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

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

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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/20230208071708.4048-1-namit@vmware.com/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
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:
 		/*

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

[Steven Rostedt]

On Wed, 15 Feb 2023 09:10:24 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> 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/20230208071708.4048-1-namit@vmware.com/
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
> Changes in v2:
>   - Remove meaningless immediate size branches too.
>

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

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

[Nadav Amit]

> On Feb 15, 2023, at 2:10 AM, Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> 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/20230208071708.4048-1-namit@vmware.com/
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks for incorporating my feedback.

Acked-by: Nadav Amit <namit@vmware.com>