tytso@xxxxxxx wrote:
> On Thu, Mar 04, 2010 at 02:40:22PM +0900, Akira Fujita wrote:
>> <user-space>
>> filefrag:
>> fiemap->fm_start(0) fiemap->fm_length(~0ULL)
>>
>> <kernel-space>
>> fs/ioctl.c ioctl_fimap():
>>
>> filemap_check_ranges():
>> len(~0ULL)
>> new_len(4398046511103 = s_maxbytes) <--- Because 'len > s_maxbytes'
>>
>> fs/ext4/extents.c ext4_fiemap():
>> last_blk = start(0) + len(4398046511103) - 1 >> s_blocksize_bits(11)
>> = 4294967295 (0xFFFFFFFF)
>> len_blks = 4294967295 + 1 (0xFFFFFFFF + 0x00000001)
>> = 4294967296 (0x100000000) <--- _OVERFLOW!!_
>>
>> ext4_ext_walk_space():
>> num = 0
>>
>> This overflow leads to incorrect output like the below,
>> even though 2 extents exist.
>
> Akira-san,
>
> Thank you for your clear explanation; you're absolutely correct. I've
> replaced the patch with the following, which I think is a bit clearer.
> I've tested to make sure it does the right thing, including a number
> of corner cases.
Ted, I think those testcases you used would be great xfstests candidates,
hint hint. :)
-Eric
> Regards,
>
> - Ted
>
> commit aca92ff6f57c000d1b4523e383c8bd6b8269b8b1
> Author: Leonard Michlmayr <leonard.michlmayr@xxxxxxxxx>
> Date: Thu Mar 4 17:07:28 2010 -0500
>
> ext4: correctly calculate number of blocks for fiemap
>
> ext4_fiemap() rounds the length of the requested range down to
> blocksize, which is is not the true number of blocks that cover the
> requested region. This problem is especially impressive if the user
> requests only the first byte of a file: not a single extent will be
> reported.
>
> We fix this by calculating the last block of the region and then
> subtract to find the number of blocks in the extents.
>
> Signed-off-by: Leonard Michlmayr <leonard.michlmayr@xxxxxxxxx>
> Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index bd80891..7d54850 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3768,7 +3768,6 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> __u64 start, __u64 len)
> {
> ext4_lblk_t start_blk;
> - ext4_lblk_t len_blks;
> int error = 0;
>
> /* fallback to generic here if not in extents fmt */
> @@ -3782,8 +3781,14 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> if (fieinfo->fi_flags & FIEMAP_FLAG_XATTR) {
> error = ext4_xattr_fiemap(inode, fieinfo);
> } else {
> + ext4_lblk_t len_blks;
> + __u64 last_blk;
> +
> start_blk = start >> inode->i_sb->s_blocksize_bits;
> - len_blks = len >> inode->i_sb->s_blocksize_bits;
> + last_blk = (start + len - 1) >> inode->i_sb->s_blocksize_bits;
> + if (last_blk >= EXT_MAX_BLOCK)
> + last_blk = EXT_MAX_BLOCK-1;
> + len_blks = ((ext4_lblk_t) last_blk) - start_blk + 1;
>
> /*
> * Walk the extent tree gathering extent information.
> --
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