Re: [PATCH bpf-next 7/7] selftests/bpf: Add a new test based on loop6.c

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Mar 29, 2023 at 10:56:36PM -0700, Yonghong Song wrote:
> With LLVM commit [1], loop6.c will fail verification without Commit 3c2611bac08a
> ("selftests/bpf: Fix trace_virtqueue_add_sgs test issue with LLVM 17.").
> Also, there is an effort to fix LLVM since LLVM17 may be used by old kernels
> for bpf development.
> 
> A new test is added by manually doing similar transformation in [1]
> so it can be used to test related verifier changes.
> 
>   [1] https://reviews.llvm.org/D143726
> 
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  .../bpf/prog_tests/bpf_verif_scale.c          |   5 +
>  tools/testing/selftests/bpf/progs/loop7.c     | 102 ++++++++++++++++++
>  2 files changed, 107 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/progs/loop7.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c b/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c
> index 731c343897d8..cb708235e654 100644
> --- a/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c
> +++ b/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c
> @@ -180,6 +180,11 @@ void test_verif_scale_loop6()
>  	scale_test("loop6.bpf.o", BPF_PROG_TYPE_KPROBE, false);
>  }
>  
> +void test_verif_scale_loop7()
> +{
> +	scale_test("loop7.bpf.o", BPF_PROG_TYPE_KPROBE, false);
> +}
> +
>  void test_verif_scale_strobemeta()
>  {
>  	/* partial unroll. 19k insn in a loop.
> diff --git a/tools/testing/selftests/bpf/progs/loop7.c b/tools/testing/selftests/bpf/progs/loop7.c
> new file mode 100644
> index 000000000000..b234ed6f0038
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/loop7.c
> @@ -0,0 +1,102 @@
> +// 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>
> +#include "bpf_misc.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")
> +int 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, upper;
> +
> +	if (config != 0)
> +		return 0;
> +
> +	upper = out_sgs < VIRTIO_MAX_SGS ? out_sgs : VIRTIO_MAX_SGS;
> +	for (i = 0; i < upper; i++) {

since this test is doing manual hoistMinMax, let's keep __sink() in test 6,
so we guaranteed to have both flavors regardless of compiler choices?