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GNU C Library master sources branch master updated. glibc-2.23-449-gac187dc
- From: hjl at sourceware dot org
- To: glibc-cvs at sourceware dot org
- Date: 9 Jun 2016 11:44:12 -0000
- Subject: GNU C Library master sources branch master updated. glibc-2.23-449-gac187dc
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- Log -----------------------------------------------------------------
http://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=ac187dc4abde9ca6504c646106e2a7f7b2806262
commit ac187dc4abde9ca6504c646106e2a7f7b2806262
Author: H.J. Lu <hjl.tools@gmail.com>
Date: Thu Jun 9 04:43:16 2016 -0700
Always indirect branch to __libc_start_main via GOT
Since __libc_start_main in libc.so is called very early, lazy binding
isn't relevant. Always call __libc_start_main with indirect branch via
GOT to avoid extra branch to PLT slot. In case of static executable,
ld in binutils 2.26 or above can convert indirect branch into direct
branch:
0000000000400a80 <_start>:
400a80: 31 ed xor %ebp,%ebp
400a82: 49 89 d1 mov %rdx,%r9
400a85: 5e pop %rsi
400a86: 48 89 e2 mov %rsp,%rdx
400a89: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
400a8d: 50 push %rax
400a8e: 54 push %rsp
400a8f: 49 c7 c0 20 1b 40 00 mov $0x401b20,%r8
400a96: 48 c7 c1 90 1a 40 00 mov $0x401a90,%rcx
400a9d: 48 c7 c7 c0 03 40 00 mov $0x4003c0,%rdi
400aa4: 67 e8 96 09 00 00 addr32 callq 401440 <__libc_start_main>
400aaa: f4 hlt
* sysdeps/x86_64/start.S (_start): Always indirect branch to
__libc_start_main via GOT.
diff --git a/ChangeLog b/ChangeLog
index 9c417f2..f72f22e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
2016-06-09 H.J. Lu <hongjiu.lu@intel.com>
+ * sysdeps/x86_64/start.S (_start): Always indirect branch to
+ __libc_start_main via GOT.
+
+2016-06-09 H.J. Lu <hongjiu.lu@intel.com>
+
* sysdeps/x86_64/memcopy.h: New file.
* sysdeps/x86_64/wordcopy.c: Likewise.
diff --git a/sysdeps/x86_64/start.S b/sysdeps/x86_64/start.S
index 2369b69..f1b961f 100644
--- a/sysdeps/x86_64/start.S
+++ b/sysdeps/x86_64/start.S
@@ -102,23 +102,22 @@ ENTRY (_start)
mov __libc_csu_init@GOTPCREL(%rip), %RCX_LP
mov main@GOTPCREL(%rip), %RDI_LP
-
- /* Call the user's main function, and exit with its value.
- But let the libc call main. Since __libc_start_main is
- called very early, lazy binding isn't relevant here. Use
- indirect branch via GOT to avoid extra branch to PLT slot. */
- call *__libc_start_main@GOTPCREL(%rip)
#else
/* Pass address of our own entry points to .fini and .init. */
mov $__libc_csu_fini, %R8_LP
mov $__libc_csu_init, %RCX_LP
mov $main, %RDI_LP
+#endif
/* Call the user's main function, and exit with its value.
- But let the libc call main. */
- call __libc_start_main
-#endif
+ But let the libc call main. Since __libc_start_main in
+ libc.so is called very early, lazy binding isn't relevant
+ here. Use indirect branch via GOT to avoid extra branch
+ to PLT slot. In case of static executable, ld in binutils
+ 2.26 or above can convert indirect branch into direct
+ branch. */
+ call *__libc_start_main@GOTPCREL(%rip)
hlt /* Crash if somehow `exit' does return. */
END (_start)
-----------------------------------------------------------------------
Summary of changes:
ChangeLog | 5 +++++
sysdeps/x86_64/start.S | 17 ++++++++---------
2 files changed, 13 insertions(+), 9 deletions(-)
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