Re: [PATCH bpf-next v10 2/9] bpf: Introduce global percpu data

[Leon Hwang <leon.hwang@linux.dev>]

On 17/7/26 14:25, Emil Tsalapatis wrote:
> On Wed Jul 15, 2026 at 11:32 AM EDT, Leon Hwang wrote:

[...]

>>  
>> +static int percpu_array_map_direct_value_addr(const struct bpf_map *map, u64 *imm, u32 off)
>> +{
>> +	struct bpf_array *array = container_of(map, struct bpf_array, map);
>> +
>> +	if (map->max_entries != 1)
>> +		return -EOPNOTSUPP;
>> +	if (off >= map->value_size)
>> +		return -EINVAL;
>> +	if (!bpf_jit_supports_percpu_insn())
>> +		return -EOPNOTSUPP;
> 
> Nit: This should be first, if the JIT doesn't support the instruction
> all else is moot. Same below.

Ack.

> 
>> +
>> +	*imm = (u64)(__force unsigned long) array->pptrs[0];
>> +	return 0;
>> +}
>> +
>> +static int percpu_array_map_direct_value_meta(const struct bpf_map *map, u64 imm, u32 *off)
>> +{
>> +	struct bpf_array *array = container_of(map, struct bpf_array, map);
>> +	u64 base = (u64)(__force unsigned long) array->pptrs[0];
>> +
>> +	if (map->max_entries != 1)
>> +		return -EOPNOTSUPP;
>> +	if (imm < base || imm >= base + array->elem_size)
>> +		return -ENOENT;
>> +	if (!bpf_jit_supports_percpu_insn())
>> +		return -EOPNOTSUPP;
>> +
>> +	*off = imm - base;
>> +	return 0;
>> +}
>> +

[...]

>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index e1244a721194..ae442ea217c4 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -5615,6 +5615,8 @@ int bpf_map_direct_read(struct bpf_map *map, int off, int size, u64 *val,
>>  	u64 addr;
>>  	int err;
>>  
>> +	if (map->map_type == BPF_MAP_TYPE_INSN_ARRAY || map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
>> +		return -EINVAL;
>>  	err = map->ops->map_direct_value_addr(map, &addr, off);
>>  	if (err)
>>  		return err;
>> @@ -6174,6 +6176,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
>>  			if (tnum_is_const(reg->var_off) &&
>>  			    bpf_map_is_rdonly(map) &&
>>  			    map->ops->map_direct_value_addr &&
>> +			    map->map_type != BPF_MAP_TYPE_PERCPU_ARRAY &&
>>  			    map->map_type != BPF_MAP_TYPE_INSN_ARRAY) {
> 
> An issue I have with the patch is that it takes the special-casing we
> already have for INSN_ARRAY and makes it worse. The instruction arrays
> implement direct_value_addr, but it's not actually valid to call
> direct_value_addr from a bunch of places where it's called, so we have
> to special case check the map type to prevent it. Case in point the
> special casing in check_arg_const_str in the thunk below.
> 
> Imo we should consider making a different map method different from
> direct_value_addr that INSN_ARRAY and PERCPU_ARRAY implement. That way
> we remove most of the map type checks in those paths. I'm not exactly
> sure how that would look like, but I could take a look at it tomorrow.


Seems reasonable to me. I don't have good candidates for the new map
method name.

Instead of making a new map method, how about factoring out a helper for
the 'if (t == BPF_READ && value_regno >= 0) {}' block? It will simplify
these map_type checks in the helper.

> 
>>  				int map_off = off + reg->var_off.value;
>>  				u64 val = 0;
>> @@ -8176,6 +8179,12 @@ static int check_arg_const_str(struct bpf_verifier_env *env,
>>  		return -EACCES;
>>  	}
>>  
>> +	if (map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY) {
>> +		verbose(env, "%s points to percpu_array map which cannot be used as const string\n",
>> +			reg_arg_name(env, argno));
>> +		return -EACCES;
>> +	}
>> +
>>  	if (!bpf_map_is_rdonly(map)) {
>>  		verbose(env, "%s does not point to a readonly map'\n", reg_arg_name(env, argno));
>>  		return -EACCES;
>> @@ -18279,6 +18288,12 @@ static int check_and_resolve_insns(struct bpf_verifier_env *env)
>>  					return -EINVAL;
>>  				}
>>  
>> +				if (map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY &&
>> +				    !env->prog->jit_requested) {
>> +					verbose(env, "JIT is required to use global percpu data\n");
>> +					return -EOPNOTSUPP;
>> +				}
>> +
> 
> Another place where we add map-specific checks where there are none.


I think this check is unnecessary, because 'prog->jit_required = true'
has been set for global percpu data.

Will drop this if.

Thanks,
Leon

> 
>>  				err = map->ops->map_direct_value_addr(map, &addr, off);
>>  				if (err) {
>>  					verbose(env, "invalid access to map value pointer, value_size=%u off=%u\n",
>