bpf: allow states pruning for misc/invalid slots in iterator loops

Within an iterator or callback based loop, it should be safe to prune
the current state if the old state stack slot is marked as
STACK_INVALID or STACK_MISC:
- either all branches of the old state lead to a program exit;
- or some branch of the old state leads the current state.

This is the same logic as applied in non-loop cases when
states_equal() is called in NOT_EXACT mode.

The test case that exercises stacksafe() and demonstrates the
difference in verification performance is included in the next patch.
I'm not sure if it is possible to prepare a test case that exercises
regsafe(); it appears that the compute_live_registers() pass makes
this impossible.

Nevertheless, for code readability reasons, I think that stacksafe()
and regsafe() should handle STACK_INVALID / NOT_INIT symmetrically.
Hence, this commit changes both functions.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251230-loop-stack-misc-pruning-v1-1-585cfd6cec51@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0baae7828af220accd4086b9bad270e745f4aff9..3d44c5d066239f1f86ec8d2f40d3a6abac222d66 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19086,11 +19086,9 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
        if (exact == EXACT)
                return regs_exact(rold, rcur, idmap);
 
-       if (rold->type == NOT_INIT) {
-               if (exact == NOT_EXACT || rcur->type == NOT_INIT)
-                       /* explored state can't have used this */
-                       return true;
-       }
+       if (rold->type == NOT_INIT)
+               /* explored state can't have used this */
+               return true;
 
        /* Enforce that register types have to match exactly, including their
         * modifiers (like PTR_MAYBE_NULL, MEM_RDONLY, etc), as a general
@@ -19259,7 +19257,7 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 
                spi = i / BPF_REG_SIZE;
 
-               if (exact != NOT_EXACT &&
+               if (exact == EXACT &&
                    (i >= cur->allocated_stack ||
                     old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
                     cur->stack[spi].slot_type[i % BPF_REG_SIZE]))