[bug #42466] GRUB cannot load symlinks of length 60 on ext4

[KOVACS Krisztian]

URL:
  <http://savannah.gnu.org/bugs/?42466>

                 Summary: GRUB cannot load symlinks of length 60 on ext4
                 Project: GNU GRUB
            Submitted by: hidden_balabit
            Submitted on: Fri 30 May 2014 01:24:13 PM GMT
                Category: Filesystem
                Severity: Major
                Priority: 5 - Normal
              Item Group: Software Error
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 
                 Release: Git master
         Reproducibility: Every Time
         Planned Release: None

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Details:

We have a setup where the kernel to be loaded is accessed through a path
containing a symbolic link. The config is something like this:

- 8< -
search --set=root --label boot

linux '/boot/active/kernel'
initrd '/boot/active/initrd'
- 8< -

If '/boot/active' is a symbolic link to a directory whose name is exactly 60
characters long, loading the kernel fails and GRUB cannot boot. Directories
whose name is less than 60 or more than 60 work perfectly.

It seems to be that there is a difference in how the Linux ext4 implementation
handles in-inode symbolic links and what the GRUB implementation expects.

In the Linux implementation of ext4, symbolic links whose target is *shorter*
than 60 bytes are stored directly in the inode. (See
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/namei.c?id=455c6fdbd219161bd09b1165f11699d6d73de11c#n2831)

It's worth noting that the code performs the comparison '(l > 60)', where l
equals strlen(symname)+1. That means in practice that if the lenth of the
symlink target is 60, the comparison is true, meaning that it is stored as a
non-fast symlink in a separate block.

In the GRUB implementation, however, the following comparison is made to check
if the symlink target is stored in the inode structure: diro->inode.size <=
60. (sizeof(diro->inode.symlink) == 60) Unfortunately, if size is 60, this
leads to GRUB looking for the symlink target in the inode, but the ext4
implementation has not stored that info there.

I've attached a patch that seems to fix the issue by making sure that GRUB
handles symlink targets of length 60 as non-fast.



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File Attachments:


-------------------------------------------------------
Date: Fri 30 May 2014 01:24:13 PM GMT  Name:
grub-ext4-symlink-target-of-length-60.diff  Size: 671B   By: hidden_balabit
Patch fixing the issue with 60 char symlink targets.
<http://savannah.gnu.org/bugs/download.php?file_id=31471>

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