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Subject[PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text (but move it)
From: Steven Rostedt <srostedt@redhat.com>

If a probe is placed on a ftrace nop (or ftrace_caller), simply move the
probe to the next instruction instead of rejecting it. This will allow
kprobes not to be affected by ftrace using the -mfentry gcc option which
will put the ftrace nop at the beginning of the function. As a very common
case for kprobes is to add a probe to the very beginning of a function
we need a way to handle the case when ftrace takes the first instruction.

Added KPROBE_FLAG_MOVED (as suggested by Masami) that is set when the address
is moved to get around an ftrace nop.

Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
include/linux/ftrace.h | 2 ++
include/linux/kprobes.h | 1 +
kernel/kprobes.c | 13 ++++++++++++-
3 files changed, 15 insertions(+), 1 deletion(-)
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 9310993..211ae45 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -414,6 +414,8 @@ static inline int ftrace_text_reserved(void *start, void *end)
#define ftrace_set_notrace(ops, buf, len, reset) ({ -ENODEV; })
#define ftrace_free_filter(ops) do { } while (0)

+static inline unsigned long ftrace_location(unsigned long ip) { return 0; }
+
static inline ssize_t ftrace_filter_write(struct file *file, const char __user *ubuf,
size_t cnt, loff_t *ppos) { return -ENODEV; }
static inline ssize_t ftrace_notrace_write(struct file *file, const char __user *ubuf,
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index b6e1f8c..23cf41e 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -128,6 +128,7 @@ struct kprobe {
* NOTE:
* this flag is only for optimized_kprobe.
*/
+#define KPROBE_FLAG_MOVED 8 /* probe was moved passed ftrace nop */

/* Has this kprobe gone ? */
static inline int kprobe_gone(struct kprobe *p)
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index c62b854..952619b 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1319,10 +1319,22 @@ int __kprobes register_kprobe(struct kprobe *p)
struct kprobe *old_p;
struct module *probed_mod;
kprobe_opcode_t *addr;
+ unsigned long ftrace_addr;

addr = kprobe_addr(p);
if (IS_ERR(addr))
return PTR_ERR(addr);
+
+ /*
+ * If the address is located on a ftrace nop, set the
+ * breakpoint to the following instruction.
+ */
+ ftrace_addr = ftrace_location((unsigned long)addr);
+ if (unlikely(ftrace_addr)) {
+ addr = (kprobe_opcode_t *)(ftrace_addr + MCOUNT_INSN_SIZE);
+ p->flags |= KPROBE_FLAG_MOVED;
+ }
+
p->addr = addr;

ret = check_kprobe_rereg(p);
@@ -1333,7 +1345,6 @@ int __kprobes register_kprobe(struct kprobe *p)
preempt_disable();
if (!kernel_text_address((unsigned long) p->addr) ||
in_kprobes_functions((unsigned long) p->addr) ||
- ftrace_text_reserved(p->addr, p->addr) ||
jump_label_text_reserved(p->addr, p->addr)) {
ret = -EINVAL;
goto cannot_probe;
--
1.7.9.5



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