Re: [PATCH bpf-next v7 4/5] bpf: verifier: Support eliding map lookup nullness

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Tue, 2025-01-14 at 13:28 -0700, Daniel Xu wrote:
> This commit allows progs to elide a null check on statically known
> map
> lookup keys. In other words, if the verifier can statically prove
> that
> the lookup will be in-bounds, allow the prog to drop the null check.
> 
> This is useful for two reasons:
> 
> 1. Large numbers of nullness checks (especially when they cannot
> fail)
>    unnecessarily pushes prog towards BPF_COMPLEXITY_LIMIT_JMP_SEQ.
> 2. It forms a tighter contract between programmer and verifier.
> 
> For (1), bpftrace is starting to make heavier use of percpu scratch
> maps. As a result, for user scripts with large number of unrolled
> loops,
> we are starting to hit jump complexity verification errors.  These
> percpu lookups cannot fail anyways, as we only use static key values.
> Eliding nullness probably results in less work for verifier as well.
> 
> For (2), percpu scratch maps are often used as a larger stack, as the
> currrent stack is limited to 512 bytes. In these situations, it is
> desirable for the programmer to express: "this lookup should never
> fail,
> and if it does, it means I messed up the code". By omitting the null
> check, the programmer can "ask" the verifier to double check the
> logic.
> 
> Tests also have to be updated in sync with these changes, as the
> verifier is more efficient with this change. Notable, iters.c tests
> had
> to be changed to use a map type that still requires null checks, as
> it's
> exercising verifier tracking logic w.r.t iterators.
> 
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>  kernel/bpf/verifier.c                         | 92
> ++++++++++++++++++-
>  tools/testing/selftests/bpf/progs/iters.c     | 14 +--
>  .../selftests/bpf/progs/map_kptr_fail.c       |  2 +-
>  .../selftests/bpf/progs/verifier_map_in_map.c |  2 +-
>  .../testing/selftests/bpf/verifier/map_kptr.c |  2 +-
>  5 files changed, 99 insertions(+), 13 deletions(-)

[...]

> @@ -9158,6 +9216,7 @@ static int check_func_arg(struct
> bpf_verifier_env *env, u32 arg,
>  	enum bpf_arg_type arg_type = fn->arg_type[arg];
>  	enum bpf_reg_type type = reg->type;
>  	u32 *arg_btf_id = NULL;
> +	u32 key_size;
>  	int err = 0;
>  
>  	if (arg_type == ARG_DONTCARE)
> @@ -9291,8 +9350,13 @@ static int check_func_arg(struct
> bpf_verifier_env *env, u32 arg,
>  			verbose(env, "invalid map_ptr to access map-
> >key\n");
>  			return -EACCES;
>  		}
> -		err = check_helper_mem_access(env, regno, meta-
> >map_ptr->key_size,
> -					      BPF_READ, false,
> NULL);
> +		key_size = meta->map_ptr->key_size;
> +		err = check_helper_mem_access(env, regno, key_size,
> BPF_READ, false, NULL);
> +		if (err)
> +			return err;
> +		meta->const_map_key = get_constant_map_key(env, reg,
> key_size);
> +		if (meta->const_map_key < 0 && meta->const_map_key
> != -EOPNOTSUPP)
> +			return meta->const_map_key;

Mark Hartmayer reported a problem that after this commit the verifier
started refusing to load libvirt's virCgroupV2DevicesLoadProg(), which
contains the following snippet:

53: (b7) r1 = -1                      ; R1_w=-1
54: (7b) *(u64 *)(r10 -8) = r1        ; R1_w=-1 R10=fp0 fp-8_w=-1
55: (bf) r2 = r10                     ; R2_w=fp0 R10=fp0
56: (07) r2 += -8                     ; R2_w=fp-8
57: (18) r1 = 0x9553c800              ; R1_w=map_ptr(ks=8,vs=4)
59: (85) call bpf_map_lookup_elem#1

IIUC here the actual constant value is -1, which this code confuses
with an error.