netdev - Re: [RFC 0/3] bpf: Add d

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Message-ID: <20200406031602.GR23230@ZenIV.linux.org.uk>
Date:   Mon, 6 Apr 2020 04:16:02 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andrii Nakryiko <andriin@...com>, bgregg@...flix.com
Subject: Re: [RFC 0/3] bpf: Add d_path helper

On Fri, Apr 03, 2020 at 11:08:28AM +0200, Jiri Olsa wrote:

> 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..?

IDGI...
	1) d_path(), by definition, is dependent upon the
process' root - the same <mount,dentry> pair will yield
different strings if caller is chrooted.  You *can't* just
use a random process' root
	2) we are *NOT* making rename_lock and mount_lock
disable interrupts.  Not happening.

So it has to be process-synchronous anyway.  Could you describe
where that thing is going to be callable?