Re: [PATCH v3 06/15] ext2fs: add new APIs needed for fast commits

["Theodore Ts'o" <tytso@mit.edu>]

On Wed, Jan 20, 2021 at 01:26:32PM -0800, Harshad Shirwadkar wrote:
> From: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> 
> This patch adds the following new APIs:
> 
> Count the total number of blocks occupied by inode including
> intermediate extent tree nodes.
> extern blk64_t ext2fs_count_blocks(ext2_filsys fs, ext2_ino_t ino,
>                                        struct ext2_inode *inode);

I wonder if this should be something like this instead:

extern errcode_t ext2fs_count_blocks(ext2_filsys fs, ext2_ino_t ino,
                                     struct ext2_inode *inode, blk64_t *ret_count);

The problem is that ext2fs_count_blocks() calls a whole series of
ext2fs functions which could return errors:

> +	errcode = ext2fs_extent_open2(fs, ino, inode, &handle);
> +	if (errcode)
> +		goto out;
> +
> +	errcode = ext2fs_extent_get(handle, EXT2_EXTENT_ROOT, &extent);
> +	if (errcode)
> +		goto out;

... and any of these functions could return an error.  So we need to
make sure errors are faithfully returned to the caller and handled
correctly, instead of just having ext2fs_count_blocks returning a
value of 0.


I then started taking a look at the users of ext2fs_count_blocks() in
e2fsck, and I ran into more concerns.  One of the problems here is
that some of these functions get called by kernel code --- and kernel
code has a different error handling convetion of negative errno's.

And in some cases, I see we are doing this:

static int ext4_fc_handle_inode(e2fsck_t ctx, struct ext4_fc_tl *tl)
{
	...
	
	ret = ext2fs_read_inode_full(ctx->fs, ino, (struct ext2_inode *)inode,
					inode_len);
	if (ret)
		goto out;
	...
out:
	ext2fs_free_mem(&inode);
	return ret;
}

The problem here is that ext2fs_read_inode_full() returns an
errcode_t, and this is getting cast to an int and returned as if it
were a kernel error code.

Also note that ext4_fc_replay() can return 0 or 1:

#define JBD2_FC_REPLAY_STOP		0
#define JBD2_FC_REPLAY_CONTINUE		1

Fortunately, none of the functions that ext4_fc_*() call seem to be
ones which could return in an ext2fs library returning EPERM (which is
errno 1), but you see the potential risks of conflating an errcode_t
and kernel negative errno convention.

This is going to be a bit tricky to deal with, since an errcode_t can
be a errno code, but it can also be one of the codes defined in
lib/ext2fs/ext2_err.et, which get translated to numbers like:

#define EXT2_ET_DIR_CORRUPTED                    (2133571363L)
#define EXT2_ET_SHORT_READ                       (2133571364L)
#define EXT2_ET_SHORT_WRITE                      (2133571365L)

(See lib/ext2fs/ext2_err.h in the build directory of e2fsprogs and the
com_err library found in lib/et.)

So what we may need to do is to create a function which does a simple
mapping of errcode_t values to negative errno's.  It doesn't need to
be exact; in fact, a first pass might just map all errcode_t's greater
than 256 to something like -EFAULT, and all normal errno's to -errno.

We might also want to have it print a diagnistic message to stderr
that prints error_message(retval) was encoutered in function __func__
at line __LINE__.  Hopefully in actual practice they won't happen
(unless a malicious attacker is feeding us a fuzzed file sytem), but
if it does, it would be good if there is a useful message so we can
actually debug what happened.

      	   	  	     	    	     - Ted