x86, debug: remove the confusing entry in call trace

Impact: improve backtrace quality

avoid the confusion in call trace because of the lack of padding at the
tail of function.

When do_exit gets called, the return address behind call instruction is
pushed into stack. If something get wrong in do_exit, for x86_64, the
entry "kernel_execve +0x00/0xXX" rather than "child_rip +0xYY/0xZZ" is
in the call trace.

That looks confusing, so add a u2d to make the return address still part
of the original call site. (This also catches any instances of us returning
from that function somehow.)

Signed-off-by: jia zhang <jia.zhang2008@gmail.com>
Acked-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 2 insertions, 0 deletions

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 28b597ef9ca1..f6402c4ba10d 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1051,6 +1051,7 @@ ENTRY(kernel_thread_helper)
         push %eax
         CFI_ADJUST_CFA_OFFSET 4
         call do_exit
+        ud2                     # padding for call trace
         CFI_ENDPROC
 ENDPROC(kernel_thread_helper)
 
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index ddeeb1052583..4a16bf31c783 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1172,6 +1172,7 @@ child_rip:
         # exit
         mov %eax, %edi
         call do_exit
+        ud2                     # padding for call trace
         CFI_ENDPROC
 ENDPROC(child_rip)