Re: [PATCH bpf-next v2 01/13] bpf: move linked-scalar flags out of bpf_reg_state->id [NFC]
From: Vineet Gupta
Date: Mon Sep 14 2026 - 21:17:26 EST
On 9/12/26 11:50 AM, Alexei Starovoitov wrote:
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 twow3 gets ?
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@xxxxxxxxx>
Signed-off-by: Vineet Gupta <vineet.gupta@xxxxxxxxx>
---
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
OK.
+ */Why drop memcmp() ? Doesn't look correct.
+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);
regs_exact has the exact same memcmp so I don't think we are dropping it, but .....
| static bool regs_exact(const struct bpf_reg_state *rold,
| const struct bpf_reg_state *rcur,
| struct bpf_idmap *idmap)
| {
| return memcmp(rold, rcur, offsetof(struct bpf_reg_state, id)) == 0 &&
| check_ids(rold->id, rcur->id, idmap) &&
| check_ids(rold->parent_id, rcur->parent_id, idmap);
| }
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.
... Right: reverting back to what we had before.
FWIW Eduard had suggested to use regs_exact; I'm not sure if he had something else in mind which we might be overlooking.
This patch should have been refactoring, if so, this change looks
unrelated and dubious.
Back when I started, keeping it NFC made more sense. But with all the nuances discovered in the process, I'm inclined to drop the NFC stance and make functional changes - there's at least one which is the following accepted pre-series and now rejected.
old {r2.id=A+delta32} vs cur {r2.id=B+delta64}
The rest looks fine.
Thanks for taking a look.
-Vineet