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2 README for Linux 2.6 kernels
6 NOTE: This information is only valid if you are using Dazuko with syscall
7 hooking. i.e. you are configuring Dazuko with --enable-syscalls.
8 If you are using Dazuko with RedirFS (recommended), then
9 you will not have any of the problems listed here.
12 In order to lookup full file paths (even in chroot environments), Dazuko
13 uses the kernel function __d_path(). Unfortunately this function is no
14 longer exported to kernel modules in the Linux 2.6 kernel.
16 Dazuko has its own copy of this function but unfortunately it is only safe
17 for UP (uni-processor or non-SMP) kernels. This is because the real __d_path()
18 function uses a kernel lock, which is not available to kernel modules.
20 If your kernel is not configured as SMP, then it is safe to use Dazuko's
21 copy of the __d_path() function. This is default.
23 If your kernel is SMP then you should _not_ use Dazuko's copy of __d_path().
24 You can disable Dazuko's local copy of __d_path() by configuring Dazuko with:
26 $ ./configure --disable-local-dpath
28 Since Dazuko requires __d_path(), your kernel will have to export it if you
29 disable the local copy. Some GNU/Linux distributions have a modified kernel
30 that does export this function. The "vanilla" Linux kernel does _not_ export
31 this function. You can check if your kernel exports this function with:
33 $ grep __d_path /proc/kallsyms
35 The results can be interpreted as:
37 xxxxx T __d_path <= exported
38 xxxxx t __d_path <= not exported
40 If you are running an SMP kernel and __d_path() is not exported, then you
41 will have to modify your kernel source code to export the symbol and
42 rebuild your kernel. The modification is very simply and can be made by
43 using the included patch to modify the fs/dcache.c file. Here is an example:
45 $ patch /lib/modules/`uname -r`/build/fs/dcache.c patch_dpath.diff
47 The location of your Linux kernel source code may be different.
49 Once you have patched the fs/dcache.c file, you can rebuild and install your
50 kernel and kernel modules.
52 If your GNU/Linux distribution doesn't export the __d_path() function, you
53 may want to write them email suggesting that this function be exported in
54 the next release.
56 Note: As a workaround, Dazuko now offers the configuration option
57 --disable-chroot-support. This will disable Dazuko from trying to
58 resolve chroot'd filenames, thus making Dazuko safe for SMP kernels
59 that do not export __d_path(). However, if this option is used,
60 registered applications will only be able to work with chroot'd
61 filenames if they resolve the filenames from within userland. If
62 the application you plan to use with Dazuko requires handling
63 chroot'd filenames (and you have an SMP kernel that cannot export
64 __d_path()), then you should contact the application developer to
65 make sure it supports this option.