On Tue, 2024-09-24 at 04:40 -0600, Daniel Xu wrote:
This commit allows progs to elide a null check on statically known map lookup keys. In other words, if the verifier can statically prove that the lookup will be in-bounds, allow the prog to drop the null check.
This is useful for two reasons:
- Large numbers of nullness checks (especially when they cannot fail) unnecessarily pushes prog towards BPF_COMPLEXITY_LIMIT_JMP_SEQ.
- It forms a tighter contract between programmer and verifier.
For (1), bpftrace is starting to make heavier use of percpu scratch maps. As a result, for user scripts with large number of unrolled loops, we are starting to hit jump complexity verification errors. These percpu lookups cannot fail anyways, as we only use static key values. Eliding nullness probably results in less work for verifier as well.
For (2), percpu scratch maps are often used as a larger stack, as the currrent stack is limited to 512 bytes. In these situations, it is desirable for the programmer to express: "this lookup should never fail, and if it does, it means I messed up the code". By omitting the null check, the programmer can "ask" the verifier to double check the logic.
Tests also have to be updated in sync with these changes, as the verifier is more efficient with this change. Notable, iters.c tests had to be changed to use a map type that still requires null checks, as it's exercising verifier tracking logic w.r.t iterators.
Signed-off-by: Daniel Xu dxu@dxuuu.xyz
Acked-by: Eduard Zingerman eddyz87@gmail.com
[...]
+/* Returns constant key value if possible, else -1 */ +static long get_constant_map_key(struct bpf_verifier_env *env,
struct bpf_reg_state *key)+{
- struct bpf_func_state *state = func(env, key);
- struct bpf_reg_state *reg;
- int stack_off;
- int slot;
- int spi;
- if (key->type != PTR_TO_STACK)
return -1;- if (!tnum_is_const(key->var_off))
return -1;- stack_off = key->off + key->var_off.value;
- slot = -stack_off - 1;
- if (slot < 0)
/* Stack grew upwards */return -1;
Nitpick: I'd also add a test like below:
SEC("socket") __failure __msg("invalid indirect access to stack R2 off=4096 size=4") __naked void key_lookup_at_invalid_fp(void) { asm volatile (" \ r1 = %[map_array] ll; \ r2 = r10; \ r2 += 4096; \ call %[bpf_map_lookup_elem]; \ r0 = *(u64*)(r0 + 0); \ exit; \ " : : __imm(bpf_map_lookup_elem), __imm_addr(map_array) : __clobber_all); }
(double checked with v2 and this test does cause page fault)
[...]