Re: [PATCH 2/3] net: bpf jit: x86: optimize choose_load_func error path

[David Miller <davem@davemloft.net>]

From: Vladimir Murzin <murzin.v@gmail.com>
Date: Tue,  8 Oct 2013 20:31:49 +0400

> Macro CHOOSE_LOAD_FUNC returns handler for "any offset" if checks for K
> were not passed. At the same time handlers for "any offset" cases make
> the same checks against r_addr at run-time, that will always lead to
> bpf_error.
> 
> Run-time checks are still necessary for indirect load operations, but
> error path for absolute and mesh loads are worth to optimize during bpf
> compile time.
> 
> Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
> 
> Cc: Jan Seiffert <kaffeemonster@googlemail.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: "David S. Miller" <davem@davemloft.net
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> 
> ---
>  arch/x86/net/bpf_jit_comp.c |   15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 79c216a..28ac17f 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -123,7 +123,7 @@ static inline void bpf_flush_icache(void *start, void *end)
>  }
>  
>  #define CHOOSE_LOAD_FUNC(K, func) \
> -	((int)K < 0 ? ((int)K >= SKF_LL_OFF ? func##_negative_offset : func) : func##_positive_offset)
> +	((int)K < 0 ? ((int)K >= SKF_LL_OFF ? func##_negative_offset : NULL) : func##_positive_offset)
>  
>  /* Helper to find the offset of pkt_type in sk_buff
>   * We want to make sure its still a 3bit field starting at a byte boundary.
> @@ -611,7 +611,13 @@ void bpf_jit_compile(struct sk_filter *fp)
>  			}
>  			case BPF_S_LD_W_ABS:
>  				func = CHOOSE_LOAD_FUNC(K, sk_load_word);
> -common_load:			seen |= SEEN_DATAREF;
> +common_load:
> +				if (!func) {
> +					CLEAR_A();
> +					EMIT_JMP(cleanup_addr - addrs[i]);
> +					break;
> +				}
> +				seen |= SEEN_DATAREF;
>  				t_offset = func - (image + addrs[i]);
>  				EMIT1_off32(0xbe, K); /* mov imm32,%esi */
>  				EMIT1_off32(0xe8, t_offset); /* call */
> @@ -625,10 +631,7 @@ common_load:			seen |= SEEN_DATAREF;
>  			case BPF_S_LDX_B_MSH:
>  				func = CHOOSE_LOAD_FUNC(K, sk_load_byte_msh);
>  				seen |= SEEN_DATAREF | SEEN_XREG;
> -				t_offset = func - (image + addrs[i]);
> -				EMIT1_off32(0xbe, K);	/* mov imm32,%esi */
> -				EMIT1_off32(0xe8, t_offset); /* call sk_load_byte_msh */
> -				break;
> +				goto common_load;

This second hunk will set SEEN_DATAREF even if common_load takes the
!func path, that is not the intention at all here.

There's a reason why these two code blocks aren't shared.