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authorH.J. Lu <hjl.tools@gmail.com>2016-06-09 04:43:16 -0700
committerH.J. Lu <hjl.tools@gmail.com>2016-06-09 04:43:31 -0700
commitac187dc4abde9ca6504c646106e2a7f7b2806262 (patch)
tree6399e3373227afacaa5575f18f9304ad9ddd7a86
parent75437079e4c5a7416052940ca641a41ddbf6f5fd (diff)
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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.
-rw-r--r--ChangeLog5
-rw-r--r--sysdeps/x86_64/start.S17
2 files changed, 13 insertions, 9 deletions
diff --git a/ChangeLog b/ChangeLog
index 9c417f2321..f72f22e2df 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 2369b69ec3..f1b961f5ba 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)