Re: [PATCH bpf-next 3/3] selftests/bpf: Add tests for arena fault reporting

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Thu, 7 Aug 2025 at 15:25, <puranjay@kernel.org> wrote:
>
> Yonghong Song <yonghong.song@linux.dev> writes:
>
> > On 8/6/25 1:58 AM, Puranjay Mohan wrote:
> >> Add selftests for testing the reporting of arena page faults through BPF
> >> streams. Two new bpf programs are added that read and write to an
> >> unmapped arena address and the fault reporting is verified in the
> >> userspace through streams.
> >>
> >> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> >> ---
> >>   .../testing/selftests/bpf/prog_tests/stream.c | 24 ++++++++++++
> >>   tools/testing/selftests/bpf/progs/stream.c    | 37 +++++++++++++++++++
> >>   2 files changed, 61 insertions(+)
> >>
> >> diff --git a/tools/testing/selftests/bpf/prog_tests/stream.c b/tools/testing/selftests/bpf/prog_tests/stream.c
> >> index d9f0185dca61b..4bdde56de35b1 100644
> >> --- a/tools/testing/selftests/bpf/prog_tests/stream.c
> >> +++ b/tools/testing/selftests/bpf/prog_tests/stream.c
> >> @@ -41,6 +41,22 @@ struct {
> >>              "([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
> >>              "|[ \t]+[^\n]+\n)*",
> >>      },
> >> +    {
> >> +            offsetof(struct stream, progs.stream_arena_read_fault),
> >> +            "ERROR: Arena READ access at unmapped address 0x.*\n"
> >> +            "CPU: [0-9]+ UID: 0 PID: [0-9]+ Comm: .*\n"
> >> +            "Call trace:\n"
> >> +            "([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
> >> +            "|[ \t]+[^\n]+\n)*",
> >> +    },
> >> +    {
> >> +            offsetof(struct stream, progs.stream_arena_write_fault),
> >> +            "ERROR: Arena WRITE access at unmapped address 0x.*\n"
> >> +            "CPU: [0-9]+ UID: 0 PID: [0-9]+ Comm: .*\n"
> >> +            "Call trace:\n"
> >> +            "([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
> >> +            "|[ \t]+[^\n]+\n)*",
> >> +    },
> >>   };
> >>
> >>   static int match_regex(const char *pattern, const char *string)
> >> @@ -85,6 +101,14 @@ void test_stream_errors(void)
> >>                      continue;
> >>              }
> >>   #endif
> >> +#if !defined(__x86_64__) && !defined(__aarch64__)
> >> +            ASSERT_TRUE(1, "Arena fault reporting unsupported, skip.");
> >> +            if (i == 2 || i == 3) {
> >> +                    ret = bpf_prog_stream_read(prog_fd, 2, buf, sizeof(buf), &ropts);
> >> +                    ASSERT_EQ(ret, 0, "stream read");
> >> +                    continue;
> >> +            }
> >> +#endif
> >>
> >>              ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDERR, buf, sizeof(buf), &ropts);
> >>              ASSERT_GT(ret, 0, "stream read");
> >> diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
> >> index 35790897dc879..58ebff60cd96a 100644
> >> --- a/tools/testing/selftests/bpf/progs/stream.c
> >> +++ b/tools/testing/selftests/bpf/progs/stream.c
> >> @@ -1,10 +1,15 @@
> >>   // SPDX-License-Identifier: GPL-2.0
> >>   /* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> >> +#define BPF_NO_KFUNC_PROTOTYPES
> >
> > Do we have to defineBPF_NO_KFUNC_PROTOTYPES in the above? Without the above, we do not need
> > below extern bpf_res_spin_lock and bpf_res_spin_unlock.
> >
>
> If we don't define BPF_NO_KFUNC_PROTOTYPES then there are build failures
> for bpf_arena_alloc/free_pages() because the prototypes in vmlinux.h
> lack __arena attribute.

I would address this by dropping the alloc/free.
Instead to work around "addr_space_cast insn in program without arena error",
insert a dummy store "ptr = &arena" in the program, where ptr is a
global void *.

>
> >> +
> >> +    page = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
> >> +    bpf_arena_free_pages(&arena, page, 1);
> >> +
>
> Thanks,
> Puranjay