netdev - Re: [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

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Date:   Sat, 23 Nov 2019 05:35:14 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Wenbo Zhang <ethercflow@...il.com>, bpf@...r.kernel.org,
        ast@...nel.org.com, daniel@...earbox.net, yhs@...com,
        andrii.nakryiko@...il.com, netdev@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for
 mapping a file descriptor to a pathname

On Fri, Nov 22, 2019 at 09:19:21PM -0800, Alexei Starovoitov wrote:

> hard to tell. It will be run out of bpf prog that attaches to kprobe or
> tracepoint. What is the concern about locking?
> d_path() doesn't take any locks and doesn't depend on any locks. Above 'if'
> checks that plain d_path() is used and not some specilized callback with
> unknown logic.

It sure as hell does.  It might end up taking rename_lock and/or mount_lock
spinlock components.  It'll try not to, but if the first pass ends up with
seqlock mismatch, it will just grab the spinlock the second time around.

> > with this number; quite possibly never before that function had been called
> > _and_ not once after it has returned.
> 
> Right. TOCTOU is not a concern here. It's tracing. It's ok for full path to be
> 'one time deal'.

It might very well be a full path of something completely unrelated to what
the syscall ends up operating upon.  It's not that the file might've been
moved; it might be a different file.  IOW, results of that tracing might be
misleading.