selftests/bpf: test case for callback_depth states pruning logic

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@linux.dev/

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240222154121.6991-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
index 5905e036e0eaca6aa7c38e06412aabf0e2b1b4b9..a955a6358206eac8a4b5065b531e0171d4cc2a62 100644 (file)
--- a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
+++ b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
@@ -239,4 +239,74 @@ int bpf_loop_iter_limit_nested(void *unused)
        return 1000 * a + b + c;
 }
 
+struct iter_limit_bug_ctx {
+       __u64 a;
+       __u64 b;
+       __u64 c;
+};
+
+static __naked void iter_limit_bug_cb(void)
+{
+       /* This is the same as C code below, but written
+        * in assembly to control which branches are fall-through.
+        *
+        *   switch (bpf_get_prandom_u32()) {
+        *   case 1:  ctx->a = 42; break;
+        *   case 2:  ctx->b = 42; break;
+        *   default: ctx->c = 42; break;
+        *   }
+        */
+       asm volatile (
+       "r9 = r2;"
+       "call %[bpf_get_prandom_u32];"
+       "r1 = r0;"
+       "r2 = 42;"
+       "r0 = 0;"
+       "if r1 == 0x1 goto 1f;"
+       "if r1 == 0x2 goto 2f;"
+       "*(u64 *)(r9 + 16) = r2;"
+       "exit;"
+       "1: *(u64 *)(r9 + 0) = r2;"
+       "exit;"
+       "2: *(u64 *)(r9 + 8) = r2;"
+       "exit;"
+       :
+       : __imm(bpf_get_prandom_u32)
+       : __clobber_all
+       );
+}
+
+SEC("tc")
+__failure
+__flag(BPF_F_TEST_STATE_FREQ)
+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);
+
+       /* This is the same as C code below,
+        * written in assembly to guarantee checks order.
+        *
+        *   if (ctx.a == 42 && ctx.b == 42 && ctx.c == 7)
+        *     asm volatile("r1 /= 0;":::"r1");
+        */
+       asm volatile (
+       "r1 = *(u64 *)%[ctx_a];"
+       "if r1 != 42 goto 1f;"
+       "r1 = *(u64 *)%[ctx_b];"
+       "if r1 != 42 goto 1f;"
+       "r1 = *(u64 *)%[ctx_c];"
+       "if r1 != 7 goto 1f;"
+       "r1 /= 0;"
+       "1:"
+       :
+       : [ctx_a]"m"(ctx.a),
+         [ctx_b]"m"(ctx.b),
+         [ctx_c]"m"(ctx.c)
+       : "r1"
+       );
+       return 0;
+}
+
 char _license[] SEC("license") = "GPL";