Subject: Re: [PATCH net] bpf: x86: fix epilogue generation for eBPF programs

From: Daniel Borkmann <[email-masked]>
Date: Thu, 27 Nov 2014 10:52:49 +0100

On 11/27/2014 06:02 AM, Alexei Starovoitov wrote:
> classic BPF has a restriction that last insn is always BPF_RET.
> eBPF doesn't have BPF_RET instruction and this restriction.
> It has BPF_EXIT insn which can appear anywhere in the program
> one or more times and it doesn't have to be last insn.
> Fix eBPF JIT to emit epilogue when first BPF_EXIT is seen
> and all other BPF_EXIT instructions will be emitted as jump.
>
> Signed-off-by: Alexei Starovoitov <[email-masked]>
> ---
> Note, this bug is applicable only to native eBPF programs
> which first were introduced in 3.18, so no need to send it
> to stable and therefore no 'Fixes' tag.

Btw, even if it's not sent to -stable, a 'Fixes:' tag is useful
information for backporting and regression tracking, preferably
always mentioned where it can clearly be identified.

> arm64 JIT has the same problem, but the fix is not as trivial,
> so will be done as separate patch.
>
> Since 3.18 can only load eBPF programs and cannot execute them,
> this patch can even be done in net-next only, but I think it's worth
> to apply it to 3.18(net), so that JITed output for native eBPF
> programs is correct when bpf syscall loads it with net.core.bpf_jit_enable=2

Yes, sounds good to me, the condition insn_cnt - 1 is still held
with BPF to eBPF transformations.

>   arch/x86/net/bpf_jit_comp.c |    6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 3f62734..7e90244 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -178,7 +178,7 @@ static void jit_fill_hole(void *area, unsigned int size)
>   }
>
>   struct jit_context {
> -	unsigned int cleanup_addr; /* epilogue code offset */
> +	int cleanup_addr; /* epilogue code offset */

Why this type change here? This seems a bit out of context (I would
have expected a mention of this in the commit message, otherwise).

>   	bool seen_ld_abs;
>   };
>
> @@ -192,6 +192,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
>   	struct bpf_insn *insn = bpf_prog->insnsi;
>   	int insn_cnt = bpf_prog->len;
>   	bool seen_ld_abs = ctx->seen_ld_abs | (oldproglen == 0);
> +	bool seen_exit = false;
>   	u8 temp[BPF_MAX_INSN_SIZE + BPF_INSN_SAFETY];
>   	int i;
>   	int proglen = 0;
> @@ -854,10 +855,11 @@ common_load:
>   			goto common_load;
>
>   		case BPF_JMP | BPF_EXIT:
> -			if (i != insn_cnt - 1) {
> +			if (seen_exit) {
>   				jmp_offset = ctx->cleanup_addr - addrs[i];
>   				goto emit_jmp;
>   			}
> +			seen_exit = true;
>   			/* update cleanup_addr */
>   			ctx->cleanup_addr = proglen;
>   			/* mov rbx, qword ptr [rbp-X] */
>