linux-kernel - [PATCH v2 3/9] x86/entry/32: fix the end of the stack for newly forked tasks

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Message-Id: <ecafcd882676bf48ceaf50483782552bb98476e5.1474480779.git.jpoimboe@redhat.com>
Date:   Wed, 21 Sep 2016 16:04:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: [PATCH v2 3/9] x86/entry/32: fix the end of the stack for newly forked tasks

Thanks to all the recent x86 entry code refactoring, most tasks' kernel
stacks start at the same offset right below their saved pt_regs,
regardless of which syscall was used to enter the kernel.  That creates
a nice convention which makes it straightforward to identify the end of
the stack, which can be useful for the unwinder to verify the stack is
sane.

Calling schedule_tail() directly breaks that convention because its an
asmlinkage function so its argument has to be pushed on the stack.  Add
a wrapper which creates a proper "end of stack" frame header before the
call.

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/entry/entry_32.S | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index dae8684..82b55ef 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -44,6 +44,7 @@
 #include <asm/alternative-asm.h>
 #include <asm/asm.h>
 #include <asm/smap.h>
+#include <asm/frame.h>
 
 	.section .entry.text, "ax"
 
@@ -237,6 +238,23 @@ 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
@@ -244,9 +262,7 @@ END(__switch_to_asm)
  * edi: kernel thread arg
  */
 ENTRY(ret_from_fork)
-	pushl	%eax
-	call	schedule_tail
-	popl	%eax
+	call	schedule_tail_wrapper
 
 	testl	%ebx, %ebx
 	jnz	1f		/* kernel threads are uncommon */
-- 
2.7.4