Re: [RFC PATCH 1/7] static_call/x86: Add __static_call_returnl0()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 10, 2020 at 01:56:03AM +0100, Frederic Weisbecker wrote:

> [fweisbec: s/disp16/data16, integrate into text_get_insn(), elaborate
>  comment on the resulting insn, emulate on int3 trap, provide validation,
>  uninline __static_call_return0() for HAVE_STATIC_CALL]

> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index b7421780e4e9..1250f440d1be 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -65,6 +65,9 @@ extern void text_poke_finish(void);
>  #define JMP8_INSN_SIZE		2
>  #define JMP8_INSN_OPCODE	0xEB
>  
> +#define XOR5RAX_INSN_SIZE	5
> +#define XOR5RAX_INSN_OPCODE	0x31
> +
>  #define DISP32_SIZE		4
>  
>  static __always_inline int text_opcode_size(u8 opcode)
> @@ -80,6 +83,7 @@ static __always_inline int text_opcode_size(u8 opcode)
>  	__CASE(CALL);
>  	__CASE(JMP32);
>  	__CASE(JMP8);
> +	__CASE(XOR5RAX);
>  	}
>  
>  #undef __CASE
> @@ -99,8 +103,21 @@ static __always_inline
>  void *text_gen_insn(u8 opcode, const void *addr, const void *dest)
>  {
>  	static union text_poke_insn insn; /* per instance */
> -	int size = text_opcode_size(opcode);
> +	int size;
>  
> +	if (opcode == XOR5RAX_INSN_OPCODE) {
> +		/*
> +		 * data16 data16 xorq %rax, %rax - a single 5 byte instruction that clears %rax
> +		 * The REX.W cancels the effect of any data16.
> +		 */
> +		static union text_poke_insn xor5rax = {
> +			.text = { 0x66, 0x66, 0x48, 0x31, 0xc0 },
> +		};
> +
> +		return &xor5rax.text;
> +	}
> +
> +	size = text_opcode_size(opcode);
>  	insn.opcode = opcode;
>  
>  	if (size > 1) {
> @@ -165,6 +182,13 @@ void int3_emulate_ret(struct pt_regs *regs)
>  	unsigned long ip = int3_emulate_pop(regs);
>  	int3_emulate_jmp(regs, ip);
>  }
> +
> +static __always_inline
> +void int3_emulate_xor5rax(struct pt_regs *regs)
> +{
> +	regs->ax = 0;
> +}
> +
>  #endif /* !CONFIG_UML_X86 */
>  
>  #endif /* _ASM_X86_TEXT_PATCHING_H */
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 2400ad62f330..37592f576a10 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1125,6 +1125,10 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
>  		int3_emulate_jmp(regs, (long)ip + tp->rel32);
>  		break;
>  
> +	case XOR5RAX_INSN_OPCODE:
> +		int3_emulate_xor5rax(regs);
> +		break;
> +
>  	default:
>  		BUG();
>  	}
> @@ -1291,6 +1295,7 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
>  	switch (tp->opcode) {
>  	case INT3_INSN_OPCODE:
>  	case RET_INSN_OPCODE:
> +	case XOR5RAX_INSN_OPCODE:
>  		break;
>  
>  	case CALL_INSN_OPCODE:

Why did you add full emulation of this? The patch I send to you used the
text_poke_bp(.emulate) argument to have it emulate an actual call to the
out-of-line version of that function.

That should work fine and is a lot less code.