[RESEND][PATCH v15 7/7] ARM: kprobes: enable OPTPROBES for ARM 32

(2014/12/09 18:14), Jon Medhurst (Tixy) wrote:
[...]

quoted

quoted

diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
index 3a58db4..a4ec240 100644
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c

@@ -163,19 +163,31 @@ void __kprobes arch_arm_kprobe(struct kprobe *p)
  * memory. It is also needed to atomically set the two half-words of a 32-bit
  * Thumb breakpoint.
  */
-int __kprobes __arch_disarm_kprobe(void *p)
-{
-	struct kprobe *kp = p;
-	void *addr = (void *)((uintptr_t)kp->addr & ~1);
-
-	__patch_text(addr, kp->opcode);
+struct patch {
+	void *addr;
+	unsigned int insn;
+};
 
+static int __kprobes_remove_breakpoint(void *data)
+{
+	struct patch *p = data;
+	__patch_text(p->addr, p->insn);
 	return 0;
 }
 
+void __kprobes kprobes_remove_breakpoint(void *addr, unsigned int insn)
+{
+	struct patch p = {
+		.addr = addr,
+		.insn = insn,
+	};
+	stop_machine(__kprobes_remove_breakpoint, &p, cpu_online_mask);
+}

Hmm, I think finally we should fix patch_text() in patch.c to forcibly use stop_machine
by adding "bool stop" parameter, instead of introducing new another patch_text()
implementation, because we'd better avoid two private "patch" data structures.

That was my first thought too, then I realised that breaks encapsulation
of the patch_text implementation, because its use of stop_machine is an
implementation detail and it could be rewritten to not use stop machine.
(That is sort of on my long term todo list
https://lkml.org/lkml/2014/9/4/188)

Indeed. OK, now let it goes. :)

Whereas stop machine is used by kprobes to avoid race conditions with
the undefined instruction exception handler and something like that
would be needed even if patch_text didn't use stop_machine.

At this point, it's OK.

However, I'm not convinced completely. Perhaps, it depends on cache-coherent bus
implementation, but there may be some implementations which can allow us to
change one instruction atomically without stop_machine.

I'm actually interested in PREEMPT_RT on arm32, and this stop_machine() is a barrier
to apply kprobes on real-time systems.

Thank you,


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt at hitachi.com