From: Roland McGrath When we're stopped at syscall entry tracing, ptrace can change the %rax value from -ENOSYS to something else. If no system call is actually made because the syscall number (now in orig_rax) is bad, then we now always reset %rax to -ENOSYS again. This changes it to leave the return value alone after entry tracing. That way, the %rax value set by ptrace is there to be seen in user mode (or in syscall exit tracing). This is consistent with what the 32-bit kernel does. Signed-off-by: Roland McGrath Cc: Ingo Molnar Cc: Thomas Gleixner Cc: Andi Kleen Signed-off-by: Andrew Morton --- arch/x86/kernel/entry_64.S | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff -puN arch/x86/kernel/entry_64.S~x86_64-ptrace-vs-enosys arch/x86/kernel/entry_64.S --- a/arch/x86/kernel/entry_64.S~x86_64-ptrace-vs-enosys +++ a/arch/x86/kernel/entry_64.S @@ -319,19 +319,17 @@ badsys: /* Do syscall tracing */ tracesys: SAVE_REST - movq $-ENOSYS,RAX(%rsp) + movq $-ENOSYS,RAX(%rsp) /* ptrace can change this for a bad syscall */ FIXUP_TOP_OF_STACK %rdi movq %rsp,%rdi call syscall_trace_enter LOAD_ARGS ARGOFFSET /* reload args from stack in case ptrace changed it */ RESTORE_REST cmpq $__NR_syscall_max,%rax - movq $-ENOSYS,%rcx - cmova %rcx,%rax - ja 1f + ja int_ret_from_sys_call /* RAX(%rsp) set to -ENOSYS above */ movq %r10,%rcx /* fixup for C */ call *sys_call_table(,%rax,8) -1: movq %rax,RAX-ARGOFFSET(%rsp) + movq %rax,RAX-ARGOFFSET(%rsp) /* Use IRET because user could have changed frame */ /* _