From: Al Viro <viro@zeniv.linux.org.uk>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Wenbo Zhang <ethercflow@gmail.com>,
bpf@vger.kernel.org, ast@kernel.org.com, daniel@iogearbox.net,
yhs@fb.com, andrii.nakryiko@gmail.com, netdev@vger.kernel.org,
linux-fsdevel@vger.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
Date: Sat, 23 Nov 2019 05:35:14 +0000 [thread overview]
Message-ID: <20191123053514.GJ26530@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20191123051919.dsw7v6jyad4j4ilc@ast-mbp.dhcp.thefacebook.com>
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.
next prev parent reply other threads:[~2019-11-23 5:35 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1574162990.git.ethercflow@gmail.com>
[not found] ` <e8b1281b7405eb4b6c1f094169e6efd2c8cc95da.1574162990.git.ethercflow@gmail.com>
2019-11-23 3:18 ` [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname Alexei Starovoitov
2019-11-23 4:43 ` Al Viro
2019-11-23 4:51 ` Al Viro
2019-11-23 5:19 ` Alexei Starovoitov
2019-11-23 5:35 ` Al Viro [this message]
2019-11-23 6:04 ` Alexei Starovoitov
2019-12-13 19:51 ` Brendan Gregg
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