Re: [RFC PATCH bpf-next v2 1/4] bpf, x64: Emit nops for X86_PATCH

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Oct 11, 2023 at 11:27:22PM +0800, Leon Hwang wrote:
> For next commit to reuse emit_nops(), move emit_nops() before
> emit_prologue().
> 
> By the way, change memcpy(prog, x86_nops[5], X86_PATCH_SIZE) to
> emit_nops(&prog, X86_PATCH_SIZE).

I find the subject of the commit a bit bogus. Could you change it to
something like:

use emit_nops() to produce nop5 instead memcpy'ing x86_nops[5]

I also feel that you should be consistent and address other spots that are
the same as the one that you are touching in emit_prologue() - there are
two more from what i see.

> 
> Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 41 ++++++++++++++++++-------------------
>  1 file changed, 20 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 8c10d9abc2394..c2a0465d37da4 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -304,6 +304,25 @@ static void pop_callee_regs(u8 **pprog, bool *callee_regs_used)
>  	*pprog = prog;
>  }
>  
> +static void emit_nops(u8 **pprog, int len)
> +{
> +	u8 *prog = *pprog;
> +	int i, noplen;
> +
> +	while (len > 0) {
> +		noplen = len;
> +
> +		if (noplen > ASM_NOP_MAX)
> +			noplen = ASM_NOP_MAX;
> +
> +		for (i = 0; i < noplen; i++)
> +			EMIT1(x86_nops[noplen][i]);
> +		len -= noplen;
> +	}
> +
> +	*pprog = prog;
> +}
> +
>  /*
>   * Emit x86-64 prologue code for BPF program.
>   * bpf_tail_call helper will skip the first X86_TAIL_CALL_OFFSET bytes
> @@ -319,8 +338,7 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
>  	 * but let's waste 5 bytes for now and optimize later
>  	 */
>  	EMIT_ENDBR();
> -	memcpy(prog, x86_nops[5], X86_PATCH_SIZE);
> -	prog += X86_PATCH_SIZE;
> +	emit_nops(&prog, X86_PATCH_SIZE);
>  	if (!ebpf_from_cbpf) {
>  		if (tail_call_reachable && !is_subprog)
>  			/* When it's the entry of the whole tailcall context,
> @@ -989,25 +1007,6 @@ static void detect_reg_usage(struct bpf_insn *insn, int insn_cnt,
>  	}
>  }
>  
> -static void emit_nops(u8 **pprog, int len)
> -{
> -	u8 *prog = *pprog;
> -	int i, noplen;
> -
> -	while (len > 0) {
> -		noplen = len;
> -
> -		if (noplen > ASM_NOP_MAX)
> -			noplen = ASM_NOP_MAX;
> -
> -		for (i = 0; i < noplen; i++)
> -			EMIT1(x86_nops[noplen][i]);
> -		len -= noplen;
> -	}
> -
> -	*pprog = prog;
> -}
> -
>  /* emit the 3-byte VEX prefix
>   *
>   * r: same as rex.r, extra bit for ModRM reg field
> -- 
> 2.41.0
>