netdev - Re: [PATCH v16 02/12] namei: allow nd_jump

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Message-ID: <20191116180934.fajrkc4jqcewiuqd@yavin.dot.cyphar.com>
Date:   Sun, 17 Nov 2019 05:09:34 +1100
From:   Aleksa Sarai <asarai@...e.de>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Aleksa Sarai <cyphar@...har.com>, Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
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Subject: Re: [PATCH v16 02/12] namei: allow nd_jump_link() to produce errors

On 2019-11-16, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sat, Nov 16, 2019 at 11:27:52AM +1100, Aleksa Sarai wrote:
> > +	error = nd_jump_link(&path);
> > +	if (error)
> > +		path_put(&path);
> 
> > +	error = nd_jump_link(&ns_path);
> > +	if (error)
> > +		path_put(&ns_path);
> 
> > +	error = nd_jump_link(&path);
> > +	if (error)
> > +		path_put(&path);
> 
> 3 calls.  Exact same boilerplate in each to handle a failure case.
> Which spells "wrong calling conventions"; it's absolutely clear
> that we want that path_put() inside nd_jump_link().
> 
> The rule should be this: reference that used to be held in
> *path is consumed in any case.  On success it goes into
> nd->path, on error it's just dropped, but in any case, the
> caller has the same refcounting environment to deal with.
> 
> If you need the same boilerplate cleanup on failure again and again,
> the calling conventions are wrong and need to be fixed.

Will do.

> And I'm not sure that int is the right return type here, to be honest.
> void * might be better - return ERR_PTR() or NULL, so that the value
> could be used as return value of ->get_link() that calls that thing.

I don't agree, given that the few callers of ns_get_path() are
inconsistent with regards to whether they should use IS_ERR() or check
for NULL, not to mention that "void *error" reads to me as being very
odd given how common "int error" is throughout the kernel. There's also
the "error == ERR_PTR(-EAGAIN)" checks which also read as being quite
odd too.

But the main motivating factor for changing it was that the one use
where "void *" is useful (proc_ns_get_link) becomes needlessly ugly
because of the "nd_jump_link() can return errors" change:

	error = ERR_PTR(nd_jump_link(&ns_path));

Or probably (if you don't want to rely on ERR_PTR(0) == NULL):

	int err = nd_jump_link(&ns_path);
	if (err)
		error = ERR_PTR(err);

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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