2023-02-15 00:11:00

by Masami Hiramatsu

Subject: [PATCH -tip v2] x86/kprobes: Remove unneeded casting from immediate value

From: Masami Hiramatsu (Google) <[email protected]>

Remove unneeded casting from immediate value assignments for relative
jump offset. Since the immediate values in the 'insn' data structure are
assigned from immediate bytes as a signed value to insn.immediate.value
by insn_field_set(). Thus, if we need to access that value as a signed
value (in this kprobe's case), we don't need to cast it.
This is a kind of clean up (should not change behavior) follows Nadav's
bugfix.

Link: https://lore.kernel.org/all/[email protected]/

Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
---
Changes in v2:
- Remove meaningless immediate size branches too.
---
arch/x86/kernel/kprobes/core.c | 19 ++++---------------
1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 695873c0f50b..41ffdf04d9c4 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -606,20 +606,12 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
break;
case 0xe8: /* near call relative */
p->ainsn.emulate_op = kprobe_emulate_call;
- if (insn->immediate.nbytes == 2)
- p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
- else
- p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
+ p->ainsn.rel32 = insn->immediate.value;
break;
case 0xeb: /* short jump relative */
case 0xe9: /* near jump relative */
p->ainsn.emulate_op = kprobe_emulate_jmp;
- if (insn->immediate.nbytes == 1)
- p->ainsn.rel32 = *(s8 *)&insn->immediate.value;
- else if (insn->immediate.nbytes == 2)
- p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
- else
- p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
+ p->ainsn.rel32 = insn->immediate.value;
break;
case 0x70 ... 0x7f:
/* 1 byte conditional jump */
@@ -633,10 +625,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
/* 2 bytes Conditional Jump */
p->ainsn.emulate_op = kprobe_emulate_jcc;
p->ainsn.jcc.type = opcode & 0xf;
- if (insn->immediate.nbytes == 2)
- p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
- else
- p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
+ p->ainsn.rel32 = insn->immediate.value;
} else if (opcode == 0x01 &&
X86_MODRM_REG(insn->modrm.bytes[0]) == 0 &&
X86_MODRM_MOD(insn->modrm.bytes[0]) == 3) {
@@ -651,7 +640,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
p->ainsn.emulate_op = kprobe_emulate_loop;
p->ainsn.loop.type = opcode & 0x3;
p->ainsn.loop.asize = insn->addr_bytes * 8;
- p->ainsn.rel32 = *(s8 *)&insn->immediate.value;
+ p->ainsn.rel32 = insn->immediate.value;
break;
case 0xff:
/*

2023-02-15 04:44:38

by Steven Rostedt

Subject: Re: [PATCH -tip v2] x86/kprobes: Remove unneeded casting from immediate value

On Wed, 15 Feb 2023 09:10:24 +0900
"Masami Hiramatsu (Google)" <[email protected]> wrote:

> From: Masami Hiramatsu (Google) <[email protected]>
>
> Remove unneeded casting from immediate value assignments for relative
> jump offset. Since the immediate values in the 'insn' data structure are
> assigned from immediate bytes as a signed value to insn.immediate.value
> by insn_field_set(). Thus, if we need to access that value as a signed
> value (in this kprobe's case), we don't need to cast it.
> This is a kind of clean up (should not change behavior) follows Nadav's
> bugfix.
>
> Link: https://lore.kernel.org/all/[email protected]/
>
> Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
> ---
> Changes in v2:
> - Remove meaningless immediate size branches too.
>

Reviewed-by: Steven Rostedt (Google) <[email protected]>

-- Steve

2023-02-15 08:08:14

by Nadav Amit

Subject: Re: [PATCH -tip v2] x86/kprobes: Remove unneeded casting from immediate value

> On Feb 15, 2023, at 2:10 AM, Masami Hiramatsu (Google) <[email protected]> wrote:
>
> From: Masami Hiramatsu (Google) <[email protected]>
>
> Remove unneeded casting from immediate value assignments for relative
> jump offset. Since the immediate values in the 'insn' data structure are
> assigned from immediate bytes as a signed value to insn.immediate.value
> by insn_field_set(). Thus, if we need to access that value as a signed
> value (in this kprobe's case), we don't need to cast it.
> This is a kind of clean up (should not change behavior) follows Nadav's
> bugfix.
>
> Link: https://lore.kernel.org/all/[email protected]/
>
> Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

Thanks for incorporating my feedback.

Acked-by: Nadav Amit <[email protected]>