From: Sultan Alsawaf <[email protected]>
We should be using the program fd here, not the perf event fd.
Fixes: 63f2f5ee856ba ("libbpf: add ability to attach/detach BPF program to perf event")
Signed-off-by: Sultan Alsawaf <[email protected]>
---
tools/lib/bpf/libbpf.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d43cc3f29dae..3d20d57d4af5 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -9538,7 +9538,7 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
if (!link)
return ERR_PTR(-ENOMEM);
link->detach = &bpf_link__detach_perf_event;
- link->fd = pfd;
+ link->fd = prog_fd;
if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
err = -errno;
--
2.30.2
On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf <[email protected]> wrote:
>
> From: Sultan Alsawaf <[email protected]>
>
> We should be using the program fd here, not the perf event fd.
Why? Can you elaborate on what issue you ran into with the current code?
>
> Fixes: 63f2f5ee856ba ("libbpf: add ability to attach/detach BPF program to perf event")
> Signed-off-by: Sultan Alsawaf <[email protected]>
> ---
> tools/lib/bpf/libbpf.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d43cc3f29dae..3d20d57d4af5 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -9538,7 +9538,7 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
> if (!link)
> return ERR_PTR(-ENOMEM);
> link->detach = &bpf_link__detach_perf_event;
> - link->fd = pfd;
> + link->fd = prog_fd;
>
> if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
> err = -errno;
> --
> 2.30.2
>
On Fri, Mar 12, 2021 at 05:31:14PM -0800, Andrii Nakryiko wrote:
> On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf <[email protected]> wrote:
> >
> > From: Sultan Alsawaf <[email protected]>
> >
> > We should be using the program fd here, not the perf event fd.
>
> Why? Can you elaborate on what issue you ran into with the current code?
bpf_link__pin() would fail with -EINVAL when using tracepoints, kprobes, or
uprobes. The failure would happen inside the kernel, in bpf_link_get_from_fd()
right here:
if (f.file->f_op != &bpf_link_fops) {
fdput(f);
return ERR_PTR(-EINVAL);
}
Since bpf wasn't looking for the perf event fd, I swapped it for the program fd
and bpf_link__pin() worked.
Sultan
On Fri, Mar 12, 2021 at 6:22 PM Sultan Alsawaf <[email protected]> wrote:
>
> On Fri, Mar 12, 2021 at 05:31:14PM -0800, Andrii Nakryiko wrote:
> > On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf <[email protected]> wrote:
> > >
> > > From: Sultan Alsawaf <[email protected]>
> > >
> > > We should be using the program fd here, not the perf event fd.
> >
> > Why? Can you elaborate on what issue you ran into with the current code?
>
> bpf_link__pin() would fail with -EINVAL when using tracepoints, kprobes, or
> uprobes. The failure would happen inside the kernel, in bpf_link_get_from_fd()
> right here:
> if (f.file->f_op != &bpf_link_fops) {
> fdput(f);
> return ERR_PTR(-EINVAL);
> }
kprobe/tracepoint/perf_event attachments behave like bpf_link (so
libbpf uses user-space high-level bpf_link APIs for it), but they are
not bpf_link-based in the kernel. So bpf_link__pin() won't work for
such types of programs until we actually have bpf_link-backed
attachment support in the kernel itself. I never got to implementing
this because we already had auto-detachment properties from perf_event
FD itself. But it would be nice to have that done as a real bpf_link
in the kernel (with all the observability, program update,
force-detach support).
Looking for volunteers to make this happen ;)
>
> Since bpf wasn't looking for the perf event fd, I swapped it for the program fd
> and bpf_link__pin() worked.
But you were pinning the BPF program, not a BPF link. Which is not
what should have happen.
>
> Sultan
On Fri, Mar 12, 2021 at 06:33:01PM -0800, Andrii Nakryiko wrote:
> On Fri, Mar 12, 2021 at 6:22 PM Sultan Alsawaf <[email protected]> wrote:
> >
> > On Fri, Mar 12, 2021 at 05:31:14PM -0800, Andrii Nakryiko wrote:
> > > On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf <[email protected]> wrote:
> > > >
> > > > From: Sultan Alsawaf <[email protected]>
> > > >
> > > > We should be using the program fd here, not the perf event fd.
> > >
> > > Why? Can you elaborate on what issue you ran into with the current code?
> >
> > bpf_link__pin() would fail with -EINVAL when using tracepoints, kprobes, or
> > uprobes. The failure would happen inside the kernel, in bpf_link_get_from_fd()
> > right here:
> > if (f.file->f_op != &bpf_link_fops) {
> > fdput(f);
> > return ERR_PTR(-EINVAL);
> > }
>
> kprobe/tracepoint/perf_event attachments behave like bpf_link (so
> libbpf uses user-space high-level bpf_link APIs for it), but they are
> not bpf_link-based in the kernel. So bpf_link__pin() won't work for
> such types of programs until we actually have bpf_link-backed
> attachment support in the kernel itself. I never got to implementing
> this because we already had auto-detachment properties from perf_event
> FD itself. But it would be nice to have that done as a real bpf_link
> in the kernel (with all the observability, program update,
> force-detach support).
>
> Looking for volunteers to make this happen ;)
>
>
> >
> > Since bpf wasn't looking for the perf event fd, I swapped it for the program fd
> > and bpf_link__pin() worked.
>
> But you were pinning the BPF program, not a BPF link. Which is not
> what should have happen.
This is the code in question:
link = bpf_program__attach(prog);
// make sure `link` is valid, blah blah...
bpf_link__pin(link, some_path);
Are you saying that this usage is incorrect?
Sultan
On Fri, Mar 12, 2021 at 6:43 PM Sultan Alsawaf <[email protected]> wrote:
>
> On Fri, Mar 12, 2021 at 06:33:01PM -0800, Andrii Nakryiko wrote:
> > On Fri, Mar 12, 2021 at 6:22 PM Sultan Alsawaf <[email protected]> wrote:
> > >
> > > On Fri, Mar 12, 2021 at 05:31:14PM -0800, Andrii Nakryiko wrote:
> > > > On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf <[email protected]> wrote:
> > > > >
> > > > > From: Sultan Alsawaf <[email protected]>
> > > > >
> > > > > We should be using the program fd here, not the perf event fd.
> > > >
> > > > Why? Can you elaborate on what issue you ran into with the current code?
> > >
> > > bpf_link__pin() would fail with -EINVAL when using tracepoints, kprobes, or
> > > uprobes. The failure would happen inside the kernel, in bpf_link_get_from_fd()
> > > right here:
> > > if (f.file->f_op != &bpf_link_fops) {
> > > fdput(f);
> > > return ERR_PTR(-EINVAL);
> > > }
> >
> > kprobe/tracepoint/perf_event attachments behave like bpf_link (so
> > libbpf uses user-space high-level bpf_link APIs for it), but they are
> > not bpf_link-based in the kernel. So bpf_link__pin() won't work for
> > such types of programs until we actually have bpf_link-backed
> > attachment support in the kernel itself. I never got to implementing
> > this because we already had auto-detachment properties from perf_event
> > FD itself. But it would be nice to have that done as a real bpf_link
> > in the kernel (with all the observability, program update,
> > force-detach support).
> >
> > Looking for volunteers to make this happen ;)
> >
> >
> > >
> > > Since bpf wasn't looking for the perf event fd, I swapped it for the program fd
> > > and bpf_link__pin() worked.
> >
> > But you were pinning the BPF program, not a BPF link. Which is not
> > what should have happen.
>
> This is the code in question:
> link = bpf_program__attach(prog);
> // make sure `link` is valid, blah blah...
> bpf_link__pin(link, some_path);
>
> Are you saying that this usage is incorrect?
Right, for kprobe/tracepoint/perf_event attachments it's not
supported. cgroup, xdp, raw_tracepoint and
fentry/fexit/fmod_ret/freplace (and a few more) attachments are
bpf_links in the kernel, so it works for them.
>
> Sultan