linux-kernel - [PATCH] xen/x86: Adjust stack pointer in xen

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Date:	Fri, 13 Nov 2015 18:18:26 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	konrad.wilk@...cle.com, david.vrabel@...rix.com
Cc:	xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
	luto@...nel.org, boris.ostrovsky@...cle.com
Subject: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c
("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack
frame that is passed to xen_sysexit is no longer a "standard" one (i.e.
it's not pt_regs).

We need to adjust it so that subsequent xen_iret can use it.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
---

Alternatively, we could return 0 from do_fast_syscall_32() if paravirt_enabled()
is true since Xen PV guests will end up using xen_iret one way or the other. And
then we won't need xen_sysexit at all.

 arch/x86/xen/xen-asm_32.S |   23 ++++++++++++++++-------
 1 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S
index fd92a64..c70ec37 100644
--- a/arch/x86/xen/xen-asm_32.S
+++ b/arch/x86/xen/xen-asm_32.S
@@ -36,15 +36,24 @@ check_events:
 
 /*
  * We can't use sysexit directly, because we're not running in ring0.
- * But we can easily fake it up using iret.  Assuming xen_sysexit is
- * jumped to with a standard stack frame, we can just strip it back to
- * a standard iret frame and use iret.
+ * But we can easily fake it up using iret.
+ * We came here from the opportunistic SYSEXIT path in entry_SYSENTER_32
+ * which left the stack looking like this:
+ * 	$__USER_DS
+ * 	%ecx
+ * 	eflags
+ *	$__USER_CS
+ *	%eip
+ *	%eax
+ *	%gs
+ *	%fs
+ *	%es
+ *	%ds	<-- %esp
+ *
+ * so we need to adjust it to look like a standard iret frame
  */
 ENTRY(xen_sysexit)
-	movl PT_EAX(%esp), %eax			/* Shouldn't be necessary? */
-	orl $X86_EFLAGS_IF, PT_EFLAGS(%esp)
-	lea PT_EIP(%esp), %esp
-
+	add $5*4, %esp
 	jmp xen_iret
 ENDPROC(xen_sysexit)
 
-- 
1.7.1

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