Merge branch 'bpf-improve-linked-register-tracking'

Puranjay Mohan says:

====================
bpf: Improve linked register tracking

V3: https://lore.kernel.org/all/20260203222643.994713-1-puranjay@kernel.org/
Changes in v3->v4:
- Add a call to reg_bounds_sync() in sync_linked_regs() to sync bounds after alu op.
- Add a __sink(path[0]); in the C selftest so compiler doesn't say "error: variable 'path' set but
not used"

V2: https://lore.kernel.org/all/20260113152529.3217648-1-puranjay@kernel.org/
Changes in v2->v3:
- Added another selftest showing a real usage pattern
- Rebased on bpf-next/master

v1: https://lore.kernel.org/bpf/20260107203941.1063754-1-puranjay@kernel.org/
Changes in v1->v2:
- Add support for alu32 operations in linked register tracking (Alexei)
- Squash the selftest fix with the first patch (Eduard)
- Add more selftests to detect edge cases

This series extends the BPF verifier's linked register tracking to handle
negative offsets, BPF_SUB operations, and alu32 operations, enabling better bounds propagation for
common arithmetic patterns.

The verifier previously only tracked positive constant deltas between linked
registers using BPF_ADD. This meant patterns using negative offsets or
subtraction couldn't benefit from bounds propagation:

void alu32_negative_offset(void)
{
volatile char path[5];
volatile int offset = bpf_get_prandom_u32();
int off = offset;

if (off >= 5 && off < 10)
path[off - 5] = '.';
}

this gets compiled to:

0000000000000478 <alu32_negative_offset>:
143: call 0x7
144: *(u32 *)(r10 - 0xc) = w0
145: w1 = *(u32 *)(r10 - 0xc)
146: w2 = w1 // w2 and w1 share the same id
147: w2 += -0x5 // verifier knows w1 = w2 + 5
148: if w2 > 0x4 goto +0x5 <L0> // in fall-through: verifier knows w2 ∈ [0,4] => w1 ∈ [5, 9]
149: r2 = r10
150: r2 += -0x5 // r2 = fp - 5
151: r2 += r1 // r2 = fp - 5 + r1 (∈ [5, 9]) => r2 ∈ [fp, fp + 4]
152: w1 = 0x2e
153: *(u8 *)(r2 - 0x5) = w1 // r2 ∈ [fp, fp + 4] => r2 - 5 ∈ [fp - 5, fp - 1]
<L0>:
154: exit

After the changes, the verifier could link 32-bit scalars and also supported -ve offsets for linking:

146: w2 = w1
147: w2 += -0x5

It allowed the verifier to correctly propagate bounds, without the
changes in this patchset, verifier would reject this program with:

invalid unbounded variable-offset write to stack R2

This program has been added as a selftest in the second patch.

Veristat comparison on programs from sched_ext, selftests, and some meta internal programs:

Scx Progs
File Program Verdict (A) Verdict (B) Verdict (DIFF) Insns (A) Insns (B) Insns (DIFF)
----------------- ---------------- ----------- ----------- -------------- --------- --------- -------------
scx_layered.bpf.o layered_runnable success success MATCH 5674 6077 +403 (+7.10%)

FB Progs
File Program Verdict (A) Verdict (B) Verdict (DIFF) Insns (A) Insns (B) Insns (DIFF)
------------ ---------------- ----------- ----------- -------------- --------- --------- -----------------
bpf232.bpf.o layered_dump success success MATCH 1151 1218 +67 (+5.82%)
bpf257.bpf.o layered_runnable success success MATCH 5743 6143 +400 (+6.97%)
bpf252.bpf.o layered_runnable success success MATCH 5677 6075 +398 (+7.01%)
bpf227.bpf.o layered_dump success success MATCH 915 982 +67 (+7.32%)
bpf239.bpf.o layered_runnable success success MATCH 5459 5861 +402 (+7.36%)
bpf246.bpf.o layered_runnable success success MATCH 5562 6008 +446 (+8.02%)
bpf229.bpf.o layered_runnable success success MATCH 2559 3011 +452 (+17.66%)
bpf231.bpf.o layered_runnable success success MATCH 2559 3011 +452 (+17.66%)
bpf234.bpf.o layered_runnable success success MATCH 2549 3001 +452 (+17.73%)
bpf019.bpf.o do_sendmsg success success MATCH 124823 153523 +28700 (+22.99%)
bpf019.bpf.o do_parse success success MATCH 124809 153509 +28700 (+23.00%)
bpf227.bpf.o layered_runnable success success MATCH 1915 2356 +441 (+23.03%)
bpf228.bpf.o layered_runnable success success MATCH 1700 2152 +452 (+26.59%)
bpf232.bpf.o layered_runnable success success MATCH 1499 1951 +452 (+30.15%)

bpf312.bpf.o mount_exit success success MATCH 19253 62883 +43630 (+226.61%)
bpf312.bpf.o umount_exit success success MATCH 19253 62883 +43630 (+226.61%)
bpf311.bpf.o mount_exit success success MATCH 19226 62863 +43637 (+226.97%)
bpf311.bpf.o umount_exit success success MATCH 19226 62863 +43637 (+226.97%)

The above four programs have specific patters that make the verifier explore a lot more states:

for (; depth < MAX_DIR_DEPTH; depth++) {
const unsigned char* name = BPF_CORE_READ(dentry, d_name.name);
if (offset >= MAX_PATH_LEN - MAX_DIR_LEN) {
return depth;
}
int len = bpf_probe_read_kernel_str(&path[offset], MAX_DIR_LEN, name);
offset += len;
if (len == MAX_DIR_LEN) {
if (offset - 2 < MAX_PATH_LEN) { // <---- (a)
path[offset - 2] = '.';
}
if (offset - 3 < MAX_PATH_LEN) { // <---- (b)
path[offset - 3] = '.';
}
if (offset - 4 < MAX_PATH_LEN) { // <---- (c)
path[offset - 4] = '.';
}
}
}

When at some depth == N false branches of conditions (a), (b) and (c) are scheduled for
verification, constraints for offset at depth == N+1 are:
1. offset >= MAX_PATH_LEN + 2
2. offset >= MAX_PATH_LEN + 3
3. offset >= MAX_PATH_LEN + 4 (visited before others)

And after offset += len it becomes:
1. offset >= MAX_PATH_LEN - 4093
2. offset >= MAX_PATH_LEN - 4092
3. offset >= MAX_PATH_LEN - 4091 (visited before others)

Because of the DFS states exploration logic, the states above are visited in order 3, 2, 1; 3 is not
a subset of 2 and 1 is not a subset of 2, so pruning logic does not kick in.
Previously this was not a problem, because range for offset was not propagated through the
statements (a), (b), (c).

As the root cause of this regression is understood, this is not a blocker for this change.

Selftest Progs
File Program Verdict (A) Verdict (B) Verdict (DIFF) Insns (A) Insns (B) Insns (DIFF)
---------------------------------- ------------------------ ----------- ----------- -------------- --------- --------- --------------
linked_list_peek.bpf.o list_peek success success MATCH 152 88 -64 (-42.11%)
verifier_iterating_callbacks.bpf.o cond_break2 success success MATCH 110 88 -22 (-20.00%)

These are the added selftests that failed earlier but are passing now:

verifier_linked_scalars.bpf.o alu32_negative_offset failure success MISMATCH 11 13 +2 (+18.18%)
verifier_linked_scalars.bpf.o scalars_alu32_big_offset failure success MISMATCH 7 10 +3 (+42.86%)
verifier_linked_scalars.bpf.o scalars_neg_alu32_add failure success MISMATCH 7 10 +3 (+42.86%)
verifier_linked_scalars.bpf.o scalars_neg_alu32_sub failure success MISMATCH 7 10 +3 (+42.86%)
verifier_linked_scalars.bpf.o scalars_neg failure success MISMATCH 7 10 +3 (+42.86%)
verifier_linked_scalars.bpf.o scalars_neg_sub failure success MISMATCH 7 10 +3 (+42.86%)
verifier_linked_scalars.bpf.o scalars_sub_neg_imm failure success MISMATCH 7 10 +3 (+42.86%)

iters.bpf.o iter_obfuscate_counter success success MATCH 83 119 +36 (+43.37%)
bpf_cubic.bpf.o bpf_cubic_acked success success MATCH 243 430 +187 (+76.95%)
====================

Link: https://patch.msgid.link/20260204151741.2678118-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>