linux-kernel - ROMFS error reading 0 byte files

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Date:	Fri, 25 Jul 2008 14:54:37 -0500
From:	Chris Fester <cfester@....com>
To:	linux-kernel@...r.kernel.org
Subject: ROMFS error reading 0 byte files

Hi,

I've been poking around at a problem I've been having with the
latest kernels and romfs.  The symptom is getting an Input/output
error when reading a zero length file:

# find . -size 0
./usr/lib/gconv/gconv-modules.cache
# cat ./usr/lib/gconv/gconv-modules.cache 
cat: ./usr/lib/gconv/gconv-modules.cache: Input/output error

The problem seems to have occurred somewhere between 2.6.22 and
2.6.23.  Not much changed in the romfs code then, but the
mm/filemap.c had a bit of changes.  Personally I saw this problem
with Debian Lenny kernels 2.6.24 and 2.6.25.  Looking at the
kernel.org source I suspect the problem is still in 2.6.26.

I'm not a filesystem or mm expert, but I would like to get a good
solution going... if it means I get more familiar with mm and 
filesystems then so be it.  :)

Matt Waddel proposed this patch back in November of 2007 on the
uclinux-dev mail list:

diff --git a/fs/romfs/inode.c b/fs/romfs/inode.c
index dae7945..ed04d73 100644
--- a/fs/romfs/inode.c
+++ b/fs/romfs/inode.c
@@ -430,7 +430,7 @@ romfs_readpage(struct file *file, struct page * page)

		/* 32 bit warning -- but not for us :) */
		offset = page_offset(page);
-		if (offset < i_size_read(inode)) {
+		if (offset <= i_size_read(inode)) {
			avail = inode->i_size-offset;
			readlen = min_t(unsigned long, avail, PAGE_SIZE);
			if (romfs_copyfrom(inode, buf, ROMFS_I(inode)->i_dataoffset+offset, readlen) == readlen) {

I mailed Matt recently and he's been using this patch for a while now
with no failures.

I'm not sure if this patch is the best solution.  I was looking at 
cramfs's inode.c:cramfs_readpage, and it looks like it's structured
a little more optimally in the 0 length case.  

For reference to the original discussion, see the uclinux-dev mail list:
http://mailman.uclinux.org/pipermail/uclinux-dev/2007-December/044836.html

Since the maintainer for romfs seems to have moved on, could someone with
vfs knowledge please direct me on what I should do next?  The company I
work for uses romfs extensively in their products.  So far they've been
lucky because the newest kernel being used is 2.6.18.  But there's 
rumblings to upgrade to a newer kernel, and I'd like to be prepared with
a good solid fix in mainline rather than patching.

Thanks in advance for any help!
Chris Fester

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