[Xen-devel] [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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@xxxxxxxxxx>
---

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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