Re: [PATCH 2/2] selftest/bpf: Test returning zero from a perf bpf program suppresses SIGIO.

[Kyle Huey]

On Tue, Dec 5, 2023 at 8:54 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
> On 12/4/23 3:14 PM, Kyle Huey wrote:
> > The test sets a hardware breakpoint and uses a bpf program to suppress the
> > I/O availability signal if the ip matches the expected value.
> >
> > Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
> > ---
> >   .../selftests/bpf/prog_tests/perf_skip.c      | 95 +++++++++++++++++++
> >   .../selftests/bpf/progs/test_perf_skip.c      | 23 +++++
> >   2 files changed, 118 insertions(+)
> >   create mode 100644 tools/testing/selftests/bpf/prog_tests/perf_skip.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/test_perf_skip.c
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/perf_skip.c b/tools/testing/selftests/bpf/prog_tests/perf_skip.c
> > new file mode 100644
> > index 000000000000..b269a31669b7
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/perf_skip.c
> > @@ -0,0 +1,95 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#define _GNU_SOURCE
> > +#include <test_progs.h>
> > +#include "test_perf_skip.skel.h"
> > +#include <linux/hw_breakpoint.h>
> > +#include <sys/mman.h>
> > +
> > +#define BPF_OBJECT            "test_perf_skip.bpf.o"
> > +
> > +static void handle_sig(int)
>
> I hit a warning here:
> home/yhs/work/bpf-next/tools/testing/selftests/bpf/prog_tests/perf_skip.c:10:27: error: omitting the parameter name in a function definition is a C23 extension [-Werror,-Wc23-extensions]

Yeah, Meta's kernel-ci bot sent me off-list email about this one.

>
>     10 | static void handle_sig(int)
>        |
>
> Add a parameter and marked as unused can resolve the issue.
>
> #define __always_unused         __attribute__((__unused__))
>
> static void handle_sig(int unused __always_unused)
> {
>          ASSERT_OK(1, "perf event not skipped");
> }
>
>
> > +{
> > +     ASSERT_OK(1, "perf event not skipped");
> > +}
> > +
> > +static noinline int test_function(void)
> > +{
> > +     return 0;
> > +}
> > +
> > +void serial_test_perf_skip(void)
> > +{
> > +     sighandler_t previous;
> > +     int duration = 0;
> > +     struct test_perf_skip *skel = NULL;
> > +     int map_fd = -1;
> > +     long page_size = sysconf(_SC_PAGE_SIZE);
> > +     uintptr_t *ip = NULL;
> > +     int prog_fd = -1;
> > +     struct perf_event_attr attr = {0};
> > +     int perf_fd = -1;
> > +     struct f_owner_ex owner;
> > +     int err;
> > +
> > +     previous = signal(SIGIO, handle_sig);
> > +
> > +     skel = test_perf_skip__open_and_load();
> > +     if (!ASSERT_OK_PTR(skel, "skel_load"))
> > +             goto cleanup;
> > +
> > +     prog_fd = bpf_program__fd(skel->progs.handler);
> > +     if (!ASSERT_OK(prog_fd < 0, "bpf_program__fd"))
> > +             goto cleanup;
> > +
> > +     map_fd = bpf_map__fd(skel->maps.ip);
> > +     if (!ASSERT_OK(map_fd < 0, "bpf_map__fd"))
> > +             goto cleanup;
> > +
> > +     ip = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, map_fd, 0);
> > +     if (!ASSERT_OK_PTR(ip, "mmap bpf map"))
> > +             goto cleanup;
> > +
> > +     *ip = (uintptr_t)test_function;
> > +
> > +     attr.type = PERF_TYPE_BREAKPOINT;
> > +     attr.size = sizeof(attr);
> > +     attr.bp_type = HW_BREAKPOINT_X;
> > +     attr.bp_addr = (uintptr_t)test_function;
> > +     attr.bp_len = sizeof(long);
> > +     attr.sample_period = 1;
> > +     attr.sample_type = PERF_SAMPLE_IP;
> > +     attr.pinned = 1;
> > +     attr.exclude_kernel = 1;
> > +     attr.exclude_hv = 1;
> > +     attr.precise_ip = 3;
> > +
> > +     perf_fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
> > +     if (CHECK(perf_fd < 0, "perf_event_open", "err %d\n", perf_fd))
> > +             goto cleanup;
> > +
> > +     err = fcntl(perf_fd, F_SETFL, O_ASYNC);
> > +     if (!ASSERT_OK(err, "fcntl(F_SETFL, O_ASYNC)"))
> > +             goto cleanup;
> > +
> > +     owner.type = F_OWNER_TID;
> > +     owner.pid = gettid();
>
> I hit a compilation failure here:
>
> /home/yhs/work/bpf-next/tools/testing/selftests/bpf/prog_tests/perf_skip.c:75:14: error: call to undeclared function 'gettid'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>     75 |         owner.pid = gettid();
>        |                     ^
>
> If you looked at some other examples, the common usage is do 'syscall(SYS_gettid)'.

Not clear why this works for me but sure I'll change that.

>
> So the following patch should fix the compilation error:
>
> #include <sys/syscall.h>
> ...
>          owner.pid = syscall(SYS_gettid);
> ...
>
> > +     err = fcntl(perf_fd, F_SETOWN_EX, &owner);
> > +     if (!ASSERT_OK(err, "fcntl(F_SETOWN_EX)"))
> > +             goto cleanup;
> > +
> > +     err = ioctl(perf_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
> > +     if (!ASSERT_OK(err, "ioctl(PERF_EVENT_IOC_SET_BPF)"))
> > +             goto cleanup;
> > +
> > +     test_function();
>
> As Andrii has mentioned in previous comments, we will have
> issue is RELEASE version of selftest is built
>    RELEASE=1 make ...
>
> See https://lore.kernel.org/bpf/20231127050342.1945270-1-yonghong.song@xxxxxxxxx

Not sure I follow this one. Are you saying adding asm volatile ("") in
test_function() is *not* sufficient?

- Kyle

>
> > +
> > +cleanup:
> > +     if (perf_fd >= 0)
> > +             close(perf_fd);
> > +     if (ip)
> > +             munmap(ip, page_size);
> > +     if (skel)
> > +             test_perf_skip__destroy(skel);
> > +
> > +     signal(SIGIO, previous);
> > +}
> > diff --git a/tools/testing/selftests/bpf/progs/test_perf_skip.c b/tools/testing/selftests/bpf/progs/test_perf_skip.c
> > new file mode 100644
> > index 000000000000..ef01a9161afe
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/progs/test_perf_skip.c
> > @@ -0,0 +1,23 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include "vmlinux.h"
> > +#include <bpf/bpf_helpers.h>
> > +#include <bpf/bpf_tracing.h>
> > +
> > +struct {
> > +     __uint(type, BPF_MAP_TYPE_ARRAY);
> > +     __uint(max_entries, 1);
> > +     __uint(map_flags, BPF_F_MMAPABLE);
> > +     __type(key, uint32_t);
> > +     __type(value, uintptr_t);
> > +} ip SEC(".maps");
> > +
> > +SEC("perf_event")
> > +int handler(struct bpf_perf_event_data *data)
> > +{
> > +     const uint32_t index = 0;
> > +     uintptr_t *v = bpf_map_lookup_elem(&ip, &index);
> > +
> > +     return !(v && *v == PT_REGS_IP(&data->regs));
> > +}
> > +
> > +char _license[] SEC("license") = "GPL";