x86/32: Remove schedule_tail_wrapper()

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lkml.kernel.org/r/20230623225529.34590-2-brgerst@gmail.com

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 91397f58ac3008556fc0cfbb2437eee45da706ff..e56123f03a79671515dc27091bf207abdbc65e68 100644 (file)
--- 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.
  *
@@ -748,16 +728,23 @@ 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
+       FRAME_END
 
        testl   %ebx, %ebx
        jnz     1f              /* kernel threads are uncommon */
 
 2:
        /* When we fork, we trace the syscall return in the child, too. */
-       movl    %esp, %eax
+       leal    4(%esp), %eax
        call    syscall_exit_to_user_mode
-       jmp     .Lsyscall_32_done
+       RET
 
        /* kernel thread */
 1:     movl    %edi, %eax