[PATCH -tip ] kprobes/x86: Just return error for sanity check failureinstead of using BUG_ON

[Masami Hiramatsu]

Return an error from __copy_instruction and use printk to
give us more productive message, since this is just an error
case which we can handle it and also the BUG_ON() never
tell us why and what happened,

This is related to the below bug-report.
https://bugzilla.redhat.com/show_bug.cgi?id=910649

Note, this patch is for help to analyze what happened.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Frank Ch. Eigler <fche@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 arch/x86/kernel/kprobes/core.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 7bfe318..a86f83d 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -353,7 +353,11 @@ int __kprobes __copy_instruction(u8 *dest, u8 *src)
 		 * have given.
 		 */
 		newdisp = (u8 *) src + (s64) insn.displacement.value - (u8 *) dest;
-		BUG_ON((s64) (s32) newdisp != newdisp); /* Sanity check.  */
+		if ((s64) (s32) newdisp != newdisp) {
+			pr_err("Kprobes error: new displacement is not fit in s32 (%llx)\n", newdisp);
+			pr_err("\tSrc: %p, Dest: %p, old disp: %x\n", src, dest, insn.displacement.value);
+			return 0;
+		}
 		disp = (u8 *) dest + insn_offset_displacement(&insn);
 		*(s32 *) disp = (s32) newdisp;
 	}

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