On 8/23/22 4:15 PM, John Fastabend wrote:
Eduard Zingerman wrote:
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).
I think the idea makes sense. Perhaps Yonhong can comment seeing he was active
on the LLVM thread. I just scanned the LLVM side for now will take a look
in more detail in a bit.
The issue is discovered when making some changes in llvm compiler.
I think it is good to add support in verifier so in the future
if compiler generates such code patterns, user won't get
surprised verification failure.
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
kernel/bpf/verifier.c | 39 ++++++++++++++++++++++++++++++++++++++-
1 file changed, 38 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2c1f8069f7b7..c48d34625bfd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -472,6 +472,11 @@ static bool type_may_be_null(u32 type)
return type & PTR_MAYBE_NULL;
}
+static bool type_is_pointer(enum bpf_reg_type type)
+{
+ return type != NOT_INIT && type != SCALAR_VALUE;
+}
+
Instead of having another helper is_pointer_value() could work here?
Checking if we need NOT_INIT in that helper now.
static bool is_acquire_function(enum bpf_func_id func_id,
const struct bpf_map *map)
{
@@ -10046,6 +10051,7 @@ 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;
@@ -10155,7 +10161,7 @@ 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
+ * object, I suppose, see the next if block), because
* otherwise the different base pointers mean the offsets aren't
* comparable.
*/
@@ -10199,6 +10205,37 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
opcode, is_jmp32);
}
+ /* 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 &&
+ type_is_pointer(src_reg->type) && type_is_pointer(dst_reg->type) &&
+ 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]);
+ }
+ }
+
if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&
!WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {
find_equal_scalars(this_branch, dst_reg);
--
2.37.1
