bpf: fix bug in eBPF verifier

while comparing for verifier state equivalency the comparison
was missing a check for uninitialized register.
Make sure it does so and add a testcase.

Fixes: f1bca824dabb ("bpf: add search pruning optimization to verifier")
Cc: Hannes Frederic Sowa <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Hannes Frederic Sowa <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

kernel/bpf/verifier.c

···
 		if (memcmp(&old->regs[i], &cur->regs[i],
 			   sizeof(old->regs[0])) != 0) {
 			if (old->regs[i].type == NOT_INIT ||
-			    old->regs[i].type == UNKNOWN_VALUE)
+			    (old->regs[i].type == UNKNOWN_VALUE &&
+			     cur->regs[i].type != NOT_INIT))
 				continue;
 			return false;
 		}

samples/bpf/test_verifier.c

···
 		.result = REJECT,
 	},
 	{
+		"program doesn't init R0 before exit in all branches",
+		.insns = {
+			BPF_JMP_IMM(BPF_JGE, BPF_REG_1, 0, 2),
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 2),
+			BPF_EXIT_INSN(),
+		},
+		.errstr = "R0 !read_ok",
+		.result = REJECT,
+	},
+	{
 		"stack out of bounds",
 		.insns = {
 			BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0),