Messages in this thread | | | Date | Fri, 05 Nov 2010 10:24:29 +0000 | From | Pádraig Brady <> | Subject | Re: Detecting bind-mounts |
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On 04/11/10 20:45, Michael Tokarev wrote: > Hello. > > There are quite some talks on the 'net - questions, not > answers - about detecting bind mounts - be it a directory > or a file. > > There are 2 (mostly) different kinds of applications. One > is cp/tar/find with --same-filesystem option (or equivalent), > that should not cross mountpoints. And one more, apps like > mountpoint(1) from sysvinit - a utility to determine if a > given path is a mountpoint. > > Neither of the two work when two directores on the same > filesystem are bind-mounted. > > The usual idiom is to compare st_dev of current directory and > the parent - if they're different that's a mount point. But > in this case, two st_devs will be the same, so such a mount > point will not be detected. > > It is even worse for bind-mounted files (as opposed to dirs): > there's no path/file/.. entry to stat(2), and cutting the > last component from the pathname does not work reliable due > to symlinks (it may be a symlink from different filesystem). > > So far I know only one way to detect a bind mount like this, > and it is unreliable anyway. It is to parse /proc/mounts > and try to find the object(s) in question. Unreliable because > of, again, symlinks, and possible complex mounts and bind- > mounts. And this is also very slow - imagine using this way > for find/tar/cp --one-file-system. > > Is there some simpler and more reliable way? Maybe use mount > syscall, like we use kill($pid, 0) to check existance of a > process? > > And as far as I understand, the same applies to multiple > mounts of the same filesystem.
The `stat` command recently got support for printing the mount point for a file: http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commit;h=ddf6fb86
`stat` will output the alias for a bind mounted file while `df` will output the initial mount point of its backing device So you could do something like:
file=. df_mnt=$(df -P "$file" | sed -n '2s/.* \([^ ]*$\)/\1/p') stat_mnt=$(stat -c%m "$file") test "$df_mnt" = "$stat_mnt" || echo "bind mount"
cheers, Pádraig. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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