Re: [RFC bpf-next 5/6] bpf: support low-32 subreg scalar linking for sign-extending movs

From: Eduard Zingerman

Date: Wed Aug 19 2026 - 02:19:42 EST

On Fri, 2026-08-14 at 16:19 -0700, Vineet Gupta wrote:

...

> diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
> index ef71999c4695..6aaedde6e9d1 100644
> --- a/kernel/bpf/states.c
> +++ b/kernel/bpf/states.c

...

> @@ -570,9 +572,18 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
> * on a path that predates this series, which is a pruning change
> * that wants measuring on its own; it is deliberately left
> * alone here.
> + *
> + * Only demand a match when the old state carries a link at all.
> + * These flags are only ever set together with an ->id, so
> + * rold->id == 0 implies none is set, and the only case this
> + * admits is "old knows no low-32 relationship, cur does" -- cur
> + * is then strictly more constrained than old, which is the safe
> + * direction for pruning. The reverse is still rejected. Without
> + * this a register that first acquires a link inside a loop would
> + * never match its pre-loop state and pruning would not converge.
> */
> if (rold->id &&
> -     (rold->flags & BPF_FLAG_SUBREG_ZEXT) != (rcur->flags & BPF_FLAG_SUBREG_ZEXT))
> +     (rold->flags & BPF_FLAG_SUBREG) != (rcur->flags & BPF_FLAG_SUBREG))
> return false;

Same comment as for ZEXT patch.

>
> if (env->explore_alu_limits) {
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8a802d49d0a4..45cb67dc3999 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14976,6 +14976,8 @@ static int adjust_reg_min_max_vals(struct bpf_verifier_env *env,
> return 0;
> }
>
> +static void reconstruct_sext32(struct bpf_reg_state *reg, struct bpf_reg_state *src);

Nit: is it possible to avoid forward declaration?

> +
> /* check validity of 32-bit and 64-bit arithmetic operations */
> static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> {
> @@ -15052,15 +15054,65 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> insn->src_reg);
> return -EACCES;
> } else if (src_reg->type == SCALAR_VALUE) {
> + int sz = insn->off >> 3;
> bool no_sext;
> + bool subreg_link;
>
> no_sext = reg_umax(src_reg) < (1ULL << (insn->off - 1));
> - if (no_sext)
> + /*
> + * When no_sext, dst == src exactly, so link them
> + * (existing behavior). When !no_sext for a 32-bit sign
> + * extension the low 32 bits are still identical (sext
> + * preserves them), so form a BPF_FLAG_SUBREG_SEXT
> + * link: a later narrowing of the low 32 bits
> + * propagates here, and sync_linked_regs() rebuilds
> + * the high half via reconstruct_sext32().
> + *
> + * An ADD_CONST-linked src is excluded for the same
> + * reason as in the zero-extending arm below:
> + * assign_scalar_id_before_mov() would clear its
> + * base+delta link, and a combined subreg+delta link
> + * isn't modeled anyway. Unlike that arm a self-mov is
> + * NOT excluded -- r0 = (s32)r0 is the case this is
> + * here for.
> + */
> + subreg_link = (sz == 4) &&
> +       !(src_reg->flags & BPF_FLAG_ADD_CONST);
> +
> + if (no_sext || subreg_link)
> assign_scalar_id_before_mov(env, src_reg);
> *dst_reg = *src_reg;
> - if (!no_sext)
> - clear_scalar_id(dst_reg);
> - coerce_reg_to_size_sx(dst_reg, insn->off >> 3);
> + if (!no_sext) {
> + if (subreg_link && src_reg->id) {
> + /* ->id already copied above */
> + dst_reg->flags = (dst_reg->flags & ~BPF_FLAG_SUBREG) |
> + BPF_FLAG_SUBREG_SEXT;
> + } else {
> + clear_scalar_id(dst_reg);
> + }
> + }
> + /*
> + * coerce_reg_to_size_sx() falls back to the full sext
> + * range when smin/smax straddle the sign boundary (e.g.
> + * an errno-or-zero value clamped to [-4095, 0]). For a
> + * register tracked as the sign-extension of its low 32
> + * bits the high half IS that sign-extension, so rebuild
> + * the tighter 64-bit range from the low bounds, taken
> + * from a snapshot because coerce overwrites them.
> + *
> + * Gated on sz == 4, not on the flag alone: an (s8)/(s16)
> + * mov whose src is already SEXT-linked copies the flag
> + * across in the *dst_reg = *src_reg above, and a 32-bit
> + * reconstruction must not run for a narrower operation.
> + */
> + if (sz == 4 && (dst_reg->flags & BPF_FLAG_SUBREG_SEXT)) {
> + struct bpf_reg_state sext_src = *dst_reg;
> +
> + coerce_reg_to_size_sx(dst_reg, sz);
> + reconstruct_sext32(dst_reg, &sext_src);

It does not make sense to maintain two functions that do register sign
extension. If coerce_reg_to_size_sx() is not precise enough for the
32-bit case, then it should be adapted instead of special-cased.

> + } else {
> + coerce_reg_to_size_sx(dst_reg, sz);
> + }
> } else {
> mark_reg_unknown(env, regs, insn->dst_reg);
> }
> @@ -15107,7 +15159,15 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> if (!is_src_reg_u32) {
> if (wide_subreg_link && src_reg->id) {
> /* ->id already copied above */
> - dst_reg->flags |= BPF_FLAG_SUBREG_ZEXT;
> + /*
> + * Zero-extension: high bits are 0, not a
> + * sign-extension of the low field. Drop any
> + * SUBREG_SEXT copied from a sext-linked src
> + * so sync_linked_regs() rebuilds dst by
> + * zero-extension, not reconstruct_sext32().
> + */
> + dst_reg->flags = (dst_reg->flags & ~BPF_FLAG_SUBREG) |
> + BPF_FLAG_SUBREG_ZEXT;

Nit: please find a way to reduce indentation (e.g. less if-nesting, or a utility function).

> } else {
> clear_scalar_id(dst_reg);
> }
> @@ -15961,6 +16021,32 @@ static void collect_linked_regs(struct bpf_verifier_env *env,
> }
> }
>
> +/*
> + * Set @reg to the sign-extension of the low 32 bits currently held by @src.
> + * A BPF_FLAG_SUBREG_SEXT-linked register came from a 32-bit sign
> + * extension (r0 = (s32)r0): it shares @src's low 32 bits and its high bits are
> + * the sign-extension of that low field. Only the value fields are written;
> + * @reg's linkage fields (id, delta, flags) are left intact by
> + * the caller (___mark_reg_known touches only var_off/r64/r32). Callers must
> + * ensure no ADD_CONST delta is involved (see sync_linked_regs()).
> + */
> +static void reconstruct_sext32(struct bpf_reg_state *reg, struct bpf_reg_state *src)

Nit: let's rename src -> known_reg, to make reading sync_linked_regs() simpler.

> +{
> + s32 s32min = reg_s32_min(src);
> + s32 s32max = reg_s32_max(src);
> +
> + if (s32min == s32max) {
> + /* Low 32 bits are constant -> the whole value is the sext constant. */
> + ___mark_reg_known(reg, (u64)(s64)s32min);
> + } else {
> + /* Sign-extension is monotonic over the signed-32 range. */
> + reg_set_srange64(reg, (s64)s32min, (s64)s32max);
> + reg_set_srange32(reg, s32min, s32max);
> + reg->var_off = tnum_range((u64)(s64)s32min, (u64)(s64)s32max);

Note that known lower 32-bits of the known_reg->var_off are lost,
we might benefit from adding a dedicated tnum_sext().

> + reg_bounds_sync(reg);
> + }
> +}
> +
> /* For all R in linked_regs, copy known_reg range into R
> * if R->id == known_reg->id.
> */
> @@ -15984,17 +16070,21 @@ static void sync_linked_regs(struct bpf_verifier_env *env, struct bpf_verifier_s
> * A low-32 linked register shares only the base's low 32 bits;
> * the flag says how its high bits are derived. For
> * BPF_FLAG_SUBREG_ZEXT they are zero (32-bit zero-extending mov).
> + * For BPF_FLAG_SUBREG_SEXT they are the sign-extension of the low
> + * field (32-bit sign extension).
> * Rebuild it from known_reg's low 32 bits accordingly, but only
> * when neither side carries an ADD_CONST delta -- with a delta
> * the low bits differ from the base by that delta and the combined
> * subreg+ADD_CONST reconstruction isn't modeled here, so leave reg
> * unchanged (sound, just less precise).
> */
> - if (reg->flags & BPF_FLAG_SUBREG_ZEXT) {
> + if (reg->flags & BPF_FLAG_SUBREG) {
> if (!((reg->flags | known_reg->flags) & BPF_FLAG_ADD_CONST)) {
> - {
> + if (reg->flags & BPF_FLAG_SUBREG_SEXT) {
> + reconstruct_sext32(reg, known_reg);
> + } else {

Let's move this branch to a dedicated utility function as well.

> u32 saved_id = reg->id;
> - u8 saved_subreg = reg->flags & BPF_FLAG_SUBREG_ZEXT;
> + u8 saved_subreg = reg->flags & BPF_FLAG_SUBREG;
>
> /*
> * reg = zext32(known_reg): its low 32 bits come from

...