[PATCH] kprobes/x86: Use the current code when modified by Kprobes using ftrace

[Petr Mladek <pmladek@suse.cz>]

can_probe() checks if the given address points to the beginning of
an instruction. It analyzes all the instructions from the beginning
of the function until the given address. The code might be modified
by another Kprobe. In this case, the current code is read into a buffer,
int3 breakpoint is replaced by the saved opcode in the buffer, and
can_probe() analyzes the buffer instead.

There is a bug that __recover_probed_insn() tries to restore
the original code even for Kprobes using the ftrace framework.
But in this case, the opcode is not stored. See the difference
between arch_prepare_kprobe() and arch_prepare_kprobe_ftrace().
The opcode is stored by arch_copy_kprobe() only from
arch_prepare_kprobe().

This patch makes Kprobe to use the current code when it is modified
by ftrace. It is not the original one but it is a valid instruction
of the same length. Therefore it is perfectly fine for the check.

Note that I found this problem when playing with Kprobes. I did it
on x86_64 with gcc-4.8.3 that supported -mfentry. I modified
samples/kprobes/kprobe_example.c and added offset 5 to put
the probe right after the fentry area:

--- cut ---
 static struct kprobe kp = {
 	.symbol_name	= "do_fork",
+	.offset = 5,
 };
--- cut ---

Then I was able to load kprobe_example before jprobe_example
but not the other way around:

$> modprobe jprobe_example
$> modprobe kprobe_example
modprobe: ERROR: could not insert 'kprobe_example': Invalid or incomplete multibyte or wide character

It did not make much sense and debugging pointed to the bug
described above.

Signed-off-by: Petr Mladek <pmladek@suse.cz>
---
 arch/x86/kernel/kprobes/core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 98f654d466e5..ef321caae3ba 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -225,8 +225,14 @@ __recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
 	struct kprobe *kp;
 
 	kp = get_kprobe((void *)addr);
-	/* There is no probe, return original address */
-	if (!kp)
+	/*
+	 * Use the current code if it is not modified by Kprobe
+	 * or when the Kprobe is using ftrace. In the second case
+	 * we do not have any information about the original code
+	 * but it is not a problem. Ftrace has put there a valid
+	 * instruction of the same length.
+	 */
+	if (!kp || kprobe_ftrace(kp))
 		return addr;
 
 	/*
-- 
1.8.5.6