selftests/bpf: trigger verifier.c:maybe_exit_scc() for a speculative state

This is a test case minimized from a syzbot reproducer from [1].
The test case triggers verifier.c:maybe_exit_scc() w/o
preceding call to verifier.c:maybe_enter_scc() on a speculative
symbolic execution path.

Here is verifier log for the test case:

Live regs before insn:
0: .......... (b7) r0 = 100
1 1: 0......... (7b) *(u64 *)(r10 -512) = r0
1 2: 0......... (b5) if r0 <= 0x0 goto pc-2
3: 0......... (95) exit
0: R1=ctx() R10=fp0
0: (b7) r0 = 100 ; R0_w=100
1: (7b) *(u64 *)(r10 -512) = r0 ; R0_w=100 R10=fp0 fp-512_w=100
2: (b5) if r0 <= 0x0 goto pc-2
mark_precise: ...
2: R0_w=100
3: (95) exit

from 2 to 1 (speculative execution): R0_w=scalar() R1=ctx() R10=fp0 fp-512_w=100
1: R0_w=scalar() R1=ctx() R10=fp0 fp-512_w=100
1: (7b) *(u64 *)(r10 -512) = r0
processed 5 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

- Non-speculative execution path 0-3 does not allocate any checkpoints
(and hence does not call maybe_enter_scc()), and schedules a
speculative jump from 2 to 1.
- Speculative execution path stops immediately because of an infinite
loop detection and triggers verifier.c:update_branch_counts() ->
maybe_exit_scc() calls.

[1] https://lore.kernel.org/bpf/[e-mail]/

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

tools/testing/selftests/bpf/progs/verifier_loops1.c

···
 	: __clobber_all);
 }
 
+/*
+ * This test case triggered a bug in verifier.c:maybe_exit_scc().
+ * Speculative execution path reaches stack access instruction,
+ * stops and triggers maybe_exit_scc() w/o accompanying maybe_enter_scc() call.
+ */
+SEC("socket")
+__arch_x86_64
+__caps_unpriv(CAP_BPF)
+__naked void maybe_exit_scc_bug1(void)
+{
+	asm volatile (
+	"r0 = 100;"
+"1:"
+	/* Speculative execution path reaches and stops here. */
+	"*(u64 *)(r10 - 512) = r0;"
+	/* Condition is always false, but verifier speculatively executes the true branch. */
+	"if r0 <= 0x0 goto 1b;"
+	"exit;"
+	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";