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| author | H.J. Lu <hjl.tools@gmail.com> | 2016-06-09 04:43:16 -0700 |
|---|---|---|
| committer | H.J. Lu <hjl.tools@gmail.com> | 2016-06-09 04:43:31 -0700 |
| commit | ac187dc4abde9ca6504c646106e2a7f7b2806262 (patch) | |
| tree | 6399e3373227afacaa5575f18f9304ad9ddd7a86 /sysdeps/x86_64/start.S | |
| parent | 75437079e4c5a7416052940ca641a41ddbf6f5fd (diff) | |
| download | glibc-ac187dc4abde9ca6504c646106e2a7f7b2806262.tar.gz | |
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.
Diffstat (limited to 'sysdeps/x86_64/start.S')
| -rw-r--r-- | sysdeps/x86_64/start.S | 17 |
1 files changed, 8 insertions, 9 deletions
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) |