Re: [PATCH v3 bpf-next 1/3] bpf: introduce new VFS based BPF kfuncs

[Matt Bobrowski <mattbobrowski@google.com>]

On Fri, Jul 26, 2024 at 02:25:26PM -0700, Song Liu wrote:
> On Fri, Jul 26, 2024 at 1:56 AM Matt Bobrowski <mattbobrowski@google.com> wrote:
> >
> [...]
> > +       len = buf + buf__sz - ret;
> > +       memmove(buf, ret, len);
> > +       return len;
> > +}
> > +__bpf_kfunc_end_defs();
> > +
> > +BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
> > +BTF_ID_FLAGS(func, bpf_get_task_exe_file,
> > +            KF_ACQUIRE | KF_TRUSTED_ARGS | KF_SLEEPABLE | KF_RET_NULL)
> > +BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE | KF_SLEEPABLE)
> 
> Do we really need KF_SLEEPABLE for bpf_put_file?

Well, the guts of fput() is annotated w/ might_sleep(), so the calling
thread may presumably be involuntarily put to sleep? You can also see
the guts of fput() invoking various indirect function calls
i.e. ->release(), and depending on the implementation of those, they
could be initiating resource release related actions which
consequently could result in waiting for some I/O to be done? fput()
also calls dput() and mntput() and these too can also do a bunch of
teardown.

Please correct me if I've misunderstood something.

/M