Re: [External] Storing sk_buffs as kptrs in map

[Martin KaFai Lau <martin.lau@linux.dev>]

On 12/6/24 8:24 AM, Maciej Fijalkowski wrote:
>> I think we can remove the projection_of call from the
>> bpf_is_prog_ctx_type() such that it honors the exact argument
>> type written in the kernel source code. Add this particular projection_of
>> check (renamed to bpf_is_kern_ctx in the diff) to the other callers for
>> backward compat such that the caller can selectively translate
>> the argument of a subprog to the corresponding prog ctx type.
>>
>> Lightly tested only:
> I tried the kernel diff on my side and it addressed my needs. Will you
> send a patch?

There is no real kfunc taking the "struct sk_buff *" now. It is better to make 
this change together with your skb acquire/release kfunc introduction. You can 
include this patch in your future set.

> 
>> diff --git i/kernel/bpf/btf.c w/kernel/bpf/btf.c
>> index e7a59e6462a9..2d39f91617fb 100644
>> --- i/kernel/bpf/btf.c
>> +++ w/kernel/bpf/btf.c
>> @@ -5914,6 +5914,26 @@ bool btf_is_projection_of(const char *pname, const char *tname)
>>   	return false;
>>   }
>> +static bool btf_is_kern_ctx(const struct btf *btf,
>> +			    const struct btf_type *t,
>> +			    enum bpf_prog_type prog_type)
>> +{
>> +	const struct btf_type *ctx_type;
>> +	const char *tname, *ctx_tname;
>> +
>> +	t = btf_type_skip_modifiers(btf, t->type, NULL);
>> +	if (!btf_type_is_struct(t))
>> +		return false;
>> +
>> +	tname = btf_name_by_offset(btf, t->name_off);
>> +	if (!tname)
>> +		return false;
>> +
>> +	ctx_type = find_canonical_prog_ctx_type(prog_type);
>> +	ctx_tname = btf_name_by_offset(btf_vmlinux, ctx_type->name_off);
>> +	return btf_is_projection_of(ctx_tname, tname);
> We're sort of doubling the work that btf_is_prog_ctx_type() is doing also,
> maybe add a flag to btf_is_prog_ctx_type() that will allow us to skip
> btf_is_projection_of() call when needed? e.g. in get_kfunc_ptr_arg_type().

It is pretty cheap to do the btf_is_kern_ctx().

I don't have a strong opinion on either way. may be a "bool check_kern_ctx".