Date: Sat, 1 Jan 2011 18:54:04 +0000 From: Roger Leigh <rleigh@debian.org> To: Bruce Evans <brde@optusnet.com.au> Cc: freebsd-bugs@freebsd.org, freebsd-gnats-submit@freebsd.org Subject: Re: bin/153600: Path length restrictions in mount/umount tools prevent filesystem mount/unmount Message-ID: <20110101185404.GH11671@codelibre.net> In-Reply-To: <20110102050301.B1641@besplex.bde.org> References: <201101011746.p01Hk5UQ008490@red.freebsd.org> <20110102050301.B1641@besplex.bde.org>
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--to+bXLvrczl8f0V1 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 an= d/or are truncating the path at some point. >> >> The mount(2) manual page documents the max path length at 1023 character= s, 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 r= eport is very low. The documented [ENAMETOOLONG] limit in mount(2) is sens= ible, but does not appear to reflect the practical reality at the present t= ime. If the 80-85 character limit could be eliminated to allow this to wor= k 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) =3D 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 mou= nt > 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 --=20 .''`. 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. --to+bXLvrczl8f0V1 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAk0feEwACgkQVcFcaSW/uEhzKgCfRUHtNiwnSjHt//aTpbXqQNSY AUEAoM+ODWdhnvMlyk6RMRMAXSKwfpIq =bMeU -----END PGP SIGNATURE----- --to+bXLvrczl8f0V1--
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