[PATCH bpf-next v1 2/2] selftests/bpf: test with a very short loop

[Eduard Zingerman <eddyz87@gmail.com>]

The test added is a simplified reproducer from syzbot report [1].
If verifier does not insert checkpoint somewhere inside the loop,
verification of the program would take a very long time.

This would happen because mark_chain_precision() for register r7 would
constantly trace jump history of the loop back, processing many
iterations for each mark_chain_precision() call.

[1] https://lore.kernel.org/bpf/670429f6.050a0220.49194.0517.GAE@google.com/

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../bpf/progs/verifier_search_pruning.c       | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_search_pruning.c b/tools/testing/selftests/bpf/progs/verifier_search_pruning.c
index 5a14498d352f..f40e57251e94 100644
--- a/tools/testing/selftests/bpf/progs/verifier_search_pruning.c
+++ b/tools/testing/selftests/bpf/progs/verifier_search_pruning.c
@@ -2,6 +2,7 @@
 /* Converted from tools/testing/selftests/bpf/verifier/search_pruning.c */
 
 #include <linux/bpf.h>
+#include <../../../include/linux/filter.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
 
@@ -336,4 +337,26 @@ l0_%=:	r1 = 42;					\
 	: __clobber_all);
 }
 
+/* Without checkpoint forcibly inserted at the back-edge a loop this
+ * test would take a very long time to verify.
+ */
+SEC("kprobe")
+__failure __log_level(4)
+__msg("BPF program is too large.")
+__naked void short_loop1(void)
+{
+	asm volatile (
+	"   r7 = *(u16 *)(r1 +0);"
+	"1: r7 += 0x1ab064b9;"
+	"   .8byte %[jset];" /* same as 'if r7 & 0x702000 goto 1b;' */
+	"   r7 &= 0x1ee60e;"
+	"   r7 += r1;"
+	"   if r7 s> 0x37d2 goto +0;"
+	"   r0 = 0;"
+	"   exit;"
+	:
+	: __imm_insn(jset, BPF_JMP_IMM(BPF_JSET, BPF_REG_7, 0x702000, -2))
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.46.2