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From: Jiri Olsa <jolsa@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jiri Olsa <jolsa@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	Yonghong Song <yhs@fb.com>, Martin KaFai Lau <kafai@fb.com>,
	David Miller <davem@redhat.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Wenbo Zhang <ethercflow@gmail.com>,
	KP Singh <kpsingh@chromium.org>, Andrii Nakryiko <andriin@fb.com>,
	bgregg@netflix.com
Subject: Re: [RFC 0/3] bpf: Add d_path helper
Date: Fri, 3 Apr 2020 11:08:28 +0200	[thread overview]
Message-ID: <20200403090828.GF2784502@krava> (raw)
In-Reply-To: <20200402142106.GF23230@ZenIV.linux.org.uk>

On Thu, Apr 02, 2020 at 03:21:06PM +0100, Al Viro wrote:
> On Wed, Apr 01, 2020 at 01:09:04PM +0200, Jiri Olsa wrote:
> > hi,
> > adding d_path helper to return full path for 'path' object.
> > 
> > I originally added and used 'file_path' helper, which did the same,
> > but used 'struct file' object. Then realized that file_path is just
> > a wrapper for d_path, so we'd cover more calling sites if we add
> > d_path helper and allowed resolving BTF object within another object,
> > so we could call d_path also with file pointer, like:
> > 
> >   bpf_d_path(&file->f_path, buf, size);
> > 
> > This feature is mainly to be able to add dpath (filepath originally)
> > function to bpftrace, which seems to work nicely now, like:
> > 
> >   # bpftrace -e 'kretfunc:fget { printf("%s\n", dpath(args->ret->f_path));  }' 
> > 
> > I'm not completely sure this is all safe and bullet proof and there's
> > no other way to do this, hence RFC post.
> > 
> > I'd be happy also with file_path function, but I thought it'd be
> > a shame not to try to add d_path with the verifier change.
> > I'm open to any suggestions ;-)
> 
> What are the locking conditions guaranteed to that sucker?  Note that d_path()
> is *NOT* lockless - call it from an interrupt/NMI/etc. and you are fucked.
> It can grab rename_lock and mount_lock; usually it avoids that, so you won't
> see them grabbed on every call, but after the first seqlock mismatch it will
> fall back to grabbing the spinlock in question.  And then there's ->d_dname(),
> with whatever things _that_ chooses to do....

if we limit it just to task context I think it would still be
helpful for us:

  if (in_task())
	d_path..

perhaps even create a d_path version without d_dname callback
if that'd be still a problem, because it seems to be there mainly
for special filesystems..?

thanks,
jirka


  reply	other threads:[~2020-04-03  9:08 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-01 11:09 [RFC 0/3] bpf: Add d_path helper Jiri Olsa
2020-04-01 11:09 ` [PATCH 1/3] bpf: Add support to check if BTF object is nested in another object Jiri Olsa
2020-04-07  1:16   ` Alexei Starovoitov
2020-04-07  9:37     ` Jiri Olsa
2020-04-01 11:09 ` [PATCH 2/3] bpf: Add d_path helper Jiri Olsa
2020-04-02 14:02   ` Florent Revest
2020-04-03  9:01     ` Jiri Olsa
2020-04-06  2:49       ` Andrii Nakryiko
2020-04-01 11:09 ` [PATCH 3/3] selftests/bpf: Add test for " Jiri Olsa
2020-04-02 14:03 ` [RFC 0/3] bpf: Add " Florent Revest
2020-04-03  8:55   ` Jiri Olsa
2020-04-02 14:21 ` Al Viro
2020-04-03  9:08   ` Jiri Olsa [this message]
2020-04-06  3:16     ` Al Viro
2020-04-06  9:09       ` Jiri Olsa
2020-04-06 12:47         ` Al Viro
2020-04-07  1:10         ` Alexei Starovoitov
2020-04-07  8:53           ` Jiri Olsa
2020-04-07  9:27           ` KP Singh
2020-04-07  9:45             ` Jiri Olsa

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