[PATCH 5/5] powerpc/jprobes: Validate break handler invocation as being due to a jprobe_return()

[Naveen N. Rao]

Fix a circa 2005 FIXME by implementing a check to ensure that we
actually got into the jprobe break handler() due to the trap in
jprobe_return().

Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
---
This is v2 of the patch posted previously (*) to change the WARN() to a 
pr_debug() as suggested by Masami.
(*) https://patchwork.ozlabs.org/patch/762670/

- Naveen

 arch/powerpc/kernel/kprobes.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index db40b13fd3d1..5b26d5202273 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -648,24 +648,22 @@ NOKPROBE_SYMBOL(setjmp_pre_handler);
 
 void __used jprobe_return(void)
 {
-	asm volatile("trap" ::: "memory");
+	asm volatile("jprobe_return_trap:\n"
+		     "trap\n"
+		     ::: "memory");
 }
 NOKPROBE_SYMBOL(jprobe_return);
 
-static void __used jprobe_return_end(void)
-{
-}
-NOKPROBE_SYMBOL(jprobe_return_end);
-
 int longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 {
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 
-	/*
-	 * FIXME - we should ideally be validating that we got here 'cos
-	 * of the "trap" in jprobe_return() above, before restoring the
-	 * saved regs...
-	 */
+	if (regs->nip != ppc_kallsyms_lookup_name("jprobe_return_trap")) {
+		pr_debug("longjmp_break_handler NIP (0x%lx) does not match jprobe_return_trap (0x%lx)\n",
+				regs->nip, ppc_kallsyms_lookup_name("jprobe_return_trap"));
+		return 0;
+	}
+
 	memcpy(regs, &kcb->jprobe_saved_regs, sizeof(struct pt_regs));
 	/* It's OK to start function graph tracing again */
 	unpause_graph_tracing();
-- 
2.14.1