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

[Eduard Zingerman <eddyz87@gmail.com>]

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>

...

> 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.

>  	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.

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