selftests/bpf: test if state loops are detected in a tricky case

A convoluted test case for iterators convergence logic that
demonstrates that states with branch count equal to 0 might still be
a part of not completely explored loop.

E.g. consider the following state diagram:

               initial     Here state 'succ' was processed first,
                 |         it was eventually tracked to produce a
                 V         state identical to 'hdr'.
    .---------> hdr        All branches from 'succ' had been explored
    |            |         and thus 'succ' has its .branches == 0.
    |            V
    |    .------...        Suppose states 'cur' and 'succ' correspond
    |    |       |         to the same instruction + callsites.
    |    V       V         In such case it is necessary to check
    |   ...     ...        whether 'succ' and 'cur' are identical.
    |    |       |         If 'succ' and 'cur' are a part of the same loop
    |    V       V         they have to be compared exactly.
    |   succ <- cur
    |    |
    |    V
    |   ...
    |    |
    '----'

Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

tools/testing/selftests/bpf/progs/iters.c

@@ -998,6 +998,183 @@ __naked int loop_state_deps1(void)
 	);
 }
 
+SEC("?raw_tp")
+__failure
+__msg("math between fp pointer and register with unbounded")
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked int loop_state_deps2(void)
+{
+	/* This is equivalent to C program below.
+	 *
+	 * The case turns out to be tricky in a sense that:
+	 * - states with read+precise mark on c are explored only on a second
+	 *   iteration of the first inner loop and in a state which is pushed to
+	 *   states stack first.
+	 * - states with c=-25 are explored only on a second iteration of the
+	 *   second inner loop and in a state which is pushed to states stack
+	 *   first.
+	 *
+	 * Depending on the details of iterator convergence logic
+	 * verifier might stop states traversal too early and miss
+	 * unsafe c=-25 memory access.
+	 *
+	 *   j = iter_new();             // fp[-16]
+	 *   a = 0;                      // r6
+	 *   b = 0;                      // r7
+	 *   c = -24;                    // r8
+	 *   while (iter_next(j)) {
+	 *     i = iter_new();           // fp[-8]
+	 *     a = 0;                    // r6
+	 *     b = 0;                    // r7
+	 *     while (iter_next(i)) {
+	 *       if (a == 1) {
+	 *         a = 0;
+	 *         b = 1;
+	 *       } else if (a == 0) {
+	 *         a = 1;
+	 *         if (random() == 42)
+	 *           continue;
+	 *         if (b == 1) {
+	 *           *(r10 + c) = 7;     // this is not safe
+	 *           iter_destroy(i);
+	 *           iter_destroy(j);
+	 *           return;
+	 *         }
+	 *       }
+	 *     }
+	 *     iter_destroy(i);
+	 *     i = iter_new();           // fp[-8]
+	 *     a = 0;                    // r6
+	 *     b = 0;                    // r7
+	 *     while (iter_next(i)) {
+	 *       if (a == 1) {
+	 *         a = 0;
+	 *         b = 1;
+	 *       } else if (a == 0) {
+	 *         a = 1;
+	 *         if (random() == 42)
+	 *           continue;
+	 *         if (b == 1) {
+	 *           a = 0;
+	 *           c = -25;
+	 *         }
+	 *       }
+	 *     }
+	 *     iter_destroy(i);
+	 *   }
+	 *   iter_destroy(j);
+	 *   return;
+	 */
+	asm volatile (
+		"r1 = r10;"
+		"r1 += -16;"
+		"r2 = 0;"
+		"r3 = 10;"
+		"call %[bpf_iter_num_new];"
+		"r6 = 0;"
+		"r7 = 0;"
+		"r8 = -24;"
+	"j_loop_%=:"
+		"r1 = r10;"
+		"r1 += -16;"
+		"call %[bpf_iter_num_next];"
+		"if r0 == 0 goto j_loop_end_%=;"
+
+		/* first inner loop */
+		"r1 = r10;"
+		"r1 += -8;"
+		"r2 = 0;"
+		"r3 = 10;"
+		"call %[bpf_iter_num_new];"
+		"r6 = 0;"
+		"r7 = 0;"
+	"i_loop_%=:"
+		"r1 = r10;"
+		"r1 += -8;"
+		"call %[bpf_iter_num_next];"
+		"if r0 == 0 goto i_loop_end_%=;"
+	"check_one_r6_%=:"
+		"if r6 != 1 goto check_zero_r6_%=;"
+		"r6 = 0;"
+		"r7 = 1;"
+		"goto i_loop_%=;"
+	"check_zero_r6_%=:"
+		"if r6 != 0 goto i_loop_%=;"
+		"r6 = 1;"
+		"call %[bpf_get_prandom_u32];"
+		"if r0 != 42 goto check_one_r7_%=;"
+		"goto i_loop_%=;"
+	"check_one_r7_%=:"
+		"if r7 != 1 goto i_loop_%=;"
+		"r0 = r10;"
+		"r0 += r8;"
+		"r1 = 7;"
+		"*(u64 *)(r0 + 0) = r1;"
+		"r1 = r10;"
+		"r1 += -8;"
+		"call %[bpf_iter_num_destroy];"
+		"r1 = r10;"
+		"r1 += -16;"
+		"call %[bpf_iter_num_destroy];"
+		"r0 = 0;"
+		"exit;"
+	"i_loop_end_%=:"
+		"r1 = r10;"
+		"r1 += -8;"
+		"call %[bpf_iter_num_destroy];"
+
+		/* second inner loop */
+		"r1 = r10;"
+		"r1 += -8;"
+		"r2 = 0;"
+		"r3 = 10;"
+		"call %[bpf_iter_num_new];"
+		"r6 = 0;"
+		"r7 = 0;"
+	"i2_loop_%=:"
+		"r1 = r10;"
+		"r1 += -8;"
+		"call %[bpf_iter_num_next];"
+		"if r0 == 0 goto i2_loop_end_%=;"
+	"check2_one_r6_%=:"
+		"if r6 != 1 goto check2_zero_r6_%=;"
+		"r6 = 0;"
+		"r7 = 1;"
+		"goto i2_loop_%=;"
+	"check2_zero_r6_%=:"
+		"if r6 != 0 goto i2_loop_%=;"
+		"r6 = 1;"
+		"call %[bpf_get_prandom_u32];"
+		"if r0 != 42 goto check2_one_r7_%=;"
+		"goto i2_loop_%=;"
+	"check2_one_r7_%=:"
+		"if r7 != 1 goto i2_loop_%=;"
+		"r6 = 0;"
+		"r8 = -25;"
+		"goto i2_loop_%=;"
+	"i2_loop_end_%=:"
+		"r1 = r10;"
+		"r1 += -8;"
+		"call %[bpf_iter_num_destroy];"
+
+		"r6 = 0;"
+		"r7 = 0;"
+		"goto j_loop_%=;"
+	"j_loop_end_%=:"
+		"r1 = r10;"
+		"r1 += -16;"
+		"call %[bpf_iter_num_destroy];"
+		"r0 = 0;"
+		"exit;"
+		:
+		: __imm(bpf_get_prandom_u32),
+		  __imm(bpf_iter_num_new),
+		  __imm(bpf_iter_num_next),
+		  __imm(bpf_iter_num_destroy)
+		: __clobber_all
+	);
+}
+
 SEC("?raw_tp")
 __success
 __naked int triple_continue(void)