Re: [PATCH bpf-next v2 01/13] bpf: move linked-scalar flags out of bpf_reg_state->id [NFC]

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu Sep 10, 2026 at 9:46 AM PDT, Vineet Gupta wrote:
> bpf_reg_state->id doubles as a linked-register id and, in its top two
> bits, as a record of how the register relates to that set:
>
>   #define BPF_ADD_CONST64 (1U << 31)
>   #define BPF_ADD_CONST32 (1U << 30)
>
> Every user of ->id therefore has to mask, and more link kinds are coming.
> Move the two bits into a bitfield next to ->precise, which is the last
> field of the struct and outside every memcmp() window used for state
> comparison, so the layout and all byte-wise comparisons are unchanged. The
> two kinds are mutually exclusive, so a 2-bit enum captures them and makes
> ADD_CONST_32 vs ADD_CONST_64 explicit at each use.
>
> ->id becomes a plain 32-bit identifier: no masking anywhere, and
> check_scalar_ids() loses its two-level "check the compound id, then the
> base id" dance in favour of a single check_ids().
>
> While here, use regs_exact() for the explore_alu_limits case in regsafe():
> it is what that open-coded memcmp+check_scalar_ids pair amounts to, and it
> picks up the add_const comparison for free (parent_id is 0 for
> SCALAR_VALUE).
>
> check_stack_write_fixed_off() cleared ->id directly on a narrowing spill,
> which would now leave ->add_const set without an id; use
> clear_scalar_id().
>
> Moving the kind out of ->id also drops an incidental comparison in
> regs_exact(), which used to see it as part of the idmap key; the next
> patch restores it. Otherwise no functional change intended.
>
> Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
> ---
> v2: was RFC 2/6.
>  - kinds are a 2-bit enum bitfield, not a byte of flags; RFC 1/6, which
>    turned ->precise into that byte, is dropped (Eduard)
>  - use regs_exact() for the explore_alu_limits case
>  - clear_scalar_id() on the narrowing spill, which would otherwise leave
>    ->add_const set without an id
>
>  include/linux/bpf_verifier.h                  | 25 ++++++++-----
>  kernel/bpf/log.c                              |  4 +--
>  kernel/bpf/states.c                           | 35 +++++--------------
>  kernel/bpf/verifier.c                         | 35 +++++++++++--------
>  .../bpf/progs/verifier_linked_scalars.c       | 34 +++++++++---------
>  5 files changed, 65 insertions(+), 68 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 9727df5af83a..afb1e5628698 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -35,6 +35,17 @@ enum bpf_iter_state {
>  	BPF_ITER_STATE_DRAINED,
>  };
>  
> +/*
> + * Records that a register is (base + ->delta) within its ->id set:
> + * r1 += 10;   r1 gets ADD_CONST_64 delta
> + * w3 += 10;   r3 gets ADD_CONST_32 delta

w3 gets ?

> + */
> +enum bpf_add_const {
> +	ADD_CONST_NONE = 0,
> +	ADD_CONST_32,		/* delta was added with a 32-bit ALU op */
> +	ADD_CONST_64,		/* ... with a 64-bit ALU op */
> +};
> +
>  struct bpf_reg_state {
>  	/* Ordering of fields matters.  See states_equal() */
>  	enum bpf_reg_type type;
> @@ -136,16 +147,9 @@ struct bpf_reg_state {
>  	 * to a specific instance of bpf_iter.
>  	 */
>  	/*
> -	 * Upper bit of ID is used to remember relationship between "linked"
> -	 * registers. Example:
> +	 * Registers sharing an ->id are "linked":
>  	 * r1 = r2;    both will have r1->id == r2->id == N
> -	 * r1 += 10;   r1->id == N | BPF_ADD_CONST and r1->delta == 10
> -	 * r3 = r2;    both will have r3->id == r2->id == N
> -	 * w3 += 10;   r3->id == N | BPF_ADD_CONST32 and r3->delta == 10
>  	 */
> -#define BPF_ADD_CONST64 (1U << 31)
> -#define BPF_ADD_CONST32 (1U << 30)
> -#define BPF_ADD_CONST (BPF_ADD_CONST64 | BPF_ADD_CONST32)
>  	u32 id;
>  	/*
>  	 * Tracks the parent object this register was derived from.
> @@ -164,6 +168,11 @@ struct bpf_reg_state {
>  	u32 frameno;
>  	/* if (!precise && SCALAR_VALUE) min/max/tnum don't affect safety */
>  	bool precise;
> +	/*
> +	 * How this register relates to the others sharing its ->id.
> +	 * Non-zero only if ->id is.
> +	 */
> +	enum bpf_add_const add_const:2;
>  };
>  
>  static inline s64 reg_smin(const struct bpf_reg_state *reg)
> diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
> index fb032dfdc0de..f8d7a5c8052f 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->add_const)
>  		verbose(env, "%+d", reg->delta);
>  	if (reg->parent_id)
>  		verbose_a("parent_id=%d", reg->parent_id);
> diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
> index 66fb11b6c6a7..d974baad37ee 100644
> --- a/kernel/bpf/states.c
> +++ b/kernel/bpf/states.c
> @@ -369,13 +369,6 @@ static bool check_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
>   * and r7.id=0 (both independent), without temp IDs both would map old_id=X
>   * to cur_id=0 and pass. With temp IDs: r6 maps X->temp1, r7 tries to map
>   * X->temp2, but X is already mapped to temp1, so the check fails correctly.
> - *
> - * When old_id has BPF_ADD_CONST set, the compound id (base | flag) and the
> - * base id (flag stripped) must both map consistently. Example: old has
> - * r2.id=A, r3.id=A|flag (r3 = r2 + delta), cur has r2.id=B, r3.id=C|flag
> - * (r3 derived from unrelated r4). Without the base check, idmap gets two
> - * independent entries A->B and A|flag->C|flag, missing that A->C conflicts
> - * with A->B. The base ID cross-check catches this.
>   */
>  static bool check_scalar_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
>  {
> @@ -384,15 +377,7 @@ static bool check_scalar_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
>  
>  	cur_id = cur_id ? cur_id : ++idmap->tmp_id_gen;
>  
> -	if (!check_ids(old_id, cur_id, idmap))
> -		return false;
> -	if (old_id & BPF_ADD_CONST) {
> -		old_id &= ~BPF_ADD_CONST;
> -		cur_id &= ~BPF_ADD_CONST;
> -		if (!check_ids(old_id, cur_id, idmap))
> -			return false;
> -	}
> -	return true;
> +	return check_ids(old_id, cur_id, idmap);
>  }
>  
>  static void __clean_func_state(struct bpf_verifier_env *env,
> @@ -542,8 +527,7 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
>  			/* explore_alu_limits disables tnum_in() and range_within()
>  			 * logic and requires everything to be strict
>  			 */
> -			return memcmp(rold, rcur, offsetof(struct bpf_reg_state, id)) == 0 &&
> -			       check_scalar_ids(rold->id, rcur->id, idmap);
> +			return regs_exact(rold, rcur, idmap);

Why drop memcmp() ? Doesn't look correct.
Also even after above change to check_scalar_ids() the check_scalar_ids() is still
no equivalent to check_ids() that regs_exact() is doing.

This patch should have been refactoring, if so, this change looks
unrelated and dubious.

The rest looks fine.