bpf: Preserve scalar zero spills for stack reads

Stack reads can read back bytes that belong to a previously spilled scalar
constant zero. Today mark_reg_stack_read() only treats STACK_ZERO bytes as
known zero bytes, so the destination register can become unknown even
though every byte in the read range is known to be zero. This can lead to
rejecting otherwise valid programs once the loaded byte is used as a
pointer offset.

The original reproducer uses a variable-offset stack byte read emitted by
clang 22.1.6 at -O2/-O3 from a small helper-based BPF C program. Fixed
offset reads have a related mixed case as well: pure scalar-zero spill
reads are already handled, but a fixed read spanning both STACK_ZERO and
scalar const-zero STACK_SPILL bytes still falls back to unknown.

Teach mark_reg_stack_read() to also consider STACK_SPILL bytes backed by a
spilled scalar constant zero as zero bytes, and use that path for both
variable-offset stack reads and fixed-offset mixed reads. Keep the existing
pure register-fill behavior unchanged.

When a zero result depends on such a spill, mark the contributing stack
slots precise before accepting the const-zero result so pruning cannot
reuse a zero-spill state for a later non-zero spill state.

No deployed-program regression is currently known, so target bpf-next.

Assisted-by: opencode:gpt-5.5
Signed-off-by: Woojin Ji <random6.xyz@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260625-bpf-stack-var-off-zero-v1-v3-1-a068210a761b@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 39a851e690ec..76b8b7627a10 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -1243,6 +1243,11 @@ static inline void bpf_bt_set_frame_slot(struct backtrack_state *bt, u32 frame,
 	bt->stack_masks[frame] |= 1ull << slot;
 }
 
+static inline void bpf_bt_set_frame_slot_mask(struct backtrack_state *bt, u32 frame, u64 mask)
+{
+	bt->stack_masks[frame] |= mask;
+}
+
 static inline void bt_set_frame_stack_arg_slot(struct backtrack_state *bt, u32 frame, u32 slot)
 {
 	bt->stack_arg_masks[frame] |= 1 << slot;