netdev - Re: [PATCH bpf-next 07/19] bpf: verifier: remove dead code

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Message-ID: <20181220004516.ltbcer6xkhiuhbvo@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 19 Dec 2018 16:45:17 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     daniel@...earbox.net, oss-drivers@...ronome.com,
        netdev@...r.kernel.org, yhs@...com, kafai@...com
Subject: Re: [PATCH bpf-next 07/19] bpf: verifier: remove dead code

On Wed, Dec 19, 2018 at 10:29:18AM -0800, Jakub Kicinski wrote:
> Instead of overwriting dead code with jmp -1 instructions
> remove it completely for root.  Adjust verifier state and
> line info appropriately.
> 
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> ---
>  include/linux/filter.h |   1 +
>  kernel/bpf/core.c      |  12 ++++
>  kernel/bpf/verifier.c  | 139 ++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 149 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 537e9e7c6e6f..c99969022493 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -782,6 +782,7 @@ static inline bool bpf_dump_raw_ok(void)
>  
>  struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
>  				       const struct bpf_insn *patch, u32 len);
> +int bpf_remove_insns(struct bpf_prog *prog, u32 off, u32 cnt);
>  
>  void bpf_clear_redirect_map(struct bpf_map *map);
>  
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 2eb7cc9822bb..e3498d82eb74 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -461,6 +461,18 @@ struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
>  	return prog_adj;
>  }
>  
> +int bpf_remove_insns(struct bpf_prog *prog, u32 off, u32 cnt)
> +{
> +	/* Branch offsets can't overflow when program is shrinking, no need
> +	 * to call bpf_adj_branches(..., true) here
> +	 */
> +	memmove(prog->insnsi + off, prog->insnsi + off + cnt,
> +		sizeof(struct bpf_insn) * (prog->len - off - cnt));
> +	prog->len -= cnt;
> +
> +	return WARN_ON_ONCE(bpf_adj_branches(prog, off, off + cnt, off, false));
> +}
> +
>  void bpf_prog_kallsyms_del_subprogs(struct bpf_prog *fp)
>  {
>  	int i;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7db2a48ea7f7..98a276f4c4e6 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6226,6 +6226,113 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of
>  	return new_prog;
>  }
>  
> +static int adjust_subprog_starts_after_remove(struct bpf_verifier_env *env,
> +					      u32 off, u32 cnt)
> +{
> +	int i, j;
> +
> +	/* find first prog starting at or after off (first to remove) */
> +	for (i = 0; i < env->subprog_cnt; i++)
> +		if (env->subprog_info[i].start >= off)
> +			break;
> +	/* find first prog starting at or after off + cnt (first to stay) */
> +	for (j = i; j < env->subprog_cnt; j++)
> +		if (env->subprog_info[j].start >= off + cnt)
> +			break;
> +	/* if j doesn't start exactly at off + cnt, we are just removing
> +	 * the front of previous prog
> +	 */

I think this will break func_info, since it's not adjusted here.
Also iirc line_info is relying on first insn to always have line_info.
If first insn is dead, second insn might not have a line_info generated
and things won't go well.

Martin, Yonghong, please chime in.