[RFC 05/30] x86/entry/64/compat: After SYSENTER, move STI after the NT fixup
From: Andy Lutomirski
Date: Tue Sep 01 2015 - 18:52:32 EST
We eventually want to make it all the way into C code before
enabling interrupts. We need to rework our flags handling slightly
to delay enabling interrupts.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
arch/x86/entry/entry_64_compat.S | 30 ++++++++++++++++++++++--------
1 file changed, 22 insertions(+), 8 deletions(-)
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index aa76864a8a6b..1432d60a1f4a 100644
@@ -58,14 +58,9 @@ ENDPROC(native_usergs_sysret32)
* with the int 0x80 path.
- * Interrupts are off on entry.
- * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
- * it is too small to ever cause noticeable irq latency.
+ /* Interrupts are off on entry. */
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
/* Zero-extending 32-bit regs, do not remove */
movl %ebp, %ebp
@@ -76,7 +71,16 @@ ENTRY(entry_SYSENTER_compat)
/* Construct struct pt_regs on stack */
pushq $__USER32_DS /* pt_regs->ss */
pushq %rbp /* pt_regs->sp */
- pushfq /* pt_regs->flags */
+ * Push flags. This is nasty. First, interrupts are currently
+ * off, but we need pt_regs->flags to have IF set. Second, even
+ * if TF was set when SYSENTER started, it's clear by now. We fix
+ * that later using TIF_SINGLESTEP.
+ pushfq /* pt_regs->flags (except IF = 0) */
+ orl $X86_EFLAGS_IF, (%rsp) /* Fix saved flags */
pushq $__USER32_CS /* pt_regs->cs */
pushq %r10 /* pt_regs->ip = thread_info->sysenter_return */
pushq %rax /* pt_regs->orig_ax */
@@ -92,12 +96,22 @@ ENTRY(entry_SYSENTER_compat)
* Sysenter doesn't filter flags, so we need to clear NT
* ourselves. To save a few cycles, we can check whether
* NT was set instead of doing an unconditional popfq.
+ * This needs to happen before enabling interrupts so that
+ * we don't get preempted with NT set.
testl $X86_EFLAGS_NT, EFLAGS(%rsp)
+ * Re-enable interrupts. IRQ tracing already thinks that IRQs are
+ * on (since we treat user mode as having IRQs on), and the
+ * prologue above is too short for it to be worth adding a
+ * tracing round trip.
* no need to do an access_ok check here because rbp has been
* 32-bit zero extended
@@ -244,7 +258,7 @@ sysexit_audit:
- pushq $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
+ pushq $X86_EFLAGS_FIXED
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