Re: [PATCH v4 bpf-next 10/14] bpf: Add d_path helper

From: Jiri Olsa <jolsa@redhat.com>
To: KP Singh <kpsingh@chromium.org>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Jiri Olsa <jolsa@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	Martin KaFai Lau <kafai@fb.com>, David Miller <davem@redhat.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Wenbo Zhang <ethercflow@gmail.com>,
	Andrii Nakryiko <andriin@fb.com>,
	Brendan Gregg <bgregg@netflix.com>,
	Florent Revest <revest@chromium.org>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v4 bpf-next 10/14] bpf: Add d_path helper
Date: Fri, 17 Jul 2020 10:28:35 +0200	[thread overview]
Message-ID: <20200717082835.GB522549@krava> (raw)
In-Reply-To: <6f3fd6b0-cc3d-57e7-0444-dcaf399e6abd@chromium.org>

On Fri, Jul 17, 2020 at 01:13:23AM +0200, KP Singh wrote:
> 
> 
> On 6/28/20 9:42 PM, Jiri Olsa wrote:
> > On Fri, Jun 26, 2020 at 01:38:27PM -0700, Andrii Nakryiko wrote:
> >> On Thu, Jun 25, 2020 at 4:49 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >>>
> >>> Adding d_path helper function that returns full path
> >>> for give 'struct path' object, which needs to be the
> >>> kernel BTF 'path' object.
> >>>
> >>> The helper calls directly d_path function.
> >>>
> >>> Updating also bpf.h tools uapi header and adding
> >>> 'path' to bpf_helpers_doc.py script.
> >>>
> >>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> >>> ---
> >>>  include/uapi/linux/bpf.h       | 14 +++++++++-
> >>>  kernel/trace/bpf_trace.c       | 47 ++++++++++++++++++++++++++++++++++
> >>>  scripts/bpf_helpers_doc.py     |  2 ++
> >>>  tools/include/uapi/linux/bpf.h | 14 +++++++++-
> >>>  4 files changed, 75 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >>> index 0cb8ec948816..23274c81f244 100644
> >>> --- a/include/uapi/linux/bpf.h
> >>> +++ b/include/uapi/linux/bpf.h
> >>> @@ -3285,6 +3285,17 @@ union bpf_attr {
> >>>   *             Dynamically cast a *sk* pointer to a *udp6_sock* pointer.
> >>>   *     Return
> >>>   *             *sk* if casting is valid, or NULL otherwise.
> >>> + *
> >>> + * int bpf_d_path(struct path *path, char *buf, u32 sz)
> >>> + *     Description
> >>> + *             Return full path for given 'struct path' object, which
> >>> + *             needs to be the kernel BTF 'path' object. The path is
> >>> + *             returned in buffer provided 'buf' of size 'sz'.
> >>> + *
> >>> + *     Return
> >>> + *             length of returned string on success, or a negative
> >>> + *             error in case of failure
> >>
> >> It's important to note whether string is always zero-terminated (I'm
> >> guessing it is, right?).
> > 
> > right, will add
> 
> Also note that bpf_probe_read_{kernel, user}_str return the length including
> the NUL byte:
> 
>  * 	Return
>  * 		On success, the strictly positive length of the string,
>  * 		including the trailing NUL character. On error, a negative
>  * 		value.
> 
> It would be good to keep this uniform. So you will need a len += 1 here as well.

good point, will keep it that way

thanks,
jirka