[patch-RFC 23/26] Prepare x86_64 for TIF_SYSCALL_TRACE async flag set in entry.S

[Mathieu Desnoyers]

When the flag is inactive upon syscall entry and concurrently activated before
exit, we seem to reach a state where the top of stack is incorrect upon return
to user space.

Fix this by fixing the top of stack and jumping to int_ret_from_sys_call if we
detect that thread flags has been modified.

We make sure that the thread flag read is coherent between our new test and the ALLWORK_MASK test by first saving it in a register used for both comparisons.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: Andi Kleen <ak@xxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: H. Peter Anvin <hpa@xxxxxxxxx>
---
 arch/x86/kernel/entry_64.S |   12 ++++++++++++
 1 file changed, 12 insertions(+)

Index: linux-2.6-lttng/arch/x86/kernel/entry_64.S
===================================================================
--- linux-2.6-lttng.orig/arch/x86/kernel/entry_64.S	2007-11-13 09:25:25.000000000 -0500
+++ linux-2.6-lttng/arch/x86/kernel/entry_64.S	2007-11-13 09:49:48.000000000 -0500
@@ -268,6 +268,8 @@ sysret_check:		
 	/* Handle reschedules */
 	/* edx:	work, edi: workmask */	
 sysret_careful:
+	testl $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP),%edx
+	jnz ret_from_sys_call_trace
 	bt $TIF_NEED_RESCHED,%edx
 	jnc sysret_signal
 	TRACE_IRQS_ON
@@ -279,6 +281,16 @@ sysret_careful:
 	CFI_ADJUST_CFA_OFFSET -8
 	jmp sysret_check
 
+ret_from_sys_call_trace:
+	TRACE_IRQS_ON
+	sti
+	SAVE_REST
+	FIXUP_TOP_OF_STACK %rdi
+	movq %rsp,%rdi
+	LOAD_ARGS ARGOFFSET  /* reload args from stack in case ptrace changed it */
+	RESTORE_REST
+	jmp int_ret_from_sys_call
+
 	/* Handle a signal */ 
 sysret_signal:
 	TRACE_IRQS_ON

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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