Hi Andi,
I am trying to add a TIF_TRACE_KERNEL to each architectures to have a
system-wide activation of syscall_trace. However, I get the following
issue on x86_64 : a few processes segfault and others get a GPF when I
enable the flag on all processes. I am starting to think that it might
be caused by an incorrect top of stack when we return from a
syscall/interrupt in these processes. It would happen if we get into the
following race:
1 - process A enters in a syscall, TIF_KERNEL_TRACE is cleared
2 - we activate TIF_KERNEL_TRACE
3 - process A returns from syscall (with wrong top of stack ?) -> segfault.
Am I on the right track ?
Can this be a concern with TIF_SYSCALL_TRACE also ? (potential race in
ptrace ?)
Thanks for you input,
Mathieu
My x86_64 flags patch for 2.6.23.1 looks like this:
---
include/asm-x86_64/thread_info.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
Index: linux-2.6-lttng/include/asm-x86_64/thread_info.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-x86_64/thread_info.h 2007-07-30
18:46:16.000000000 -0400
+++ linux-2.6-lttng/include/asm-x86_64/thread_info.h 2007-07-30
19:13:03.000000000 -0400
@@ -107,6 +107,7 @@ static inline struct thread_info *stack_
* Warning: layout of LSW is hardcoded in entry.S
*/
#define TIF_SYSCALL_TRACE 0 /* syscall trace active */
+#define TIF_KERNEL_TRACE 1 /* kernel trace active */
#define TIF_SIGPENDING 2 /* signal pending */
#define TIF_NEED_RESCHED 3 /* rescheduling necessary */
#define TIF_SINGLESTEP 4 /* reenable singlestep on user return*/
@@ -125,6 +126,7 @@ static inline struct thread_info *stack_
#define TIF_FREEZE 23 /* is freezing for suspend */
#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
+#define _TIF_KERNEL_TRACE (1<<TIF_KERNEL_TRACE)
#define _TIF_SIGPENDING (1<<TIF_SIGPENDING)
#define _TIF_SINGLESTEP (1<<TIF_SINGLESTEP)
#define _TIF_NEED_RESCHED (1<<TIF_NEED_RESCHED)
@@ -142,7 +144,7 @@ static inline struct thread_info *stack_
/* work to do on interrupt/exception return */
#define _TIF_WORK_MASK \
- (0x0000FFFF &
~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SINGLESTEP|_TIF_SECCOMP))
+ (0x0000FFFF &
~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_KERNEL_TRACE|_TIF_SINGLESTEP|_TIF_SECCOMP))
/* work to do on any return to user space */
#define _TIF_ALLWORK_MASK (0x0000FFFF & ~_TIF_SECCOMP)
And the code to activate/deactivate the flags:
---
include/linux/sched.h | 3 +++
kernel/fork.c | 9 +++++++++
kernel/sched.c | 42 ++++++++++++++++++++++++++++++++++++++++++
3 files changed, 54 insertions(+)
Index: linux-2.6-lttng/include/linux/sched.h
===================================================================
--- linux-2.6-lttng.orig/include/linux/sched.h 2007-10-12 12:02:09.000000000
-0400
+++ linux-2.6-lttng/include/linux/sched.h 2007-10-12 12:11:23.000000000
-0400
@@ -1953,6 +1953,9 @@ static inline void inc_syscw(struct task
}
#endif
+extern void clear_kernel_trace_flag_all_tasks(void);
+extern void set_kernel_trace_flag_all_tasks(void);
+
#endif /* __KERNEL__ */
#endif
Index: linux-2.6-lttng/kernel/fork.c
===================================================================
--- linux-2.6-lttng.orig/kernel/fork.c 2007-10-12 12:02:18.000000000 -0400
+++ linux-2.6-lttng/kernel/fork.c 2007-10-12 12:11:23.000000000 -0400
@@ -1241,6 +1241,15 @@ static struct task_struct *copy_process(
!cpu_online(task_cpu(p))))
set_task_cpu(p, smp_processor_id());
+ /*
+ * The state of the parent's TIF_KTRACE flag may have changed
+ * since it was copied in dup_task_struct() so we re-copy it here.
+ */
+ if (test_thread_flag(TIF_KERNEL_TRACE))
+ set_tsk_thread_flag(p, TIF_KERNEL_TRACE);
+ else
+ clear_tsk_thread_flag(p, TIF_KERNEL_TRACE);
+
/* CLONE_PARENT re-uses the old parent */
if (clone_flags & (CLONE_PARENT|CLONE_THREAD))
p->real_parent = current->real_parent;
Index: linux-2.6-lttng/kernel/sched.c
===================================================================
--- linux-2.6-lttng.orig/kernel/sched.c 2007-10-12 12:02:17.000000000 -0400
+++ linux-2.6-lttng/kernel/sched.c 2007-10-12 12:11:55.000000000 -0400
@@ -7032,3 +7032,45 @@ struct cgroup_subsys cpu_cgroup_subsys =
};
#endif /* CONFIG_FAIR_CGROUP_SCHED */
+
+/**
+ * clear_kernel_trace_flag_all_tasks - clears all TIF_KERNEL_TRACE thread
flags.
+ *
+ * This function iterates on all threads in the system to clear their
+ * TIF_KERNEL_TRACE flag. Setting the TIF_KERNEL_TRACE flag with the
+ * tasklist_lock held in copy_process() makes sure that once we finish clearing
+ * the thread flags, all threads have their flags cleared.
+ */
+void clear_kernel_trace_flag_all_tasks(void)
+{
+ struct task_struct *p;
+ struct task_struct *t;
+
+ read_lock(&tasklist_lock);
+ do_each_thread(p, t) {
+ clear_tsk_thread_flag(t, TIF_KERNEL_TRACE);
+ } while_each_thread(p, t);
+ read_unlock(&tasklist_lock);
+}
+EXPORT_SYMBOL_GPL(clear_kernel_trace_flag_all_tasks);
+
+/**
+ * set_kernel_trace_flag_all_tasks - sets all TIF_KERNEL_TRACE thread flags.
+ *
+ * This function iterates on all threads in the system to set their
+ * TIF_KERNEL_TRACE flag. Setting the TIF_KERNEL_TRACE flag with the
+ * tasklist_lock held in copy_process() makes sure that once we finish setting
+ * the thread flags, all threads have their flags set.
+ */
+void set_kernel_trace_flag_all_tasks(void)
+{
+ struct task_struct *p;
+ struct task_struct *t;
+
+ read_lock(&tasklist_lock);
+ do_each_thread(p, t) {
+ set_tsk_thread_flag(t, TIF_KERNEL_TRACE);
+ } while_each_thread(p, t);
+ read_unlock(&tasklist_lock);
+}
+EXPORT_SYMBOL_GPL(set_kernel_trace_flag_all_tasks);
--
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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