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

[PATCH -tip] kprobes/x86: 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>
---
 arch/x86/kernel/kprobes/core.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 695873c0f50b..2440e736d0e4 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -607,19 +607,19 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
 	case 0xe8:	/* near call relative */
 		p->ainsn.emulate_op = kprobe_emulate_call;
 		if (insn->immediate.nbytes == 2)
-			p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
+			p->ainsn.rel32 = 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;
+			p->ainsn.rel32 = insn->immediate.value;
 		else if (insn->immediate.nbytes == 2)
-			p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
+			p->ainsn.rel32 = 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 */
@@ -634,9 +634,9 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
 			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;
+				p->ainsn.rel32 = 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 +651,7 @@ 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] kprobes/x86: Remove unneeded casting from immediate value

[Peter Zijlstra]

On Fri, Feb 10, 2023 at 12:36:36AM +0900, Masami Hiramatsu (Google) 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>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

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

[Nadav Amit]

On 2/9/23 5:36 PM, Masami Hiramatsu (Google) 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>
> ---
>   arch/x86/kernel/kprobes/core.c |   16 ++++++++--------
>   1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 695873c0f50b..2440e736d0e4 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -607,19 +607,19 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
>   	case 0xe8:	/* near call relative */
>   		p->ainsn.emulate_op = kprobe_emulate_call;
>   		if (insn->immediate.nbytes == 2)
> -			p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
> +			p->ainsn.rel32 = insn->immediate.value;
>   		else
> -			p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
> +			p->ainsn.rel32 = insn->immediate.value;

Hmm.. I don't get it. What the purpose of keeping the duplicated code 
(after your change)?

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

[Masami Hiramatsu]

On Fri, 10 Feb 2023 02:01:54 +0200
Nadav Amit <nadav.amit@gmail.com> wrote:

> 
> On 2/9/23 5:36 PM, Masami Hiramatsu (Google) 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>
> > ---
> >   arch/x86/kernel/kprobes/core.c |   16 ++++++++--------
> >   1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index 695873c0f50b..2440e736d0e4 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -607,19 +607,19 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
> >   	case 0xe8:	/* near call relative */
> >   		p->ainsn.emulate_op = kprobe_emulate_call;
> >   		if (insn->immediate.nbytes == 2)
> > -			p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
> > +			p->ainsn.rel32 = insn->immediate.value;
> >   		else
> > -			p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
> > +			p->ainsn.rel32 = insn->immediate.value;
> 
> Hmm.. I don't get it. What the purpose of keeping the duplicated code 
> (after your change)?
> 

Oops, good catch! I think p->ainsn.rel32 can be set without any check in general.
Let me update it.

Thank you!


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