linux-kernel - Re: [PATCH v5 5/8] virtio: Treat alloc_dax() -EOPNOTSUPP failure as non-fatal

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Message-ID: <20240213061823.GB27995@wunner.de>
Date: Tue, 13 Feb 2024 07:18:23 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnd Bergmann <arnd@...db.de>, Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Matthew Wilcox <willy@...radead.org>,
	Russell King <linux@...linux.org.uk>, linux-arch@...r.kernel.org,
	linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-xfs@...r.kernel.org,
	dm-devel@...ts.linux.dev, nvdimm@...ts.linux.dev,
	linux-s390@...r.kernel.org, Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...nel.org>,
	Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [PATCH v5 5/8] virtio: Treat alloc_dax() -EOPNOTSUPP failure as
 non-fatal

On Mon, Feb 12, 2024 at 03:02:46PM -0800, Dan Williams wrote:
> However, Lukas, I think Linus is right, your DEFINE_FREE() should use
> IS_ERR_OR_NULL().

Uh... that's a negative, sir. ;)

IS_ERR_OR_NULL() results in...
* a superfluous NULL pointer check in x509_key_preparse() and
* a superfluous IS_ERR check in x509_cert_parse().

IS_ERR() results *only* in...
* a superfluous IS_ERR check in x509_cert_parse().

I can get rid of the IS_ERR() check by using assume().

I can *not* get rid of the NULL pointer check because the compiler
is compiled with -fno-delete-null-pointer-checks.  (The compiler
seems to ignore __attribute__((returns_nonnull)) due to that.)

> I.e. the problem is trying to use
> __free(x509_free_certificate) in x509_cert_parse().
> 
> > --- a/crypto/asymmetric_keys/x509_cert_parser.c
> > +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> > @@ -60,24 +60,24 @@ void x509_free_certificate(struct x509_certificate *cert)
> >   */
> >  struct x509_certificate *x509_cert_parse(const void *data, size_t datalen)
> >  {
> > -       struct x509_certificate *cert;
> > -       struct x509_parse_context *ctx;
> > +       struct x509_certificate *cert __free(x509_free_certificate);
> 
> ...make this:
> 
>     struct x509_certificate *cert __free(kfree);

That doesn't work I'm afraid.  x509_cert_parse() needs
x509_free_certificate() to be called in the error path,
not kfree().  See the existing code in current mainline:

x509_cert_parse() populates three sub-allocations in
struct x509_certificate (pub, sig, id) and two
sub-sub-allocations (pub->key, pub->params).

So I'd have to add five additional local variables which
get freed by __cleanup().  One of them (pub->key) requires
kfree_sensitive() instead of kfree(), so I'd need an extra
DEFINE_FREE() for that.

I haven't tried it but I suspect the result would look
terrible and David Howells wouldn't like it.

> ...and Mathieu, this should be IS_ERR_OR_NULL() to skip an unnecessary
> call to virtio_fs_cleanup_dax() at function exit that the compiler
> should elide.

My recommendation is to check for !IS_ERR() in the DEFINE_FREE() clause
and amend virtio_fs_cleanup_dax() with a "if (!dax_dev) return;" for
defensiveness in case someone calls it with a NULL pointer.

That's the best solution I could come up with for the x509_certificate
conversion.

Note that even with superfluous checks avoided, __cleanup() causes
gcc-12 to always generate two return paths.  It's very visible in
the generated code that all the stack unwinding code gets duplicated
in every function using __cleanup().  The existing Assembler code
of x509_key_preparse() and x509_cert_parse(), without __cleanup()
invocation, has only a single return path.

So __cleanup() bloats the code regardless of superfluous checks,
but future gcc versions might avoid that.  clang-15 generates much
more compact code (vmlinux is a couple hundred kBytes smaller),
but does weird things such as inlining x509_free_certificate()
in x509_cert_parse().

As you may have guessed, I've spent an inordinate amount of time
down that rabbit hole. ;(

Thanks,

Lukas