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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 | |
| parent | 75437079e4c5a7416052940ca641a41ddbf6f5fd (diff) | |
| download | glibc-ac187dc4abde9ca6504c646106e2a7f7b2806262.tar.gz glibc-ac187dc4abde9ca6504c646106e2a7f7b2806262.tar.xz glibc-ac187dc4abde9ca6504c646106e2a7f7b2806262.zip | |
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-- | ChangeLog | 5 | ||||
| -rw-r--r-- | sysdeps/x86_64/start.S | 17 |
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) |