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authorRoland McGrath <roland@redhat.com>2006-07-30 03:04:06 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2006-07-31 13:28:43 -0700
commit0b0bf7a3ccb6f0b38ead71980e79f875046047b7 (patch)
tree1c7b0689d2f0f9839ff9a793ed3990d9c1591fc0 /Documentation/kbuild/makefiles.txt
parent072d3d1acb452f4abd8d3d20af661f2e28854b59 (diff)
[PATCH] vDSO hash-style fix
The latest toolchains can produce a new ELF section in DSOs and dynamically-linked executables. The new section ".gnu.hash" replaces ".hash", and allows for more efficient runtime symbol lookups by the dynamic linker. The new ld option --hash-style={sysv|gnu|both} controls whether to produce the old ".hash", the new ".gnu.hash", or both. In some new systems such as Fedora Core 6, gcc by default passes --hash-style=gnu to the linker, so that a standard invocation of "gcc -shared" results in producing a DSO with only ".gnu.hash". The new ".gnu.hash" sections need to be dealt with the same way as ".hash" sections in all respects; only the dynamic linker cares about their contents. To work with older dynamic linkers (i.e. preexisting releases of glibc), a binary must have the old ".hash" section. The --hash-style=both option produces binaries that a new dynamic linker can use more efficiently, but an old dynamic linker can still handle. The new section runs afoul of the custom linker scripts used to build vDSO images for the kernel. On ia64, the failure mode for this is a boot-time panic because the vDSO's PT_IA_64_UNWIND segment winds up ill-formed. This patch addresses the problem in two ways. First, it mentions ".gnu.hash" in all the linker scripts alongside ".hash". This produces correct vDSO images with --hash-style=sysv (or old tools), with --hash-style=gnu, or with --hash-style=both. Second, it passes the --hash-style=sysv option when building the vDSO images, so that ".gnu.hash" is not actually produced. This is the most conservative choice for compatibility with any old userland. There is some concern that some ancient glibc builds (though not any known old production system) might choke on --hash-style=both binaries. The optimizations provided by the new style of hash section do not really matter for a DSO with a tiny number of symbols, as the vDSO has. If someone wants to use =gnu or =both for their vDSO builds and worry less about that compatibility, just change the option and the linker script changes will make any choice work fine. Signed-off-by: Roland McGrath <roland@redhat.com> Cc: "Luck, Tony" <tony.luck@intel.com> Cc: Kyle McMartin <kyle@mcmartin.ca> Cc: Paul Mackerras <paulus@samba.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Jeff Dike <jdike@addtoit.com> Cc: Andi Kleen <ak@muc.de> Cc: Sam Ravnborg <sam@ravnborg.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'Documentation/kbuild/makefiles.txt')
-rw-r--r--Documentation/kbuild/makefiles.txt14
1 files changed, 14 insertions, 0 deletions
diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 14ef3868a328..0706699c9da9 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -407,6 +407,20 @@ more details, with real examples.
The second argument is optional, and if supplied will be used
if first argument is not supported.
+ ld-option
+ ld-option is used to check if $(CC) when used to link object files
+ supports the given option. An optional second option may be
+ specified if first option are not supported.
+
+ Example:
+ #arch/i386/kernel/Makefile
+ vsyscall-flags += $(call ld-option, -Wl$(comma)--hash-style=sysv)
+
+ In the above example vsyscall-flags will be assigned the option
+ -Wl$(comma)--hash-style=sysv if it is supported by $(CC).
+ The second argument is optional, and if supplied will be used
+ if first argument is not supported.
+
cc-option
cc-option is used to check if $(CC) support a given option, and not
supported to use an optional second option.