[PATCH 1/2] kprobes/x86: boost: Fix checking if there is enough room for a jump

[Eugene Shatokhin <eugene.shatokhin@rosalab.ru>]

Kprobes' "boost" feature allows to avoid single-stepping in some cases,
along with its overhead. It needs a relative jump placed in the insn
slot right after the instruction. So the length of the instruction plus
5 (the length of the near relative unconditional jump) must not exceed
the length of the slot.

However, the code currently checks if that sum is strictly less than
the length of the slot. So "boost" cannot be used for the instructions
of 10 bytes in size (with MAX_INSN_SIZE == 15), like
"movabs $0x45525f4b434f4c43,%rax" (48 b8 43 4c 4f 43 4b 5f 52 45),
"movl $0x1,0xf8dd(%rip)"          (c7 05 dd f8 00 00 01 00 00 00), etc.

This patch fixes that conditional.

Signed-off-by: Eugene Shatokhin <eugene.shatokhin@rosalab.ru>
---
 arch/x86/kernel/kprobes/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 1deffe6..0a42b76 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -881,7 +881,7 @@ static void resume_execution(struct kprobe *p, struct pt_regs *regs,
 
 	if (p->ainsn.boostable == 0) {
 		if ((regs->ip > copy_ip) &&
-		    (regs->ip - copy_ip) + 5 < MAX_INSN_SIZE) {
+		    (regs->ip - copy_ip) + 5 <= MAX_INSN_SIZE) {
 			/*
 			 * These instructions can be executed directly if it
 			 * jumps back to correct address.
-- 
2.3.2