Re: [PATCH] ext2_readdir() filp->f_pos fix

Jan Blunck
2004-07-30T10:50:27Z

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Andrew Morton wrote:
> Interesting. How did you actually notice this? Is the same problem not present
> in 2.4?

I noticed this problem because I'm "abusing" the f_pos in a VFS based
implementation of something like union-mount. There are already some
things done but I still need time until its ready for being posted
here.
Didn't checked that for 2.4 cause I'm only working on 2.6 at the moment.

> If the IS_ERR(page) returns true, should we not advance f_pos to skip this
> page?

Ok. I changed that in the attached patch. So when there is a bad page it
is possible that the f_pos points somewhere to the beginning of the next
page which might be out of range of the i_size. But I think that is ok
for a error condition.

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dir.c | 11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)

--- 1.24/fs/ext2/dir.c 2004-07-29 21:25:31 +02:00
+++ 1.26/fs/ext2/dir.c 2004-07-30 11:54:27 +02:00
@@ -251,7 +251,7 @@
loff_t pos = filp->f_pos;
struct inode *inode = filp->f_dentry->d_inode;
struct super_block *sb = inode->i_sb;
- unsigned offset = pos & ~PAGE_CACHE_MASK;
+ unsigned int offset = pos & ~PAGE_CACHE_MASK;
unsigned long n = pos >> PAGE_CACHE_SHIFT;
unsigned long npages = dir_pages(inode);
unsigned chunk_mask = ~(ext2_chunk_size(inode)-1);
@@ -270,8 +270,13 @@
ext2_dirent *de;
struct page *page = ext2_get_page(inode, n);

- if (IS_ERR(page))
+ if (IS_ERR(page)) {
+ ext2_error(sb, __FUNCTION__,
+ "bad page in #%lu",
+ inode->i_ino);
+ filp->f_pos += PAGE_CACHE_SIZE - offset;
continue;
+ }
kaddr = page_address(page);
if (need_revalidate) {
offset = ext2_validate_entry(kaddr, offset, chunk_mask);
@@ -303,6 +308,7 @@
goto success;
}
}
+ filp->f_pos += le16_to_cpu(de->rec_len);
}
ext2_put_page(page);
}
@@ -310,7 +316,6 @@
success:
ret = 0;
done:
- filp->f_pos = (n << PAGE_CACHE_SHIFT) | offset;
filp->f_version = inode->i_version;
return ret;
}