On 2/16/24 7:03 AM, Eduard Zingerman wrote:
The test case was minimized from mailing list discussion [0].
It is equivalent to the following C program:
struct iter_limit_bug_ctx { __u64 a; __u64 b; __u64 c; };
static __naked void iter_limit_bug_cb(void)
{
switch (bpf_get_prandom_u32()) {
case 1: ctx->a = 42; break;
case 2: ctx->b = 42; break;
default: ctx->c = 42; break;
}
}
int iter_limit_bug(struct __sk_buff *skb)
{
struct iter_limit_bug_ctx ctx = { 7, 7, 7 };
bpf_loop(2, iter_limit_bug_cb, &ctx, 0);
if (ctx.a == 42 && ctx.b == 42 && ctx.c == 7)
asm volatile("r1 /= 0;":::"r1");
return 0;
}
The main idea is that each loop iteration changes one of the state
variables in a non-deterministic manner. Hence it is premature to
prune the states that have two iterations left comparing them to
states with one iteration left.
E.g. {{7,7,7}, callback_depth=0} can reach state {42,42,7},
while {{7,7,7}, callback_depth=1} can't.
[0] https://lore.kernel.org/bpf/9b251840-7cb8-4d17-bd23-1fc8071d8eef@xxxxxxxxx/
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
