> from what I can see, we don't have any BPF helper with this naming
> approach(2 -> to, 4 -> for, etc). How about something like
> bpf_get_file_path?
I think bpf_get_file_path is better. I'll change to it.
> > + * Description
> > + * Get **file** atrribute from the current task by *fd*, then call
> > + * **d_path** to get it's absolute path and copy it as string into
> > + * *path* of *size*. The **path** also support pseudo filesystems
> > + * (whether or not it can be mounted). The *size* must be strictly
> > + * positive. On success, the helper makes sure that the *path* is
> > + * NUL-terminated. On failure, it is filled with zeroes.
> > + * Return
> > + * 0 on success, or a negative error in case of failure.
> Mention that we actually return a positive number on success, which is
> a size of the string + 1 for NUL byte (the +1 is not true right now,
> but I think should be).
I agree.
> I haven't followed previous discussions, so sorry if this was asked
> before. Can either fdget_raw or d_path sleep? Also, d_path seems to be
> relying on current, which in the interrupt context might not be what
> you really want. Have you considered these problems?
Yes, I've checked fdget_raw, it use atomic and rcu_read_lock/ruc_read_unlock,
so it's not sleepable. d_path use rcu_read_lock/rcu_read_unlock too.
In my mind I think this helper won't be called in the interrupt
context (Would you
please give me an example if there's an application scene). So I think
it's ok to
use d_path here.
> > + if (!f.file)
> > + goto error;
> > +
> > + p = d_path(&f.file->f_path, dst, size);
> > + if (IS_ERR_OR_NULL(p)) {
> > + ret = PTR_ERR(p);
if p can really be NULL, you'd get ret == 0 here, which is probably
not what you want.
But reading d_path, it seems like it's either valid pointer or error,
so just use IS_ERR above?
Agree, I'll fix error handling code.
> > + goto error;
> > + }
> > +
> > + ret = strlen(p);
> > + memmove(dst, p, ret);
> > + dst[ret] = '\0';
I think returning number of useful bytes (including terminating NUL)
is good and follows bpf_probe_read_str() convention. So ret++ here?
Agree. Thank you.
> + goto end;
> +
> +error:
> + memset(dst, '0', size);
> +end:
> + fdput(f);
> + return ret;
> +}
> +
[...]
Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> 于2019年10月30日周三 上午2:48写道：
>
> On Mon, Oct 28, 2019 at 1:59 PM Wenbo Zhang <ethercflow@xxxxxxxxx> wrote:
> >
> > When people want to identify which file system files are being opened,
> > read, and written to, they can use this helper with file descriptor as
> > input to achieve this goal. Other pseudo filesystems are also supported.
> >
> > This requirement is mainly discussed here:
> >
> > https://github.com/iovisor/bcc/issues/237
> >
> > v3->v4:
> > - fix missing fdput()
> > - move fd2path from kernel/bpf/trace.c to kernel/trace/bpf_trace.c
> > - move fd2path's test code to another patch
> >
> > v2->v3:
> > - remove unnecessary LOCKDOWN_BPF_READ
> > - refactor error handling section for enhanced readability
> > - provide a test case in tools/testing/selftests/bpf
> >
> > v1->v2:
> > - fix backward compatibility
> > - add this helper description
> > - fix signed-off name
> >
> > Signed-off-by: Wenbo Zhang <ethercflow@xxxxxxxxx>
> > ---
> > include/uapi/linux/bpf.h | 14 +++++++++++-
> > kernel/trace/bpf_trace.c | 40 ++++++++++++++++++++++++++++++++++
> > tools/include/uapi/linux/bpf.h | 14 +++++++++++-
> > 3 files changed, 66 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 4af8b0819a32..124632b2a697 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2775,6 +2775,17 @@ union bpf_attr {
> > * restricted to raw_tracepoint bpf programs.
> > * Return
> > * 0 on success, or a negative error in case of failure.
> > + *
> > + * int bpf_fd2path(char *path, u32 size, int fd)
>
> from what I can see, we don't have any BPF helper with this naming
> approach(2 -> to, 4 -> for, etc). How about something like
> bpf_get_file_path?
>
> > + * Description
> > + * Get **file** atrribute from the current task by *fd*, then call
> > + * **d_path** to get it's absolute path and copy it as string into
> > + * *path* of *size*. The **path** also support pseudo filesystems
> > + * (whether or not it can be mounted). The *size* must be strictly
> > + * positive. On success, the helper makes sure that the *path* is
> > + * NUL-terminated. On failure, it is filled with zeroes.
> > + * Return
> > + * 0 on success, or a negative error in case of failure.
>
> Mention that we actually return a positive number on success, which is
> a size of the string + 1 for NUL byte (the +1 is not true right now,
> but I think should be).
>
> > */
> > #define __BPF_FUNC_MAPPER(FN) \
> > FN(unspec), \
> > @@ -2888,7 +2899,8 @@ union bpf_attr {
> > FN(sk_storage_delete), \
> > FN(send_signal), \
> > FN(tcp_gen_syncookie), \
> > - FN(skb_output),
> > + FN(skb_output), \
> > + FN(fd2path),
> >
> > /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > * function eBPF program intends to call
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 571c25d60710..dd7b070df3d6 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -683,6 +683,44 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
> > .arg1_type = ARG_ANYTHING,
> > };
> >
> > +BPF_CALL_3(bpf_fd2path, char *, dst, u32, size, int, fd)
> > +{
> > + struct fd f;
> > + char *p;
> > + int ret = -EINVAL;
> > +
> > + /* Use fdget_raw instead of fdget to support O_PATH */
> > + f = fdget_raw(fd);
>
> I haven't followed previous discussions, so sorry if this was asked
> before. Can either fdget_raw or d_path sleep? Also, d_path seems to be
> relying on current, which in the interrupt context might not be what
> you really want. Have you considered these problems?
>
> > + if (!f.file)
> > + goto error;
> > +
> > + p = d_path(&f.file->f_path, dst, size);
> > + if (IS_ERR_OR_NULL(p)) {
> > + ret = PTR_ERR(p);
>
> if p can really be NULL, you'd get ret == 0 here, which is probably
> not what you want.
> But reading d_path, it seems like it's either valid pointer or error,
> so just use IS_ERR above?
>
> > + goto error;
> > + }
> > +
> > + ret = strlen(p);
> > + memmove(dst, p, ret);
> > + dst[ret] = '\0';
>
> I think returning number of useful bytes (including terminating NUL)
> is good and follows bpf_probe_read_str() convention. So ret++ here?
>
> > + goto end;
> > +
> > +error:
> > + memset(dst, '0', size);
> > +end:
> > + fdput(f);
> > + return ret;
> > +}
> > +
>
> [...]
