selftests/bpf: Add a verifier scale test with unknown bounded loop

The original bcc pull request https://github.com/iovisor/bcc/pull/3270 exposed
a verifier failure with Clang 12/13 while Clang 4 works fine.

Further investigation exposed two issues:

  Issue 1: LLVM may generate code which uses less refined value. The issue is
           fixed in LLVM patch: https://reviews.llvm.org/D97479

  Issue 2: Spills with initial value 0 are marked as precise which makes later
           state pruning less effective. This is my rough initial analysis and
           further investigation is needed to find how to improve verifier
           pruning in such cases.

With the above LLVM patch, for the new loop6.c test, which has smaller loop
bound compared to original test, I got:

  $ test_progs -s -n 10/16
  ...
  stack depth 64
  processed 390735 insns (limit 1000000) max_states_per_insn 87
      total_states 8658 peak_states 964 mark_read 6
  #10/16 loop6.o:OK

Use the original loop bound, i.e., commenting out "#define WORKAROUND", I got:

  $ test_progs -s -n 10/16
  ...
  BPF program is too large. Processed 1000001 insn
  stack depth 64
  processed 1000001 insns (limit 1000000) max_states_per_insn 91
      total_states 23176 peak_states 5069 mark_read 6
  ...
  #10/16 loop6.o:FAIL

The purpose of this patch is to provide a regression test for the above LLVM fix
and also provide a test case for further analyzing the verifier pruning issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Zhenwei Pi <pizhenwei@bytedance.com>
Link: https://lore.kernel.org/bpf/20210226223810.236472-1-yhs@fb.com

tools/testing/selftests/bpf/README.rst

@@ -111,6 +111,45 @@ available in 10.0.1. The patch is available in llvm 11.0.0 trunk.
 
 __  https://reviews.llvm.org/D78466
 
+bpf_verif_scale/loop6.o test failure with Clang 12
+==================================================
+
+With Clang 12, the following bpf_verif_scale test failed:
+  * ``bpf_verif_scale/loop6.o``
+
+The verifier output looks like
+
+.. code-block:: c
+
+  R1 type=ctx expected=fp
+  The sequence of 8193 jumps is too complex.
+
+The reason is compiler generating the following code
+
+.. code-block:: c
+
+  ;       for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
+      14:       16 05 40 00 00 00 00 00 if w5 == 0 goto +64 <LBB0_6>
+      15:       bc 51 00 00 00 00 00 00 w1 = w5
+      16:       04 01 00 00 ff ff ff ff w1 += -1
+      17:       67 05 00 00 20 00 00 00 r5 <<= 32
+      18:       77 05 00 00 20 00 00 00 r5 >>= 32
+      19:       a6 01 01 00 05 00 00 00 if w1 < 5 goto +1 <LBB0_4>
+      20:       b7 05 00 00 06 00 00 00 r5 = 6
+  00000000000000a8 <LBB0_4>:
+      21:       b7 02 00 00 00 00 00 00 r2 = 0
+      22:       b7 01 00 00 00 00 00 00 r1 = 0
+  ;       for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
+      23:       7b 1a e0 ff 00 00 00 00 *(u64 *)(r10 - 32) = r1
+      24:       7b 5a c0 ff 00 00 00 00 *(u64 *)(r10 - 64) = r5
+
+Note that insn #15 has w1 = w5 and w1 is refined later but
+r5(w5) is eventually saved on stack at insn #24 for later use.
+This cause later verifier failure. The bug has been `fixed`__ in
+Clang 13.
+
+__  https://reviews.llvm.org/D97479
+
 BPF CO-RE-based tests and Clang version
 =======================================
 

tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c

@@ -76,6 +76,7 @@ void test_bpf_verif_scale(void)
 		{ "loop2.o", BPF_PROG_TYPE_RAW_TRACEPOINT },
 		{ "loop4.o", BPF_PROG_TYPE_SCHED_CLS },
 		{ "loop5.o", BPF_PROG_TYPE_SCHED_CLS },
+		{ "loop6.o", BPF_PROG_TYPE_KPROBE },
 
 		/* partial unroll. 19k insn in a loop.
 		 * Total program size 20.8k insn.

tools/testing/selftests/bpf/progs/loop6.c

@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/ptrace.h>
+#include <stddef.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+/* typically virtio scsi has max SGs of 6 */
+#define VIRTIO_MAX_SGS	6
+
+/* Verifier will fail with SG_MAX = 128. The failure can be
+ * workarounded with a smaller SG_MAX, e.g. 10.
+ */
+#define WORKAROUND
+#ifdef WORKAROUND
+#define SG_MAX		10
+#else
+/* typically virtio blk has max SEG of 128 */
+#define SG_MAX		128
+#endif
+
+#define SG_CHAIN	0x01UL
+#define SG_END		0x02UL
+
+struct scatterlist {
+	unsigned long   page_link;
+	unsigned int    offset;
+	unsigned int    length;
+};
+
+#define sg_is_chain(sg)		((sg)->page_link & SG_CHAIN)
+#define sg_is_last(sg)		((sg)->page_link & SG_END)
+#define sg_chain_ptr(sg)	\
+	((struct scatterlist *) ((sg)->page_link & ~(SG_CHAIN | SG_END)))
+
+static inline struct scatterlist *__sg_next(struct scatterlist *sgp)
+{
+	struct scatterlist sg;
+
+	bpf_probe_read_kernel(&sg, sizeof(sg), sgp);
+	if (sg_is_last(&sg))
+		return NULL;
+
+	sgp++;
+
+	bpf_probe_read_kernel(&sg, sizeof(sg), sgp);
+	if (sg_is_chain(&sg))
+		sgp = sg_chain_ptr(&sg);
+
+	return sgp;
+}
+
+static inline struct scatterlist *get_sgp(struct scatterlist **sgs, int i)
+{
+	struct scatterlist *sgp;
+
+	bpf_probe_read_kernel(&sgp, sizeof(sgp), sgs + i);
+	return sgp;
+}
+
+int config = 0;
+int result = 0;
+
+SEC("kprobe/virtqueue_add_sgs")
+BPF_KPROBE(trace_virtqueue_add_sgs, void *unused, struct scatterlist **sgs,
+	   unsigned int out_sgs, unsigned int in_sgs)
+{
+	struct scatterlist *sgp = NULL;
+	__u64 length1 = 0, length2 = 0;
+	unsigned int i, n, len;
+
+	if (config != 0)
+		return 0;
+
+	for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
+		for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
+		     sgp = __sg_next(sgp)) {
+			bpf_probe_read_kernel(&len, sizeof(len), &sgp->length);
+			length1 += len;
+			n++;
+		}
+	}
+
+	for (i = 0; (i < VIRTIO_MAX_SGS) && (i < in_sgs); i++) {
+		for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
+		     sgp = __sg_next(sgp)) {
+			bpf_probe_read_kernel(&len, sizeof(len), &sgp->length);
+			length2 += len;
+			n++;
+		}
+	}
+
+	config = 1;
+	result = length2 - length1;
+	return 0;
+}