x86: ret_from_fork - get rid of jump back

Impact: remove dead code

If we take a closer look at the rff_trace/rff_action ret_from_fork code,
we have to realize that it does all the wrong things: for example it
checks the TIF flag - while later on jumping back to the ret-from-syscall
path - duplicating the check needlessly.

But checking for _TIF_SYSCALL_TRACE is completely unnecessary here because
we clear that flag for every freshly forked task. So the whole "tracing"
code here, for which there is a out of line jump optimization that makes
it even harder to read, is in reality completely dead code ...

Reported-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Cyrill Gorcunov <gorcunov@gmail.com>

1 files changed, 16 insertions, 15 deletions

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index e41734a537bd..3194636a4293 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -361,34 +361,35 @@ ENTRY(save_paranoid)
 END(save_paranoid)
 
 /*
- * A newly forked process directly context switches into this.
+ * A newly forked process directly context switches into this address.
+ *
+ * rdi: prev task we switched from
  */
-/* rdi: prev */
 ENTRY(ret_from_fork)
         DEFAULT_FRAME
+
         push kernel_eflags(%rip)
         CFI_ADJUST_CFA_OFFSET 8
-        popf                            # reset kernel eflags
+        popf                                    # reset kernel eflags
         CFI_ADJUST_CFA_OFFSET -8
-        call schedule_tail
+
+        call schedule_tail                      # rdi: 'prev' task parameter
+
         GET_THREAD_INFO(%rcx)
-        testl $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT),TI_flags(%rcx)
+
         CFI_REMEMBER_STATE
-        jnz rff_trace
-rff_action:
         RESTORE_REST
-        testl $3,CS-ARGOFFSET(%rsp)     # from kernel_thread?
+
+        testl $3, CS-ARGOFFSET(%rsp)            # from kernel_thread?
         je   int_ret_from_sys_call
-        testl $_TIF_IA32,TI_flags(%rcx)
+
+        testl $_TIF_IA32, TI_flags(%rcx)        # 32-bit compat task needs IRET
         jnz  int_ret_from_sys_call
+
         RESTORE_TOP_OF_STACK %rdi, -ARGOFFSET
-        jmp ret_from_sys_call
+        jmp ret_from_sys_call                   # go to the SYSRET fastpath
+
         CFI_RESTORE_STATE
-rff_trace:
-        movq %rsp,%rdi
-        call syscall_trace_leave
-        GET_THREAD_INFO(%rcx)
-        jmp rff_action
         CFI_ENDPROC
 END(ret_from_fork)