[PATCH 1/2] Revert "x86/entry: Fix the end of the stack for newly forked tasks"

[Josh Poimboeuf]

Petr Mladek reported the following warning when loading the livepatch
sample module:

  WARNING: CPU: 1 PID: 3699 at arch/x86/kernel/stacktrace.c:132 save_stack_trace_tsk_reliable+0x133/0x1a0
  ...
  Call Trace:
   __schedule+0x273/0x820
   schedule+0x36/0x80
   kthreadd+0x305/0x310
   ? kthread_create_on_cpu+0x80/0x80
   ? icmp_echo.part.32+0x50/0x50
   ret_from_fork+0x2c/0x40

That warning means the end of the stack is no longer recognized as such
for newly forked tasks.  The problem was introduced with the following
commit:

  ff3f7e2475bb ("x86/entry: Fix the end of the stack for newly forked tasks")

... which was completely misguided.  It only partially fixed the
reported issue, and it introduced another bug in the process.  None of
the other entry code saves the frame pointer before calling into C code,
so it doesn't make sense for ret_from_fork to do so either.

Contrary to what I originally thought, the original issue wasn't related
to newly forked tasks.  It was actually related to ftrace.  When entry
code calls into a function which then calls into an ftrace handler, the
stack frame looks different than normal.

The original issue will be fixed in the unwinder, in a subsequent patch.

Reported-by: Petr Mladek <pmladek@xxxxxxxx>
Fixes: ff3f7e2475bb ("x86/entry: Fix the end of the stack for newly forked tasks")
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/entry/entry_32.S | 30 +++++++++++++++++++-----------
 arch/x86/entry/entry_64.S | 11 ++++-------
 2 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 50bc269..48ef7bb 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -252,6 +252,23 @@ ENTRY(__switch_to_asm)
 END(__switch_to_asm)
 
 /*
+ * 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.
+ */
+ENTRY(schedule_tail_wrapper)
+	FRAME_BEGIN
+
+	pushl	%eax
+	call	schedule_tail
+	popl	%eax
+
+	FRAME_END
+	ret
+ENDPROC(schedule_tail_wrapper)
+/*
  * A newly forked process directly context switches into this address.
  *
  * eax: prev task we switched from
@@ -259,24 +276,15 @@ END(__switch_to_asm)
  * edi: kernel thread arg
  */
 ENTRY(ret_from_fork)
-	FRAME_BEGIN		/* help unwinder find end of stack */
-
-	/*
-	 * schedule_tail() is asmlinkage so we have to put its 'prev' argument
-	 * on the stack.
-	 */
-	pushl	%eax
-	call	schedule_tail
-	popl	%eax
+	call	schedule_tail_wrapper
 
 	testl	%ebx, %ebx
 	jnz	1f		/* kernel threads are uncommon */
 
 2:
 	/* When we fork, we trace the syscall return in the child, too. */
-	leal	FRAME_OFFSET(%esp), %eax
+	movl    %esp, %eax
 	call    syscall_return_slowpath
-	FRAME_END
 	jmp     restore_all
 
 	/* kernel thread */
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 607d72c..4a4c083 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -36,7 +36,6 @@
 #include <asm/smap.h>
 #include <asm/pgtable_types.h>
 #include <asm/export.h>
-#include <asm/frame.h>
 #include <linux/err.h>
 
 .code64
@@ -406,19 +405,17 @@ END(__switch_to_asm)
  * r12: kernel thread arg
  */
 ENTRY(ret_from_fork)
-	FRAME_BEGIN			/* help unwinder find end of stack */
 	movq	%rax, %rdi
-	call	schedule_tail		/* rdi: 'prev' task parameter */
+	call	schedule_tail			/* rdi: 'prev' task parameter */
 
-	testq	%rbx, %rbx		/* from kernel_thread? */
-	jnz	1f			/* kernel threads are uncommon */
+	testq	%rbx, %rbx			/* from kernel_thread? */
+	jnz	1f				/* kernel threads are uncommon */
 
 2:
-	leaq	FRAME_OFFSET(%rsp),%rdi	/* pt_regs pointer */
+	movq	%rsp, %rdi
 	call	syscall_return_slowpath	/* returns with IRQs disabled */
 	TRACE_IRQS_ON			/* user mode is traced as IRQS on */
 	SWAPGS
-	FRAME_END
 	jmp	restore_regs_and_iret
 
 1:
-- 
2.7.4