Bug 198301

Yes, this is a design limitation of ext4. In principle, on disk format of a symlink could be extended to allow for larger symlinks but that would require an incompatible on-disk format change and someone determined enough to implement all this in the kernel, e2fsck, libext2fs, etc. Also the motivation for the work is decreased by the fact that commonly deployed filesystems with 4k block size do not exhibit the problem. So I personally don't think fixing this is worth the effort.

Created attachment 273589 [details]
Proposed patch

Here's a proposed patch.  When the symlink's path length is longer than a block, it uses the existing code to fall back on a slow symlink.  This is the better behavior, rather than throwing an error.


Block size should only be a limitation on the path if using a fast symlink, which includes the path in the inode.  More info on fast/slow symlinks at:

https://unix.stackexchange.com/questoins/147535/fast-and-slow-symlinks

Hi Josh, did you actually test if this patch works with a symlink longer than a single block?  

The existing slow symlink code is only written to handle a single block, so just removing the check doesn't fix that. There would also need to be a new INCOMPATIBLE feature flag added, so that the kernel and e2fsck know that there are symlink a longer than a single block. Otherwise they would consider the link corrupted.

Andreas, I did test it and it appeared to work: I could create and follow the symlink where I received the error before.  I'll take another look to make sure I was doing what I think I was doing.

If you could give me a pointer to where the kernel and e2fsck expect the symlink to fit in one block, that would be a great help.

Here's the way I tested this:

1) Create a partition at sdb

2) Create an ext4 filesystem with 1k block size
$  sudo mkfs.ext4 -b 1024 /dev/sdb

3) Mount it
$  mkdir ~/smallblock
$  sudo mount /dev/sdb ~/smallblock
$  cd ~/smallblock

4) Verify configuration
$  stat -f . 
  File: "."
    ID: b3305705f96e1be2 Namelen: 255    Type: ext2/ext3
Block size: 1024        Fundamental block size: 1024
Blocks: Total: 93095    Free: 91545    Available: 84520
Inodes: Total: 25168    Free: 25157

5) Expose bug
$  ln -s 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
44f4517205b7b6e21a9cc55e2b379e2c112d0e407153d367bffb5d1bb9941522b6a6b251aa2ff91453322ec7c65a86329f4ecb16acdb4fe4d42673f69da3e8b pjdfstest_1ed95816e0eb09f8b38de6019ae5675b

This causes a name too long error

6) Patch, build, and boot into the new kernel.

7) Repeat the above, broken symlink gets created without error.

8) Remove symlink, create a folder with:
$ mkdir [The long text above] 
Create the symlink again.  Now it works.  I can move in and out of it, create files there, etc.  

If I remove the folder the link breaks but behaves as expected.  If I make the folder a little shorter and create a file in it, I can create and remove the symlink to the file, as well.

If you can suggest some other tests I should consider, I'd appreciate it.

Slight error above on step 8.  Should be:
$  mkdir -p [The long text above]

> The strange side effect of this bug is that one can not link to every file in
> a file system with a block size of e.g. 1k while PATH_MAX is 4k.

Note that you cannot necessarily link to every file in the filesystem even with a 4k block size, since the absolute path to a file can be over PATH_MAX.  PATH_MAX is a limit on the path string passed to syscalls, not a limit on the directory structure.  By using cwd-relative paths, fd-relative paths, chroots, or bind mounts, you can create a directory structure that is much deeper than PATH_MAX.  None of the major Linux filesystems enforce a directory depth limit, as far as I know -- and even if one did, it could still be mounted at a mountpoint whose absolute path is already PATH_MAX, or close to it.