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 <[email protected]>
> Signed-off-by: Vineet Gupta <[email protected]>
> ---
> 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.
