[PATCH 1/2] x86/32: Remove schedule_tail_wrapper()

Brian Gerst
2023-06-22T12:08:17Z

The unwinder expects a return address at the very top of the kernel
stack just below pt_regs and before any stack frame is created. Instead
of calling a wrapper, set up a return address as if ret_from_fork()
was called from the syscall entry code.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
arch/x86/entry/entry_32.S | 32 ++++++++++----------------------
1 file changed, 10 insertions(+), 22 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 91397f58ac30..6c1ee76adc11 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -719,26 +719,6 @@ SYM_CODE_START(__switch_to_asm)
SYM_CODE_END(__switch_to_asm)
.popsection

-/*
- * The unwinder expects the last frame on the stack to always be at the same
- * offset from the end of the page, which allows it to validate the stack.
- * Calling schedule_tail() directly would break that convention because its an
- * asmlinkage function so its argument has to be pushed on the stack. This
- * wrapper creates a proper "end of stack" frame header before the call.
- */
-.pushsection .text, "ax"
-SYM_FUNC_START(schedule_tail_wrapper)
- FRAME_BEGIN
-
- pushl %eax
- call schedule_tail
- popl %eax
-
- FRAME_END
- RET
-SYM_FUNC_END(schedule_tail_wrapper)
-.popsection
-
/*
* A newly forked process directly context switches into this address.
*
@@ -748,7 +728,13 @@ SYM_FUNC_END(schedule_tail_wrapper)
*/
.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork)
- call schedule_tail_wrapper
+ /* return address for the stack unwinder */
+ pushl $.Lsyscall_32_done
+ FRAME_BEGIN
+
+ pushl %eax
+ call schedule_tail
+ addl $4, %esp

testl %ebx, %ebx
jnz 1f /* kernel threads are uncommon */
@@ -757,7 +743,9 @@ SYM_CODE_START(ret_from_fork)
/* When we fork, we trace the syscall return in the child, too. */
movl %esp, %eax
call syscall_exit_to_user_mode
- jmp .Lsyscall_32_done
+
+ FRAME_END
+ RET

/* kernel thread */
1: movl %edi, %eax
--
2.41.0