linux-ext4 - Re: [PATCH] jbd2: gate checksum calculations on crc driver presence, not sb flags

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Date:	Thu, 1 Oct 2015 11:18:03 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Jan Kara <jack@...e.com>, "Theodore Ts'o" <tytso@....edu>,
	Nikolay Borisov <kernel@...p.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	SiteGround Operations <operations@...eground.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] jbd2: gate checksum calculations on crc driver presence,
 not sb flags

On Thu, Oct 01, 2015 at 12:35:12AM -0600, Andreas Dilger wrote:
> On Sep 30, 2015, at 11:47 AM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> > 
> > Change the journal's checksum functions to gate on whether or not the
> > crc32c driver is loaded, and gate the loading on the superblock bits.
> > This prevents a journal crash if someone loads a journal in no-csum
> > mode and then randomizes the superblock, thus flipping on the feature
> > bits.
> > 
> > Reported-by: Nikolay Borisov <kernel@...p.com>
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > ---
> > fs/jbd2/journal.c    |   12 +++++++++---
> > include/linux/jbd2.h |   10 ++++++----
> > 2 files changed, 15 insertions(+), 7 deletions(-)
> > 
> > diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> > index 8270fe9..16e3a46 100644
> > --- a/fs/jbd2/journal.c
> > +++ b/fs/jbd2/journal.c
> > @@ -122,9 +122,15 @@ EXPORT_SYMBOL(__jbd2_debug);
> > #endif
> > 
> > /* Checksumming functions */
> > +static bool journal_has_csum_v2or3_feature(journal_t *j)
> > +{
> > +	return JBD2_HAS_INCOMPAT_FEATURE(j, JBD2_FEATURE_INCOMPAT_CSUM_V2) ||
> > +	       JBD2_HAS_INCOMPAT_FEATURE(j, JBD2_FEATURE_INCOMPAT_CSUM_V3);
> > +}
> > +
> > static int jbd2_verify_csum_type(journal_t *j, journal_superblock_t *sb)
> > {
> > -	if (!jbd2_journal_has_csum_v2or3(j))
> > +	if (!journal_has_csum_v2or3_feature(j))
> > 		return 1;
> > 
> > 	return sb->s_checksum_type == JBD2_CRC32C_CHKSUM;
> > @@ -1531,7 +1537,7 @@ static int journal_get_superblock(journal_t *journal)
> > 		goto out;
> > 	}
> > 
> > -	if (jbd2_journal_has_csum_v2or3(journal) &&
> > +	if (journal_has_csum_v2or3_feature(journal) &&
> > 	    JBD2_HAS_COMPAT_FEATURE(journal, JBD2_FEATURE_COMPAT_CHECKSUM)) {
> > 		/* Can't have checksum v1 and v2 on at the same time! */
> > 		printk(KERN_ERR "JBD2: Can't enable checksumming v1 and v2/3 "
> > @@ -1545,7 +1551,7 @@ static int journal_get_superblock(journal_t *journal)
> > 	}
> > 
> > 	/* Load the checksum driver */
> > -	if (jbd2_journal_has_csum_v2or3(journal)) {
> > +	if (journal_has_csum_v2or3_feature(journal)) {
> > 		journal->j_chksum_driver = crypto_alloc_shash("crc32c", 0, 0);
> > 		if (IS_ERR(journal->j_chksum_driver)) {
> > 			printk(KERN_ERR "JBD2: Cannot load crc32c driver.\n");
> > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> > index df07e78..c74c786 100644
> > --- a/include/linux/jbd2.h
> > +++ b/include/linux/jbd2.h
> > @@ -1340,11 +1340,13 @@ extern size_t journal_tag_bytes(journal_t *journal);
> > 
> > static inline int jbd2_journal_has_csum_v2or3(journal_t *journal)
> > {
> > -	if (JBD2_HAS_INCOMPAT_FEATURE(journal, JBD2_FEATURE_INCOMPAT_CSUM_V2) ||
> > -	    JBD2_HAS_INCOMPAT_FEATURE(journal, JBD2_FEATURE_INCOMPAT_CSUM_V3))
> > -		return 1;
> > +	WARN_ON_ONCE((JBD2_HAS_INCOMPAT_FEATURE(journal,
> > +				JBD2_FEATURE_INCOMPAT_CSUM_V2) ||
> > +		 JBD2_HAS_INCOMPAT_FEATURE(journal,
> > +				JBD2_FEATURE_INCOMPAT_CSUM_V3)) &&
> > +		     journal->j_chksum_driver == NULL);
> > 
> > -	return 0;
> > +	return journal->j_chksum_driver != NULL;
> > }
> 
> Why not use:
> 
> 	WARN_ON_ONCE(journal_has_csum_v2orv3_feature() &&
> 		     journal->j_chksum_driver == NULL);
> 
> rather than open-coding it?  Yes, you would have to move that function
> to the header and give it a better name.

Sounds like a good idea, thanks.

> 
> As a side note, I've long thought about changing the macros to be shorter:
> 
> #define JBD2_HAS_INCOMPAT_FEATURE(j, name)				\
> 	((j)->j_format_version >= 2 &&					\
> 	 ((j)->j_superblock->s_feature_incompat &			\
> 	  cpu_to_be32((JBD2_HAS_INCOMPAT_FEATURE_ ## name))))
> 
> so they can be used like:
> 
> static bool jbd2_journal_has_csum_v2or3_feature(journal_t *journal)
> {
> 	return JBD2_HAS_INCOMPAT_FEATURE(journal, CSUM_V2) ||
> 	       JBD2_HAS_INCOMPAT_FEATURE(journal, CSUM_V3);
> }
> 
> This not only makes the code much shorter and more readable, it also
> avoids potentially hard-to-spot bugs like the following:
> 
> 	JBD2_HAS_INCOMPAT_FEATURE(j, JBD2_FEATURE_COMPAT_CHECKSUM)
> 
> The same would be useful for the equivalent ext4 macros as well.

Yes it will... as a separate patch.  Shorter lines and fewer opportunities
to screw things up. :)

--D

> 
> Cheers, Andreas
> 
> 
> 
> 
> 
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