The enable_single_step() logic bails out early if TF is already set.
That skips some of the bookkeeping that keeps things straight.
This makes PTRACE_SINGLEBLOCK break the behavior of a user task
that was already setting TF itself in user mode.
Fix the bookkeeping to notice the old TF setting as it should.
Test case at: http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/tests/ptrace-tests/tests/step-...
Signed-off-by: Roland McGrath <roland@redhat.com>
---
arch/x86/kernel/step.c | 22 ++++++++++++++++------
1 files changed, 16 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 92c20fe..0d2cb36 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -105,6 +105,7 @@ static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
static int enable_single_step(struct task_struct *child)
{
struct pt_regs *regs = task_pt_regs(child);
+ unsigned long oflags;
/*
* Always set TIF_SINGLESTEP - this guarantees that
@@ -113,11 +114,7 @@ static int enable_single_step(struct task_struct *child)
*/
set_tsk_thread_flag(child, TIF_SINGLESTEP);
- /*
- * If TF was already set, don't do anything else
- */
- if (regs->flags & X86_EFLAGS_TF)
- return 0;
+ oflags = regs->flags;
/* Set TF on the kernel stack.. */
regs->flags |= X86_EFLAGS_TF;
@@ -126,9 +123,22 @@ static int enable_single_step(struct task_struct *child)
* ..but if TF is changed by the instruction we will trace,
* don't mark it as being "us" that set it, so that we
* won't clear it by hand later.
+ *
+ * Note that if we don't actually execute the popf because
+ * of a signal arriving right now or suchlike, we will lose
+ * track of the fact that it really was "us" that set it.
*/
- if (is_setting_trap_flag(child, regs))
+ if (is_setting_trap_flag(child, regs)) {
+ clear_tsk_thread_flag(child, TIF_FORCED_TF);
return 0;
+ }
+
+ /*
+ * If TF was already set, check whether it was us who set it.
+ * If not, we should never attempt a block step.
+ */
+ if (oflags & X86_EFLAGS_TF)
+ return test_tsk_thread_flag(child, TIF_FORCED_TF);
set_tsk_thread_flag(child, TIF_FORCED_TF);
--
