Messages in this thread | | | Date | Sat, 06 Nov 2010 00:32:49 +0000 | From | Pádraig Brady <> | Subject | Re: Detecting bind-mounts |
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On 05/11/10 20:30, Michael Tokarev wrote: > 05.11.2010 21:30, Michael Tokarev wrote: >> 05.11.2010 13:24, Pádraig Brady wrote: >>> On 04/11/10 20:45, Michael Tokarev wrote: >> [] >>>> 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 `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" >> >> This is incorrect in two ways. >> >> First of all, stat(1), even after that commit you quote, >> still compares st_dev fields, which are the same for this >> and parent directory in case of bind mount. So this version >> of stat(1) does _not_ detect a bind mount, unfortunately. > > And this statement, in turn, is untrue. I apologize for the > misinformation, it wasn't intentional. The mentioned commit > adds the ability to detect bind mounts indeed. but... > >> Second, I asked for a low-level way to detect such a mount. >> I know how to do it not as efficient as stat(2) and not as >> reliable but much simpler than you propose above, in shell >> or in C, and I already provided that way in my original >> email: we just parse /proc/mounts file, this is faster and >> more reliable than the above shell fragment which calls a >> few external commands. > > .. the way used by stat(1) is to enumerate /proc/mounts -- > which is what I were able to come with initially. It is slow > and unreliable. Hence I asked if a faster way exist.
Sorry I was unclear.
I was just providing reference info about the changes in the new `stat -c %m` that both showed tools do now exist to determine if a file/dir is a mount point, and to validate your "slow way" of detecting (bind) mounts.
As for a fast way, I don't think one exists. BTW I'm not sure your examples are actually valid. If a file/dir is bind mounted to the same file system, then `find -xdev` should be listing it (as it has the same dev).
You want a separate option --same-mount or something, though I don't know what it would be useful for.
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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