[PATCH bpf-next v4 06/18] bpf: Treat unknown-size stack reads as reaching the frame top

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

When the size of a helper or kfunc memory argument is not a constant on
the path, the stack liveness analysis is told the call reads
MAX_BPF_STACK bytes starting at the pointer's offset. Clipped at the
top of the frame this covers everything from the offset upwards, which
is the intent, but only as long as no frame is deeper than
MAX_BPF_STACK bytes.

Return the "unknown" marker instead, which the liveness analysis already
turns into a read of every slot between the offset and the frame top,
independent of how deep the frame is.

No functional change.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2b87cb9beea0..a7b1142b0d51 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14385,11 +14385,11 @@ s64 bpf_helper_stack_access_bytes(struct bpf_verifier_env *env, struct bpf_insn
 			}
 			/*
 			 * Size arg is const on each path but differs across merged
-			 * paths. MAX_BPF_STACK is a safe upper bound for reads.
+			 * paths. Reads may extend anywhere up to the frame top.
 			 */
 			if (full_write)
 				return 0;
-			return MAX_BPF_STACK;
+			return S64_MIN;
 		}
 		return S64_MIN;
 	case ARG_PTR_TO_DYNPTR:
@@ -14475,7 +14475,8 @@ s64 bpf_kfunc_stack_access_bytes(struct bpf_verifier_env *env, struct bpf_insn *
 			size = (s64)aux->const_reg_vals[size_reg];
 			goto out;
 		}
-		return MAX_BPF_STACK;
+		/* Unknown size: the read may extend anywhere up to the frame top. */
+		return S64_MIN;
 	}
 
 	/* fixed-size pointed-to type: resolve via BTF */
-- 
2.53.0