x86/kprobes: Fix 1 byte conditional jump target

Commit 3bc753c ("kbuild: treat char as always unsigned") broke
kprobes.  Setting a probe-point on 1 byte conditional jump can cause the
kernel to crash when the (signed) relative jump offset gets treated as
unsigned.

Fix by replacing the unsigned 'immediate.bytes' (plus a cast) with the
signed 'immediate.value' when assigning to the relative jump offset.

[ dhansen: clarified changelog ]

Fixes: 3bc753c ("kbuild: treat char as always unsigned")
Suggested-by: Masami Hiramatsu (Google) <[email protected]>
Suggested-by: Dave Hansen <[email protected]>
Signed-off-by: Nadav Amit <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/all/20230208071708.4048-1-namit%40vmware.com

Author: anadav

arch/x86/kernel/kprobes/core.c

@@ -625,7 +625,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
 		/* 1 byte conditional jump */
 		p->ainsn.emulate_op = kprobe_emulate_jcc;
 		p->ainsn.jcc.type = opcode & 0xf;
-		p->ainsn.rel32 = *(char *)insn->immediate.bytes;
+		p->ainsn.rel32 = insn->immediate.value;
 		break;
 	case 0x0f:
 		opcode = insn->opcode.bytes[1];