executable would typically require
+ dynamic relocations in .text, something to be avoided. For such
+ functions, the linker emits a SHN_UNDEF symbol in the executable
+ with value equal to the PLT entry address. Normally, SHN_UNDEF
+ symbols have a value of zero, so this is a clue to ld.so that it
+ should treat these symbols specially. For relocations not in
+ ELF_RTYPE_CLASS_PLT (eg. those on function pointers), ld.so should
+ use the value of the executable SHN_UNDEF symbol, ie. the PLT entry
+ address. For relocations in ELF_RTYPE_CLASS_PLT (eg. the relocs in
+ the PLT itself), ld.so should use the value of the corresponding
+ defined symbol in the object that defines the function, ie. the
+ real function address. This complicates ld.so in that there are
+ now two possible values for a given symbol, and it gets even worse
+ because protected symbols need yet another set of rules.
+
+ On PowerPC64 we don't need any of this. The linker won't emit
+ SHN_UNDEF symbols with non-zero values. ld.so can make all
+ relocations behave "normally", ie. always use the real address
+ like PLT relocations. So always set ELF_RTYPE_CLASS_PLT. */
-#if !defined RTLD_BOOTSTRAP || USE___THREAD
-#define elf_machine_type_class(type) \
- /* This covers all the TLS relocs, though most won't appear. */ \
- (((((type) >= R_PPC64_DTPMOD64 && (type) <= R_PPC64_TPREL16_HIGHESTA) \
- || (type) == R_PPC64_ADDR24) * ELF_RTYPE_CLASS_PLT) \
- | (((type) == R_PPC64_COPY) * ELF_RTYPE_CLASS_COPY))
-#else
#define elf_machine_type_class(type) \
- ((((type) == R_PPC64_ADDR24) * ELF_RTYPE_CLASS_PLT) \
- | (((type) == R_PPC64_COPY) * ELF_RTYPE_CLASS_COPY))
-#endif
+ (ELF_RTYPE_CLASS_PLT | (((type) == R_PPC64_COPY) * ELF_RTYPE_CLASS_COPY))
/* A reloc type used for ld.so cmdline arg lookups to reject PLT entries. */
#define ELF_MACHINE_JMP_SLOT R_PPC64_JMP_SLOT
--
Alan Modra
Australia Development Lab, IBM