On Sun, Jan 02, 2011 at 05:13:16AM +1100, Bruce Evans wrote: > On Sat, 1 Jan 2011, Roger Leigh wrote: > >> t...@mason.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC amd64 >>> Description: >> When mount is asked to mount a filesystem on a node whose absolute path is >> longer than 85 characters in length, the mount fails. Umount also fails >> under some circumstances, though the testcase attached below doesn't show >> this. >> ... >>> Fix: >> I suspect that the mount/umount tools are using a fixed-length buffer and/or >> are truncating the path at some point. >> >> The mount(2) manual page documents the max path length at 1023 characters, >> and the maximum length of any single component at 255 characters. These >> limits have not been exceeded, unless the documentation is incorrect. >> >> The practical upper limit of 80-85 characters demonstrated in this bug >> report is very low. The documented [ENAMETOOLONG] limit in mount(2) is >> sensible, but does not appear to reflect the practical reality at the >> present time. If the 80-85 character limit could be eliminated to allow >> this to work as documented, this would remove a significant limitation in >> the FreeBSD system which is breaking software which requires longer paths to >> function. > > Mount name lengths are in practice limited to (MNAMELEN - 1) = 87. See > <sys/mount.h>. This isn't easy to fix, since MNAMELEN is in critical APIs > (mainly struct statfs). struct statfs has already been changed once too > often. MNAMELEN used to be (80 - 2 * sizeof(long)), which is 80 or 72, > but was changed to 88. MNAMELEN is of course mentioned in statfs(2), but > it isn't mentioned in mount(2) because it doesn't apply to the actual mount > operation but only to determining what is mounted using statfs(2). The > buffer gets truncated at mount time by mount in the kernel copying the > file name to the statfs buffer with blind truncation. > > In practice, this means that you should never use the feature of mounting > pathnames with length between MNAMELEN and (PATH_MAX - 1), since it is too > hard to manage the resulting mountpoints.
I see, thanks. This does make things somewhat more complex to fix. As a longer term (rather than immediate) solution, could I suggest taking a look at the GNU libc/linux versions of the statfs structure in <bits/statfs.h>? In this version, the fixed-length fields are entirely absent, and so the length limitations are a non-issue here. Of course, the structures are not compatible, and the missing information would need to be obtained via other means such as getmntent of /proc/mounts (for example). But, the getmntent interface is also equally unrestricted, so in practice on GNU/Linux this problem does not exist. Kind regards, Roger -- .''`. Roger Leigh : :' : Debian GNU/Linux http://people.debian.org/~rleigh/ `. `' Printing on GNU/Linux? http://gutenprint.sourceforge.net/ `- GPG Public Key: 0x25BFB848 Please GPG sign your mail.
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