Date:	Thu, 15 Sep 2011 18:04:30 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andreas Dilger <aedilger@...il.com>
Cc:	"Ted Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Amir Goldstein <amir73il@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Coly Li <colyli@...il.com>
Subject: Re: [PATCH 01/37] e2fsprogs: Read and write full-sized inodes

On Thu, Sep 15, 2011 at 03:35:45PM -0600, Andreas Dilger wrote:
> On 2011-09-15, at 2:25 PM, "Darrick J. Wong" <djwong@...ibm.com> wrote:
> > On Wed, Sep 14, 2011 at 12:39:01PM -0400, Ted Ts'o wrote:
> >> On Wed, Aug 31, 2011 at 05:35:17PM -0700, Darrick J. Wong wrote:
> >>> As part of adding inode checksums, it is necessary for all e2fsprogs to read
> >>> and write full inodes so that checksums may be calculated correctly.  Since
> >>> struct ext2_inode_large is a superset of struct ext2_inode, replace the smaller
> >>> one with the larger one.
> >> 
> >> OK, so I need to explain how the large inode support was designed.
> >> 
> >> The original assumption behind it was that most of the time, most user
> >> progams outside of programs shipped natively with e2fsprogs (which are
> >> special) don't actually need to access large inodes directly.  They
> >> are much more likely to use ext2fs_file_{open,read}().  To the extent
> >> that they need to access inodes at all, they will tend to do so
> >> read-only to fetch the size or modtime or user/group ownership fields.
> >> 
> >> And when we write an inode, we need to do a read/modify/write cycle
> >> with the inode table block anyway.  So the idea was to provide full
> >> ABI compatibility with the struct ext4_inode structure, and the
> >> functions that read and write them.  So ext4_inode is a fixed size,
> >> 128 byte structure, and functions that currently take ext4_inode must
> >> only read or write the first 128 bytes.
> >> 
> >> This also implies that probably the better place to put the checksum
> >> calculation code is in ext4_write_inode()/ext4_write_inode_large()
> >> functions.  If we need a new function ext4_write_inode_raw() which has
> >> the same function signature as ext4_write_inode_large(), but which
> >> skips the checksum calculation, so be it.
> > 
> > Okay, so here's my new game plan ... internally, libext2fs needs to read/write
> > whatever sb->inode_size is.  However, to avoid ABI breakage, it will use an
> > internal buffer (stack variable, memory block, whatever) for the IO operation
> > and the checksum operations.  As for whatever buffer+size the caller passes in,
> > it will memcpy this (potentially smaller) amount so that external programs that
> > still use the 128-byte structure won't get corrupted.  The other tools within
> > e2fsprogs can of course allocate the largest inode size they care about
> > (ext2_inode_large) if they so choose.
> > 
> > So I think this means that I can rip out both superpatches.  The inode read and
> > scan functions will need to allocate a buffer that is whatever size the
> > superblock says inodes are so that it can read, and checksum the inode.  After
> > that, the functions will copy however many bytes the caller tells us to copy
> > into the caller's buffer.  The write function will allocate a buffer that is
> > whatever size the superblock says inodes are, read that size off the disk, copy
> > however many bytes the caller gave us into this temporary buffer, then
> > calculate the checksum and write the buffer out to disk.  e2fsprogs that care
> > about extended inode fields can be modified to use ext2_inode_large, and
> > everything else can keep using ext2_inode.  This also keeps it so that client
> > programs don't have to know or care how big inodes really are.
> 
> It would be a very straight forward and useful optimization to avoid the
> extra malloc() and copy if the buffer sizes are the same. 

<shrug> I was just going to abuse the stack for that purpose... but I guess
we'd save a memcpy.

Though I guess I should ask: Is it required that e2fsprogs build on compilers
that won't do variable-sized stack arrays?  I think that limits us to C99
compilers that support the feature, but I'm not 100% sure when gcc added that.
Or if we care about other things like ... Visual Studio? ;)

(I think I asked that same question a few weeks ago, but nobody said much
either way.)

--D
> 
> > Sorry I wasn't aware that there are non-e2fsprogs users of libext2fs.
> > 
> > --D
> > 
> >> Similarly, if we put the checksum verfication in
> >> ext4_read_inode{_large}(), with a new error code if the checksum is
> >> incorrect, all existing callers will get checksum verification for
> >> free.
> >> 
> >> Now, about how ext4_{read,write}_inode_full().  This function is designed so
> >> that struct ext4_inode_large can grow in newer versions of the
> >> library.  To that end, the callers must call it as follows:
> >> 
> >>      struct ext2_inode_large inode;
> >> 
> >>      retval = ext2fs_read_inode_full(fs, ino, &inode, sizeof(inode));
> >> 
> >> If they happen to compile on a system where struct ext2_inode_large is
> >> 140 bytes, then they will pass in a bufsize of 140 bytes --- and it is
> >> up to struct ext2fs_read_inode_full() to only pass back 140 bytes, and
> >> for struct ext2fs_write_inode_full() to only read 140 bytes from the
> >> passed-in pointer.  You can think of the bufsize parameter as being a
> >> built-in versioning mechanism.
> >> 
> >> It is for a similar reason that gethostbyname() takes a socklen_t
> >> parameter.  That way it doesn't have to worry about stack smashing an
> >> legacy ipv4-only program.
> >> 
> >> So you can't just blanket replace struct ext2_inode with struct
> >> ext2_inode_large, and have ext2fs_read_inode() blindly copy out
> >> however many bytes happen to be in sizeof(struct ext2_inode_large)
> >> today.  That way lies all sorts of wierd and hard-to-debug versioning
> >> skew problems....
> >> 
> >> Does this make sense?
> >> 
> >>                    - Ted
> >> 
> >> P.S.  The ext4 wiki is down right now, due to the kernel.org security
> >> recovery efforts, but when it's back up, someone please remind me to
> >> get this text up on the wiki.  :-)
> >> 
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