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From:   Brian Gerst <brgerst@gmail.com>
To:     linux-kernel@vger.kernel.org, x86@kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        alyssa.milburn@linux.intel.com, keescook@chromium.org,
        jpoimboe@kernel.org, joao@overdrivepizza.com,
        tim.c.chen@linux.intel.com, Brian Gerst <brgerst@gmail.com>
Subject: [PATCH 1/2] x86/32: Remove schedule_tail_wrapper()
Date:   Thu, 22 Jun 2023 08:07:49 -0400
Message-ID: <20230622120750.5549-2-brgerst@gmail.com>
In-Reply-To: <20230622120750.5549-1-brgerst@gmail.com>
References: <20230622120750.5549-1-brgerst@gmail.com>
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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