Re: [PATCH] x86/kprobes: Remove dead code

[Colin Ian King <colin.king@canonical.com>]

On 24/03/2021 17:36, Muhammad Usama Anjum wrote:
> The condition in switch statement `opcode & 0xf0` cannot evaluate to
> 0xff. So this case statement will never execute. Remove it.
> 
> Fixes: 6256e668b7 ("x86/kprobes: Use int3 instead of debug trap for single-step")
> Signed-off-by: Muhammad Usama Anjum <musamaanjum@gmail.com>
> ---
>  arch/x86/kernel/kprobes/core.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 89d9f26785c7..3b7bcc077020 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -177,9 +177,6 @@ int can_boost(struct insn *insn, void *addr)
>  	case 0xf0:
>  		/* clear and set flags are boostable */
>  		return (opcode == 0xf5 || (0xf7 < opcode && opcode < 0xfe));
> -	case 0xff:
> -		/* indirect jmp is boostable */
> -		return X86_MODRM_REG(insn->modrm.bytes[0]) == 4;
>  	default:
>  		/* CS override prefix and call are not boostable */
>  		return (opcode != 0x2e && opcode != 0x9a);
> 

The 0xff case was added with some form of intention to be executed so I
suspect removing it is not an appropriate fix.