linux-ext4 - Re: [PATCH v3 13/13] fscrypt: make fscrypt_set_test_dummy

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Date:   Thu, 17 Sep 2020 08:29:07 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     linux-fscrypt@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, ceph-devel@...r.kernel.org,
        Daniel Rosenberg <drosen@...gle.com>
Subject: Re: [PATCH v3 13/13] fscrypt: make
 fscrypt_set_test_dummy_encryption() take a 'const char *'

On Thu, Sep 17, 2020 at 08:32:39AM -0400, Jeff Layton wrote:
> On Wed, 2020-09-16 at 21:11 -0700, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@...gle.com>
> > 
> > fscrypt_set_test_dummy_encryption() requires that the optional argument
> > to the test_dummy_encryption mount option be specified as a substring_t.
> > That doesn't work well with filesystems that use the new mount API,
> > since the new way of parsing mount options doesn't use substring_t.
> > 
> > Make it take the argument as a 'const char *' instead.
> > 
> > Instead of moving the match_strdup() into the callers in ext4 and f2fs,
> > make them just use arg->from directly.  Since the pattern is
> > "test_dummy_encryption=%s", the argument will be null-terminated.
> > 
> 
> Are you sure about that? I thought the point of substring_t was to give
> you a token from the string without null terminating it.
> 
> ISTM that when you just pass in ->from, you might end up with trailing
> arguments in your string like this. e.g.:
> 
>     "v2,foo,bar,baz"
> 
> ...and then that might fail to match properly
> in fscrypt_set_test_dummy_encryption.
> 

Yes I'm sure, and I had also tested it.  The use of match_token() here is to
parse one null-terminated mount option at a time.

The reason that match_token() can return multiple substrings is that the pattern
might be something like "foo=%d:%d".

But here it's just "test_dummy_encryption=%s". "%s" matches until end-of-string.

- Eric