Fix handling of embedded symbolic links (and history lesson).


Fix handling of embedded symbolic links (and history lesson).

The original filesystem release (4.2BSD) had no embedded sysmlinks.
Historically symbolic links were just a different type of file, so
the content of the symbolic link was contained in a single disk block
fragment. We observed that most symbolic links were short enough that
they could fit in the area of the inode that normally holds the block
pointers. So we created embedded symlinks where the content of the
link was held in the inode's pointer area thus avoiding the need to
seek and read a data fragment and reducing the pressure on the block
cache. At the time we had only UFS1 with 32-bit block pointers,
so the test for a fastlink was:

di_size < (NDADDR + NIADDR) * sizeof(daddr_t)

(where daddr_t would be ufs1_daddr_t today).

When embedded symlinks were added, a spare field in the superblock
with a known zero value became fs_maxsymlinklen. New filesystems
set this field to (NDADDR + NIADDR) * sizeof(daddr_t). Embedded
symlinks were assumed when di_size < fs->fs_maxsymlinklen. Thus
filesystems that preceeded this change always read from blocks
(since fs->fs_maxsymlinklen == 0) and newer ones used embedded
symlinks if they fit. Similarly symlinks created on pre-embedded
symlink filesystems always spill into blocks while newer ones will
embed if they fit.

At the same time that the embedded symbolic links were added, the
on-disk directory structure was changed splitting the former
u_int16_t d_namlen into u_int8_t d_type and u_int8_t d_namlen.
Thus fs_maxsymlinklen <= 0 (as used by the OFSFMT() macro) can
be used to distinguish old directory formats. In retrospect that
should have just been an added flag, but we did not realize we
needed to know about that change until it was already in production.

Code was split into ufs/ffs so that the log structured filesystem could
use ufs functionality while doing its own disk layout. This meant
that no ffs superblock fields could be used in the ufs code. Thus
ffs superblock fields that were needed in ufs code had to be copied
to fields in the mount structure. Since ufs_readlink needed to know
if a link was embedded, fs_maxlinklen gets copied to mnt_maxsymlinklen.

The kernel panic that arose to making this fix was triggered when a
disk error created an inode of type symlink with no allocated data
blocks but a large size. When readlink was called the uiomove was
attempted which segment faulted.

static int
struct vop_readlink_args /* {

		struct vnode *a_vp;
		struct uio *a_uio;
		struct ucred *a_cred;

} */ *ap;
struct vnode *vp = ap->a_vp;
struct inode *ip = VTOI(vp);
doff_t isize;

isize = ip->i_size;
if ((isize < vp->v_mount->mnt_maxsymlinklen) ||

	    DIP(ip, i_blocks) == 0) { /* XXX - for old fastlink support */
		return (uiomove(SHORTLINK(ip), isize, ap->a_uio));

return (VOP_READ(vp, ap->a_uio, 0, ap->a_cred));

The second part of the "if" statement that adds

DIP(ip, i_blocks) == 0) { /* XXX - for old fastlink support */

is problematic. It never appeared in BSD released by Berkeley because
as noted above mnt_maxsymlinklen is 0 for old format filesystems, so
will always fall through to the VOP_READ as it should. I had to dig
back through `git blame' to find that Rodney Grimes added it as
part of ``The big 4.4BSD Lite to FreeBSD 2.0.0 (Development) patch.''
He must have brought it across from an earlier FreeBSD. Unfortunately
the source-control logs for FreeBSD up to the merger with the
AT&T-blessed 4.4BSD-Lite conversion were destroyed as part of the
agreement to let FreeBSD remain unencumbered, so I cannot pin-point
where that line got added on the FreeBSD side.

The one change needed here is that mnt_maxsymlinklen is declared as
an int' and should be changed to be u_int64_t'.

This discovery led us to check out the code that deletes symbolic
links. Specifically

if (vp->v_type == VLNK &&

	    (ip->i_size < vp->v_mount->mnt_maxsymlinklen ||
	     datablocks == 0)) {
		if (length != 0)
			panic("ffs_truncate: partial truncate of symlink");
		bzero(SHORTLINK(ip), (u_int)ip->i_size);
		ip->i_size = 0;
		DIP_SET(ip, i_size, 0);
		if (needextclean)
			goto extclean;
		return (ffs_update(vp, waitforupdate));


Here too our broken symlink inode with no data blocks allocated
and a large size will segment fault as we are incorrectly using the
test that we have no data blocks to decide that it is an embdedded
symbolic link and attempting to bzero past the end of the inode.
The test for datablocks == 0 is unnecessary as the test for
ip->i_size < vp->v_mount->mnt_maxsymlinklen will do the right
thing in all cases.

The test for datablocks == 0 was added by David Greenman in this commit:

Author: David Greenman <dg@FreeBSD.org>
Date: Tue Aug 2 13:51:05 1994 +0000

Completed (hopefully) the kernel support for old style "fastlinks".


svn path=/head/; revision=1821

I am guessing that he likely earlier added the incorrect test in the
ufs_readlink code.

I asked David if he had any recollection of why he made this change.
Amazingly, he still had a recollection of why he had made a one-line
change more than twenty years ago. And unsurpisingly it was because
he had been stuck between a rock and a hard place.

FreeBSD was up to 1.1.5 before the switch to the 4.4BSD-Lite code
base. Prior to that, there were three years of development in all
areas of the kernel, including the filesystem code, from the combined
set of people including Bill Jolitz, Patchkit contributors, and
FreeBSD Project members. The compatibility issue at hand was caused
by the FASTLINKS patches from Curt Mayer. In merging in the 4.4BSD-Lite
changes David had to find a way to provide compatibility with both
the changes that had been made in FreeBSD 1.1.5 and with 4.4BSD-Lite.
He felt that these changes would provide compatibility with both systems.

In his words:
``My recollection is that the 'FASTLINKS' symlinks support in
FreeBSD-1.x, as implemented by Curt Mayer, worked differently than
4.4BSD. He used a spare field in the inode to duplicately store the
length. When the 4.4BSD-Lite merge was done, the optimized symlinks
support for existing filesystems (those that were initialized in
FreeBSD-1.x) were broken due to the FFS on-disk structure of
4.4BSD-Lite differing from FreeBSD-1.x. My commit was needed to
restore the backward compatibility with FreeBSD-1.x filesystems.
I think it was the best that could be done in the somewhat urgent
circumstances of the post Berkeley-USL settlement. Also, regarding
Rod's massive commit with little explanation, some context: John
Dyson and I did the initial re-port of the 4.4BSD-Lite kernel to
the 386 platform in just 10 days. It was by far the most intense
hacking effort of my life. In addition to the porting of tons of
FreeBSD-1 code, I think we wrote more than 30,000 lines of new code
in that time to deal with the missing pieces and architectural
changes of 4.4BSD-Lite. We didn't make many notes along the way.
There was a lot of pressure to get something out to the rest of the
developer community as fast as possible, so detailed discrete commits
didn't happen - it all came as a giant wad, which is why Rod's
commit message was worded the way it was.''

Reported by: Chuck Silvers
Tested by: Chuck Silvers
History by: David Greenman Lawrence
MFC after: 1 week
Sponsored by: Netflix


mckusickAuthored on May 17 2021, 12:02 AM
R10:46269d66ed02: NFSv4 server: Re-establish the delegation recall timeout