Re: [RFC PATCH 1/1] x86/kprobes: Use int3 instead of debug trap for single-step

[Peter Zijlstra]

On Tue, Mar 02, 2021 at 09:41:32AM +0100, Peter Zijlstra wrote:

> Aside of a few nits:

something like so..

--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -457,13 +457,20 @@ static void kprobe_emulate_call(struct k
 }
 NOKPROBE_SYMBOL(kprobe_emulate_call);
 
-static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
+static __always_inline 
+void __kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs, bool cond)
 {
        unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
 
-       ip += p->ainsn.rel32;
+       if (cond)
+               ip += p->ainsn.rel32;
        int3_emulate_jmp(regs, ip);
 }
+
+static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
+{
+       __kprobe_emulate_jmp(p, regs, true);
+}
 NOKPROBE_SYMBOL(kprobe_emulate_jmp);
 
 static const unsigned long jcc_mask[6] = {
@@ -488,10 +495,7 @@ static void kprobe_emulate_jcc(struct kp
                if (p->ainsn.jcc.type >= 0xe)
                        match = match && (regs->flags & X86_EFLAGS_ZF);
        }
-       if ((match && !invert) || (!match && invert))
-               kprobe_emulate_jmp(p, regs);
-       else
-               regs->ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
+       __kprobe_emulate_jmp(p, regs, (match && !invert) || (!match && invert));
 }
 NOKPROBE_SYMBOL(kprobe_emulate_jcc);
 
@@ -524,10 +528,7 @@ static void kprobe_emulate_loop(struct k
        else if (p->ainsn.loop.type == 1)       /* LOOPE */
                match = match && (regs->flags & X86_EFLAGS_ZF);
 
-       if (match)
-               kprobe_emulate_jmp(p, regs);
-       else
-               regs->ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
+       __kprobe_emulate_jmp(p, regs, match);
 }
 NOKPROBE_SYMBOL(kprobe_emulate_loop);
 
@@ -613,6 +614,12 @@ static int prepare_emulation(struct kpro
                else
                        p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
                break;
+       case 0x70 ... 0x7f:
+               /* 1 byte conditional jump */
+               p->ainsn.emulate_op = kprobe_emulate_jcc;
+               p->ainsn.jcc.type = opcode & 0xf;
+               p->ainsn.rel32 = *(char *)insn->immediate.bytes;
+               break;
        case 0x0f:
                opcode = insn->opcode.bytes[1];
                if ((opcode & 0xf0) == 0x80) {
@@ -667,12 +674,7 @@ static int prepare_emulation(struct kpro
 #endif
                break;
        default:
-               if ((opcode & 0xf0) == 0x70) {
-                       /* 1 byte conditional jump */
-                       p->ainsn.emulate_op = kprobe_emulate_jcc;
-                       p->ainsn.jcc.type = opcode & 0xf;
-                       p->ainsn.rel32 = *(char *)insn->immediate.bytes;
-               }
+               break;
        }
        p->ainsn.size = insn->length;