selftests/bpf: Test with a very short loop

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/[e-mail]/

Signed-off-by: Eduard Zingerman <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 /* 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"
 
···
 	exit;						\
 "	:
 	: __imm(bpf_ktime_get_ns)
+	: __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);
 }
 

tools/testing/selftests/bpf/veristat.cfg

···
 test_verif_scale*
 test_xdp_noinline*
 xdp_synproxy*
+verifier_search_pruning*