linux-kernel - [PATCH v2 4/6] x86/32: Remove schedule_tail

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Date:   Thu, 22 Jun 2023 16:42:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, alyssa.milburn@...ux.intel.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        samitolvanen@...gle.com, keescook@...omium.org,
        jpoimboe@...nel.org, joao@...rdrivepizza.com, brgerst@...il.com
Subject: [PATCH v2 4/6] x86/32: Remove schedule_tail_wrapper()

From: Brian Gerst <brgerst@...il.com>

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@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20230622120750.5549-2-brgerst@gmail.com
---
 arch/x86/entry/entry_32.S |   32 ++++++++++----------------------
 1 file changed, 10 insertions(+), 22 deletions(-)

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -720,26 +720,6 @@ 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.
  *
  * eax: prev task we switched from
@@ -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