Re: [PATCH bpf-next v3 03/12] bpf: Prepare linked scalars for relations narrower than 64 bits

[Vineet Gupta <vineet.gupta@linux.dev>]

On 10/29/26 3:51 PM, Eduard Zingerman wrote:
> On Tue, 2026-10-20 at 09:18 -0700, Vineet Gupta wrote:
>> Two scalars sharing an ->id are always related by full 64-bit equality
>> today: reg == base + delta over all 64 bits (this is true for
>> BPF_ADD_CONST32 as well, which implies upper bits to be all zero).
>> Sign-extension needs a genuinely low32 equal only relationship kind
>> with bit 31 supplying upper 32-bits.
>>
>> Today the following 2 relationships exist
>>
>>    #define BPF_ADD_CONST64 (1U << 31)
>>    #define BPF_ADD_CONST32 (1U << 30)
>>
>> Model them differently as
>>
>>    enum bpf_reg_link_kind { BPF_REG_LINK_NONE, BPF_REG_LINK_FULL,
>>                             BPF_REG_LINK_ZEXT, BPF_REG_LINK_SEXT };
>>
>> So BPF_ADD_CONST64 becomes BPF_REG_LINK_FULL and BPF_ADD_CONST32 becomes
>> BPF_REG_LINK_ZEXT with the exact same semantics as today. But this paves
>> the way for a future BPF_REG_LINK_SEXT in the same framework.
>>
>> The implementation needs to change as well. Today the above 2 bits are
>> shoe-horned into ->id, needing masking and OR'ing to get ->id and kind
>> bits: with more types and bits needed, move them out into an enum
>> bitfield of its own. To avoid growing the storage (and stay within the
>> 1K allocator bucket), borrow 2 bits out of ->map_uid, 31 -> 29, which is
>> still far more ids than a program can generate.
>>
>> ->id becomes a plain identifier, so check_scalar_ids() loses its "compare
>> the compound id, then the base id" dance in favour of one check_ids().
>>
>> With the kind out of ->id it also leaves the idmap key, so regs_exact()
>> and regsafe()'s explore_alu_limits arm have to compare it explicitly.
>>
>> One case changes behaviour -- old {r2.id=A+delta32} against
>> cur {r2.id=B+delta64} was accepted and is now rejected, correctly, since
>> alu32 and alu64 deltas diverge in sync_linked_regs().
>>
>> Clearing a link is no longer just ->id = 0: any stale ->link_kind needs
>> to be cleared out as well. And given this needs to be done in the newly
>> added kernel/bpf/scev.c, it is moved into bpf_verifier.h as
>> bpf_clear_scalar_id().
>>
>> The helper also zeroes ->delta, which the open-coded clear left behind.
>> Unlike ->id that field is inside the memcmp() prefix state comparison
>> uses, so a leftover value could keep two otherwise-identical states
>> apart; clearing it can only help pruning.
>>
>> Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
>> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
>> ---
> Nice refactoring.
>
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>

Thx.

> ...
>
>> diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
>> index 5432c0fc442b..a6f205653a9e 100644
>> --- a/kernel/bpf/log.c
>> +++ b/kernel/bpf/log.c
>> @@ -651,8 +651,8 @@ static void print_reg_state(struct bpf_verifier_env *env,
>>   		verbose(env, "%s", btf_type_name(reg->btf, reg->btf_id));
>>   	verbose(env, "(");
>>   	if (reg->id)
>> -		verbose_a("id=%d", reg->id & ~BPF_ADD_CONST);
>> -	if (reg->id & BPF_ADD_CONST)
>> +		verbose_a("id=%d", reg->id);
>> +	if (reg->link_kind)
>>   		verbose(env, "%+d", reg->delta);
> Nit: it would be nice to see if the link is full or zext.

Done in v4.

>>   	if (reg->parent_id)
>>   		verbose_a("parent_id=%d", reg->parent_id);
>> diff --git a/kernel/bpf/scev.c b/kernel/bpf/scev.c
>> index 48500d73105a..f0aa314233c8 100644
>> --- a/kernel/bpf/scev.c
>> +++ b/kernel/bpf/scev.c
>> @@ -2285,7 +2285,7 @@ static void scratch_widened_reg_id(struct bpf_verifier_env *env, struct bpf_reg_
>>   {
>>   	switch (base_type(reg->type)) {
>>   	case SCALAR_VALUE:
>> -		reg->id = 0;
>> +		bpf_clear_scalar_id(reg);
>>   		break;
>>   	case PTR_TO_PACKET:
>>   	case PTR_TO_PACKET_META:
>> diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
>> index 953700822fa7..a1b90a31747a 100644
>> --- a/kernel/bpf/states.c
>> +++ b/kernel/bpf/states.c
> ...
>
>> @@ -574,13 +561,14 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
>>   		 * Linked register tracking uses rold->id to detect relationships.
>>   		 * When rold->id == 0, the register is independent and any linking
>>   		 * in rcur only adds constraints. When rold->id != 0, we must verify
>> -		 * id mapping and (for BPF_ADD_CONST) offset consistency.
>> +		 * id mapping and that the link kinds agree, along with any delta
>> +		 * they carry.
>>   		 *
>>   		 * +------------------+-----------+------------------+---------------+
>> -		 * |                  | rold->id  | rold + ADD_CONST | rold->id == 0 |
>> +		 * |                  | rold->id  | rold linked kind | rold->id == 0 |
>>   		 * |------------------+-----------+------------------+---------------|
>>   		 * | rcur->id         | range,ids | false            | range         |
>> -		 * | rcur + ADD_CONST | false     | range,ids,off    | range         |
>> +		 * | rcur linked kind | false     | range,ids,delta  | range         |
>>   		 * | rcur->id == 0    | range,ids | false            | range         |
>>   		 * +------------------+-----------+------------------+---------------+
> Nit: "linked kind" -> "link_kind".
>       also, the table should show when link_kind equality is required.
> ...

Done as well. I'll carry the Ack.

Thx,
-Vineet