linux-kernel - Re: [PATCH 18/19] ext4: Add example fsinfo information [ver #16]

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Date:   Fri, 21 Feb 2020 14:43:05 +0000
From:   David Howells <dhowells@...hat.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk, raven@...maw.net,
        mszeredi@...hat.com, christian@...uner.io,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 18/19] ext4: Add example fsinfo information [ver #16]

Darrick J. Wong <darrick.wong@...cle.com> wrote:

> > +	memcpy(ctx->buffer, es->s_volume_name, sizeof(es->s_volume_name));
> 
> Shouldn't this be checking that ctx->buffer is large enough to hold
> s_volume_name?

Well, the buffer is guaranteed to be 4KiB in size.

> > +	return strlen(ctx->buffer);
> 
> s_volume_name is /not/ a null-terminated string if the label is 16
> characters long.

And the buffer is precleared, so it's automatically NULL terminated.

> > +#define FSINFO_ATTR_EXT4_TIMESTAMPS	0x400	/* Ext4 superblock timestamps */
> 
> I guess each filesystem gets ... 256 different attrs, and the third
> nibble determines the namespace?

No.  Think of it as allocating namespace in 256-number blocks.  That means
there are 16 million of them.  If a filesystem uses up an entire block, it can
always allocate another one.  I don't think it likely that we'll get
sufficient filesystems to eat them all.

David