Propagate nullness information for branches of register to register
equality compare instructions. The following rules are used:
- suppose register A maybe null
- suppose register B is not null
- for JNE A, B, ... - A is not null in the false branch
- for JEQ A, B, ... - A is not null in the true branch
E.g. for program like below:
r6 = skb->sk;
r7 = sk_fullsock(r6);
r0 = sk_fullsock(r6);
if (r0 == 0) return 0; (a)
if (r0 != r7) return 0; (b)
*r7->type; (c)
return 0;
It is safe to dereference r7 at point (c), because of (a) and (b).
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
---
kernel/bpf/verifier.c | 35 +++++++++++++++++++++++++++++++++--
1 file changed, 33 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index be24774961ab..0312d9ce292f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10267,6 +10267,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
struct bpf_verifier_state *other_branch;
struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs;
struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL;
+ struct bpf_reg_state *eq_branch_regs;
u8 opcode = BPF_OP(insn->code);
bool is_jmp32;
int pred = -1;
@@ -10376,8 +10377,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
/* detect if we are comparing against a constant value so we can adjust
* our min/max values for our dst register.
* this is only legit if both are scalars (or pointers to the same
- * object, I suppose, but we don't support that right now), because
- * otherwise the different base pointers mean the offsets aren't
+ * object, I suppose, see the PTR_MAYBE_NULL related if block below),
+ * because otherwise the different base pointers mean the offsets aren't
* comparable.
*/
if (BPF_SRC(insn->code) == BPF_X) {
@@ -10426,6 +10427,36 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]);
}
+ /* if one pointer register is compared to another pointer
+ * register check if PTR_MAYBE_NULL could be lifted.
+ * E.g. register A - maybe null
+ * register B - not null
+ * for JNE A, B, ... - A is not null in the false branch;
+ * for JEQ A, B, ... - A is not null in the true branch.
+ */
+ if (!is_jmp32 && BPF_SRC(insn->code) == BPF_X &&
+ __is_pointer_value(false, src_reg) && __is_pointer_value(false, dst_reg) &&
+ type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type)) {
+ eq_branch_regs = NULL;
+ switch (opcode) {
+ case BPF_JEQ:
+ eq_branch_regs = other_branch_regs;
+ break;
+ case BPF_JNE:
+ eq_branch_regs = regs;
+ break;
+ default:
+ /* do nothing */
+ break;
+ }
+ if (eq_branch_regs) {
+ if (type_may_be_null(src_reg->type))
+ mark_ptr_not_null_reg(&eq_branch_regs[insn->src_reg]);
+ else
+ mark_ptr_not_null_reg(&eq_branch_regs[insn->dst_reg]);
+ }
+ }
+
/* detect if R == 0 where R is returned from bpf_map_lookup_elem().
* NOTE: these optimizations below are related with pointer comparison
* which will never be JMP32.
--
2.34.1
