Re: [WTF] ->setattr() locking changes

Re: [WTF] ->setattr() locking changes

[Andries.Brouwer]

    > Hmm...  While we are at it, why don't we remove suid/sgid on truncate(2)?

    Are there any standards saying either way? But yes, it sounds logical.

"This function shall not modify the file offset for any open file
 descriptions associated with the file. Upon successful completion,
 if the file size is changed, this function shall mark for update
 the st_ctime and st_mtime fields of the file, and the S_ISUID and
 S_ISGID bits of the file mode may be cleared."

Andries

[WTF] ->setattr() locking changes

[Alexander Viro]

	a) where the hell is update to Documentation/filesystems/* ?
	b) Dave, meet grep.  grep, meet Dave.  Have a happy relationship...

_Please_, grep before doing global changes.  Trivial search for
notify_change() would show several calls under ->i_sem.  E.g. one
in fs/nfsd/vfs.c.  Or in hpfs_unlink().

All other problems with BKL brigade aside, could you at least learn to
use basic tools?  It's not a rocket science, after all - you do global
interface changes, you need to do global search.

Re: [WTF] ->setattr() locking changes

[Andrew Morton]

ext3 was missed - the removal of the BKL in notify_change
means that the filesytem fails quite quickly on SMP in -pre2.
Sorry, I should have spotted that when the patch floated past.

Please do it this way:

--- linux-2.5.8-pre2/fs/ext3/inode.c	Fri Apr  5 17:42:19 2002
+++ 25/fs/ext3/inode.c	Fri Apr  5 22:04:47 2002
@@ -2377,6 +2377,8 @@ int ext3_setattr(struct dentry *dentry, 
 			return error;
 	}
 
+	lock_kernel();
+
 	if (attr->ia_valid & ATTR_SIZE && attr->ia_size < inode->i_size) {
 		handle_t *handle;
 
@@ -2404,6 +2406,7 @@ int ext3_setattr(struct dentry *dentry, 
 
 err_out:
 	ext3_std_error(inode->i_sb, error);
+	unlock_kernel();
 	if (!error)
 		error = rc;
 	return error;


-

Re: [WTF] ->setattr() locking changes

[Linus Torvalds]

On Sat, 6 Apr 2002, Alexander Viro wrote:
>
> 	a) where the hell is update to Documentation/filesystems/* ?
> 	b) Dave, meet grep.  grep, meet Dave.  Have a happy relationship...

Al, don't blame Dave, blame me. I told Dave to use i_sem instead of a 
special semaphore, just because it seems to be the right thing.

> _Please_, grep before doing global changes.  Trivial search for
> notify_change() would show several calls under ->i_sem.  E.g. one
> in fs/nfsd/vfs.c.  Or in hpfs_unlink().

Hmm.. I don't think the fs/nfsd/vfs.c case is "obviously" under i_sem.  
Certainly not from grepping. Where?

The hpfs/namei.c one definitely needs the removal of the up/down, though. 
Altghough for the life of me I don't see why the filesystem _does_ that at 
all, since it should have been done by the upper layers already, no?

Oh, and Andrew pointed out that in the ext3 case the BKL was forgotten.

		Linus

Re: [WTF] ->setattr() locking changes

[Alexander Viro]

On Sat, 6 Apr 2002, Linus Torvalds wrote:
 
> > _Please_, grep before doing global changes.  Trivial search for
> > notify_change() would show several calls under ->i_sem.  E.g. one
> > in fs/nfsd/vfs.c.  Or in hpfs_unlink().
> 
> Hmm.. I don't think the fs/nfsd/vfs.c case is "obviously" under i_sem.  
> Certainly not from grepping. Where?

fh_lock()/fh_unlock().  Hell, it does call notify_change() with ATTR_SIZE,
so unless i_sem was taken by caller we would be in big trouble, wouldn't
we?  Old rules were "if you call it to truncate a file, grab i_sem".

> The hpfs/namei.c one definitely needs the removal of the up/down, though. 
> Altghough for the life of me I don't see why the filesystem _does_ that at 
> all, since it should have been done by the upper layers already, no?

<looking> oh, crap.

OK, here's the story:
	* HPFS can run out of space trying to remove entry from directory.
Joys of badly implemented B-Trees.  So as a last-ditch we try to truncate
victim in hope that it will give us some space.  _If_ there is nobody else
who'd have access to it.  Thus the truncate() attempt.
	* my flame re grep(1) applies to myself - first deadlock in there
had been created by new locking rules patch (in fs/namei.c).
	* Dave's patch had added one more level of the same.  Happy, happy,
joy, joy...

	Looking at that stuff, I'd suggest to
a) kill that branch in hpfs_unlink().
b) remove fh_lock()/fh_unlock() in nfsd/vfs.c::nfsd_setattr() (Trond?)
c) add ATTR_SXID that would do s[ug]id removal - under ->i_sem and switch
   the callers to it.

Comments?  If you don't see any problems with this variant I'll do it.

Re: [WTF] ->setattr() locking changes

[Alexander Viro]

On Sat, 6 Apr 2002, Alexander Viro wrote:

> 	Looking at that stuff, I'd suggest to
> a) kill that branch in hpfs_unlink().
> b) remove fh_lock()/fh_unlock() in nfsd/vfs.c::nfsd_setattr() (Trond?)
> c) add ATTR_SXID that would do s[ug]id removal - under ->i_sem and switch
>    the callers to it.
> 
> Comments?  If you don't see any problems with this variant I'll do it.

OTOH, we might be better off taking ->i_sem in all callers of notify_change().
There's only 10 of them, so it's not too much work and that would have a
benefit of allowing to do things like suid removal on write(2) in a right way.

Hmm...  While we are at it, why don't we remove suid/sgid on truncate(2)?

Re: [WTF] ->setattr() locking changes

[Linus Torvalds]

On Sat, 6 Apr 2002, Alexander Viro wrote:
> >
> > Comments?  If you don't see any problems with this variant I'll do it.
>
> OTOH, we might be better off taking ->i_sem in all callers of notify_change().

That was my first reaction on Dave's patch, but on the other hand it then
looked so simple to just let notify_change() do the locking (none of the
places I looked at wanted to do anything else), that it looked better
inside notify_change.

I agree with you that doing the locking outside would clean some stuff up,
since things like write already have the lock for other reasons.

> Hmm...  While we are at it, why don't we remove suid/sgid on truncate(2)?

Are there any standards saying either way? But yes, it sounds logical.

		Linus

Re: [WTF] ->setattr() locking changes

[Alexander Viro]

On Sat, 6 Apr 2002, Linus Torvalds wrote:

> I agree with you that doing the locking outside would clean some stuff up,
> since things like write already have the lock for other reasons.

OK, how about that?  It builds, but it's completely untested.

diff -urN C8-pre2/Documentation/filesystems/Locking C8-pre2-current/Documentation/filesystems/Locking
--- C8-pre2/Documentation/filesystems/Locking	Sat Apr  6 00:29:14 2002
+++ C8-pre2-current/Documentation/filesystems/Locking	Sat Apr  6 14:02:43 2002
@@ -65,7 +65,7 @@
 readlink:	no	no
 follow_link:	no	no
 truncate:	no	yes		(see below)
-setattr:	yes	if ATTR_SIZE
+setattr:	no	yes
 permission:	yes	no
 getattr:				(see below)
 revalidate:	no			(see below)
diff -urN C8-pre2/Documentation/filesystems/porting C8-pre2-current/Documentation/filesystems/porting
--- C8-pre2/Documentation/filesystems/porting	Tue Mar 19 16:05:44 2002
+++ C8-pre2-current/Documentation/filesystems/porting	Sat Apr  6 14:04:40 2002
@@ -116,3 +116,10 @@
 	FS_SINGLE is gone (actually, that had happened back when ->get_sb()
 went in - and hadn't been documented ;-/).  Just remove it from fs_flags
 (and see ->get_sb() entry for other actions).
+
+---
+[mandatory]
+
+->setattr() is called without BKL now.  Caller _always_ holds ->i_sem, so
+watch for ->i_sem-grabbing code that might be used by your ->setattr().
+Callers of notify_change() need ->i_sem now.
diff -urN C8-pre2/fs/attr.c C8-pre2-current/fs/attr.c
--- C8-pre2/fs/attr.c	Sat Apr  6 00:29:28 2002
+++ C8-pre2-current/fs/attr.c	Sat Apr  6 13:23:18 2002
@@ -119,6 +119,7 @@
 int notify_change(struct dentry * dentry, struct iattr * attr)
 {
 	struct inode *inode = dentry->d_inode;
+	mode_t mode = inode->i_mode;
 	int error;
 	time_t now = CURRENT_TIME;
 	unsigned int ia_valid = attr->ia_valid;
@@ -131,8 +132,25 @@
 		attr->ia_atime = now;
 	if (!(ia_valid & ATTR_MTIME_SET))
 		attr->ia_mtime = now;
+	if (ia_valid & ATTR_KILL_SUID) {
+		if (mode & S_ISUID) {
+			if (!ia_valid & ATTR_MODE) {
+				ia_valid = attr->ia_valid |= ATTR_MODE;
+				attr->ia_mode = inode->i_mode;
+			}
+			attr->ia_mode &= ~S_ISUID;
+		}
+	}
+	if (ia_valid & ATTR_KILL_SGID) {
+		if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
+			if (!ia_valid & ATTR_MODE) {
+				ia_valid = attr->ia_valid |= ATTR_MODE;
+				attr->ia_mode = inode->i_mode;
+			}
+			attr->ia_mode &= ~S_ISGID;
+		}
+	}
 
-	down(&inode->i_sem);
 	if (inode->i_op && inode->i_op->setattr) 
 		error = inode->i_op->setattr(dentry, attr);
 	else {
@@ -145,7 +163,6 @@
 				error = inode_setattr(inode, attr);
 		}
 	}
-	up(&inode->i_sem);
 	if (!error) {
 		unsigned long dn_mask = setattr_mask(ia_valid);
 		if (dn_mask)
diff -urN C8-pre2/fs/dquot.c C8-pre2-current/fs/dquot.c
--- C8-pre2/fs/dquot.c	Sat Apr  6 00:29:28 2002
+++ C8-pre2-current/fs/dquot.c	Sat Apr  6 13:26:12 2002
@@ -429,7 +429,7 @@
 	count = nr_free_dquots / priority;
 	prune_dqcache(count);
 	unlock_kernel();
-	return kmem_cache_shrink_nr(dquot_cachep);
+	return kmem_cache_shrink(dquot_cachep);
 }
 
 /* NOTE: If you change this function please check whether dqput_blocks() works right... */
diff -urN C8-pre2/fs/fat/inode.c C8-pre2-current/fs/fat/inode.c
--- C8-pre2/fs/fat/inode.c	Sat Apr  6 00:29:29 2002
+++ C8-pre2-current/fs/fat/inode.c	Sat Apr  6 13:21:12 2002
@@ -1086,7 +1086,8 @@
 			error = 0;
 		goto out;
 	}
-	if( error = inode_setattr(inode, attr) )
+	error = inode_setattr(inode, attr);
+	if (error)
 		goto out;
 
 	if (S_ISDIR(inode->i_mode))
diff -urN C8-pre2/fs/hpfs/namei.c C8-pre2-current/fs/hpfs/namei.c
--- C8-pre2/fs/hpfs/namei.c	Tue Feb 19 22:33:03 2002
+++ C8-pre2-current/fs/hpfs/namei.c	Sat Apr  6 12:20:54 2002
@@ -365,11 +365,9 @@
 			goto ret;
 		}
 		/*printk("HPFS: truncating file before delete.\n");*/
-		down(&inode->i_sem);
 		newattrs.ia_size = 0;
 		newattrs.ia_valid = ATTR_SIZE | ATTR_CTIME;
 		err = notify_change(dentry, &newattrs);
-		up(&inode->i_sem);
 		put_write_access(inode);
 		if (err)
 			goto ret;
diff -urN C8-pre2/fs/jffs2/file.c C8-pre2-current/fs/jffs2/file.c
--- C8-pre2/fs/jffs2/file.c	Sat Apr  6 00:29:29 2002
+++ C8-pre2-current/fs/jffs2/file.c	Sat Apr  6 13:21:44 2002
@@ -43,6 +43,7 @@
 #include <linux/pagemap.h>
 #include <linux/crc32.h>
 #include <linux/jffs2.h>
+#include <linux/smp_lock.h>
 #include "nodelist.h"
 
 extern int generic_file_open(struct inode *, struct file *) __attribute__((weak));
diff -urN C8-pre2/fs/ncpfs/inode.c C8-pre2-current/fs/ncpfs/inode.c
--- C8-pre2/fs/ncpfs/inode.c	Sat Apr  6 00:29:30 2002
+++ C8-pre2-current/fs/ncpfs/inode.c	Sat Apr  6 13:22:56 2002
@@ -27,6 +27,7 @@
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/init.h>
+#include <linux/smp_lock.h>
 
 #include <linux/ncp_fs.h>
 
diff -urN C8-pre2/fs/nfsd/vfs.c C8-pre2-current/fs/nfsd/vfs.c
--- C8-pre2/fs/nfsd/vfs.c	Fri Mar  8 02:09:52 2002
+++ C8-pre2-current/fs/nfsd/vfs.c	Sat Apr  6 12:39:54 2002
@@ -264,6 +264,7 @@
 		if (err)
 			goto out_nfserr;
 
+		size_change = 1;
 		err = locks_verify_truncate(inode, NULL, iap->ia_size);
 		if (err) {
 			put_write_access(inode);
@@ -279,35 +280,24 @@
 	}
 
 	/* Revoke setuid/setgid bit on chown/chgrp */
-	if ((iap->ia_valid & ATTR_UID) && (imode & S_ISUID)
-	 && iap->ia_uid != inode->i_uid) {
-		iap->ia_valid |= ATTR_MODE;
-		iap->ia_mode = imode &= ~S_ISUID;
-	}
-	if ((iap->ia_valid & ATTR_GID) && (imode & S_ISGID)
-	 && iap->ia_gid != inode->i_gid) {
-		iap->ia_valid |= ATTR_MODE;
-		iap->ia_mode = imode &= ~S_ISGID;
-	}
+	if ((iap->ia_valid & ATTR_UID) && iap->ia_uid != inode->i_uid)
+		iap->ia_valid |= ATTR_KILL_SUID;
+	if ((iap->ia_valid & ATTR_GID) && iap->ia_gid != inode->i_gid)
+		iap->ia_valid |= ATTR_KILL_SGID;
 
 	/* Change the attributes. */
 
-
 	iap->ia_valid |= ATTR_CTIME;
 
-	if (iap->ia_valid & ATTR_SIZE) {
-		fh_lock(fhp);
-		size_change = 1;
-	}
 	err = nfserr_notsync;
 	if (!check_guard || guardtime == inode->i_ctime) {
+		fh_lock(fhp);
 		err = notify_change(dentry, iap);
 		err = nfserrno(err);
-	}
-	if (size_change) {
 		fh_unlock(fhp);
-		put_write_access(inode);
 	}
+	if (size_change)
+		put_write_access(inode);
 	if (!err)
 		if (EX_ISSYNC(fhp->fh_export))
 			write_inode_now(inode, 1);
@@ -725,10 +715,11 @@
 	/* clear setuid/setgid flag after write */
 	if (err >= 0 && (inode->i_mode & (S_ISUID | S_ISGID))) {
 		struct iattr	ia;
+		ia.ia_valid = ATTR_KILL_SUID | ATTR_KILL_SGID;
 
-		ia.ia_valid = ATTR_MODE;
-		ia.ia_mode  = inode->i_mode & ~(S_ISUID | S_ISGID);
+		down(&inode->i_sem);
 		notify_change(dentry, &ia);
+		up(&inode->i_sem);
 	}
 
 	if (err >= 0 && stable) {
@@ -1157,7 +1148,9 @@
 				iap->ia_valid |= ATTR_CTIME;
 				iap->ia_mode = (iap->ia_mode&S_IALLUGO)
 					| S_IFLNK;
+				down(&dentry->d_inode->i_sem);
 				err = notify_change(dnew, iap);
+				up(&dentry->d_inode->i_sem);
 				if (!err && EX_ISSYNC(fhp->fh_export))
 					write_inode_now(dentry->d_inode, 1);
 		       }
diff -urN C8-pre2/fs/open.c C8-pre2-current/fs/open.c
--- C8-pre2/fs/open.c	Sat Apr  6 00:29:30 2002
+++ C8-pre2-current/fs/open.c	Sat Apr  6 12:37:36 2002
@@ -73,6 +73,7 @@
 
 int do_truncate(struct dentry *dentry, loff_t length)
 {
+	int err;
 	struct iattr newattrs;
 
 	/* Not pretty: "inode->i_size" shouldn't really be signed. But it is. */
@@ -81,7 +82,10 @@
 
 	newattrs.ia_size = length;
 	newattrs.ia_valid = ATTR_SIZE | ATTR_CTIME;
-	return notify_change(dentry, &newattrs);
+	down(&dentry->d_inode->i_sem);
+	err = notify_change(dentry, &newattrs);
+	up(&dentry->d_inode->i_sem);
+	return err;
 }
 
 static inline long do_sys_truncate(const char * path, loff_t length)
@@ -255,7 +259,9 @@
 		    (error = permission(inode,MAY_WRITE)) != 0)
 			goto dput_and_out;
 	}
+	down(&inode->i_sem);
 	error = notify_change(nd.dentry, &newattrs);
+	up(&inode->i_sem);
 dput_and_out:
 	path_release(&nd);
 out:
@@ -299,7 +305,9 @@
 		if ((error = permission(inode,MAY_WRITE)) != 0)
 			goto dput_and_out;
 	}
+	down(&inode->i_sem);
 	error = notify_change(nd.dentry, &newattrs);
+	up(&inode->i_sem);
 dput_and_out:
 	path_release(&nd);
 out:
@@ -449,11 +457,13 @@
 	err = -EPERM;
 	if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
 		goto out_putf;
+	down(&inode->i_sem);
 	if (mode == (mode_t) -1)
 		mode = inode->i_mode;
 	newattrs.ia_mode = (mode & S_IALLUGO) | (inode->i_mode & ~S_IALLUGO);
 	newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
 	err = notify_change(dentry, &newattrs);
+	up(&inode->i_sem);
 
 out_putf:
 	fput(file);
@@ -481,11 +491,13 @@
 	if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
 		goto dput_and_out;
 
+	down(&inode->i_sem);
 	if (mode == (mode_t) -1)
 		mode = inode->i_mode;
 	newattrs.ia_mode = (mode & S_IALLUGO) | (inode->i_mode & ~S_IALLUGO);
 	newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
 	error = notify_change(nd.dentry, &newattrs);
+	up(&inode->i_sem);
 
 dput_and_out:
 	path_release(&nd);
@@ -510,45 +522,20 @@
 	error = -EPERM;
 	if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
 		goto out;
-	if (user == (uid_t) -1)
-		user = inode->i_uid;
-	if (group == (gid_t) -1)
-		group = inode->i_gid;
-	newattrs.ia_mode = inode->i_mode;
-	newattrs.ia_uid = user;
-	newattrs.ia_gid = group;
-	newattrs.ia_valid =  ATTR_UID | ATTR_GID | ATTR_CTIME;
-	/*
-	 * If the user or group of a non-directory has been changed by a
-	 * non-root user, remove the setuid bit.
-	 * 19981026	David C Niemi <niemi@tux.org>
-	 *
-	 * Changed this to apply to all users, including root, to avoid
-	 * some races. This is the behavior we had in 2.0. The check for
-	 * non-root was definitely wrong for 2.2 anyway, as it should
-	 * have been using CAP_FSETID rather than fsuid -- 19990830 SD.
-	 */
-	if ((inode->i_mode & S_ISUID) == S_ISUID &&
-		!S_ISDIR(inode->i_mode))
-	{
-		newattrs.ia_mode &= ~S_ISUID;
-		newattrs.ia_valid |= ATTR_MODE;
+	newattrs.ia_valid =  ATTR_CTIME;
+	if (user != (uid_t) -1) {
+		newattrs.ia_valid =  ATTR_UID;
+		newattrs.ia_uid = user;
 	}
-	/*
-	 * Likewise, if the user or group of a non-directory has been changed
-	 * by a non-root user, remove the setgid bit UNLESS there is no group
-	 * execute bit (this would be a file marked for mandatory locking).
-	 * 19981026	David C Niemi <niemi@tux.org>
-	 *
-	 * Removed the fsuid check (see the comment above) -- 19990830 SD.
-	 */
-	if (((inode->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) 
-		&& !S_ISDIR(inode->i_mode))
-	{
-		newattrs.ia_mode &= ~S_ISGID;
-		newattrs.ia_valid |= ATTR_MODE;
+	if (group != (gid_t) -1) {
+		newattrs.ia_valid =  ATTR_GID;
+		newattrs.ia_gid = group;
 	}
+	if (!S_ISDIR(inode->i_mode))
+		newattrs.ia_valid |= ATTR_KILL_SUID|ATTR_KILL_SGID;
+	down(&inode->i_sem);
 	error = notify_change(dentry, &newattrs);
+	up(&inode->i_sem);
 out:
 	return error;
 }
diff -urN C8-pre2/include/linux/fs.h C8-pre2-current/include/linux/fs.h
--- C8-pre2/include/linux/fs.h	Sat Apr  6 00:29:32 2002
+++ C8-pre2-current/include/linux/fs.h	Sat Apr  6 12:58:55 2002
@@ -305,6 +305,8 @@
 #define ATTR_MTIME_SET	256
 #define ATTR_FORCE	512	/* Not a change, but a change it */
 #define ATTR_ATTR_FLAG	1024
+#define ATTR_KILL_SUID	2048
+#define ATTR_KILL_SGID	4096
 
 /*
  * This is the Inode Attributes structure, used for notify_change().  It
@@ -1313,7 +1315,7 @@
 
 extern void clear_inode(struct inode *);
 extern struct inode *new_inode(struct super_block *);
-extern void remove_suid(struct inode *inode);
+extern void remove_suid(struct dentry *);
 extern void insert_inode_hash(struct inode *);
 extern void remove_inode_hash(struct inode *);
 extern struct file * get_empty_filp(void);
diff -urN C8-pre2/mm/filemap.c C8-pre2-current/mm/filemap.c
--- C8-pre2/mm/filemap.c	Sat Apr  6 00:29:32 2002
+++ C8-pre2-current/mm/filemap.c	Sat Apr  6 12:53:58 2002
@@ -2503,18 +2503,19 @@
 	return page;
 }
 
-inline void remove_suid(struct inode *inode)
+inline void remove_suid(struct dentry *dentry)
 {
-	unsigned int mode;
+	struct iattr newattrs;
+	struct inode *inode = dentry->d_inode;
+	unsigned int mode = inode->i_mode & (S_ISUID|S_ISGID|S_IXGRP);
 
-	/* set S_IGID if S_IXGRP is set, and always set S_ISUID */
-	mode = (inode->i_mode & S_IXGRP)*(S_ISGID/S_IXGRP) | S_ISUID;
+	if (!(mode & S_IXGRP))
+		mode &= S_ISUID;
 
 	/* was any of the uid bits set? */
-	mode &= inode->i_mode;
 	if (mode && !capable(CAP_FSETID)) {
-		inode->i_mode &= ~mode;
-		mark_inode_dirty(inode);
+		newattrs.ia_valid = ATTR_KILL_SUID | ATTR_KILL_SGID;
+		notify_change(dentry, &newattrs);
 	}
 }
 
@@ -2646,7 +2647,7 @@
 	if (count == 0)
 		goto out;
 
-	remove_suid(inode);
+	remove_suid(file->f_dentry);
 	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
 	mark_inode_dirty_sync(inode);
 
diff -urN C8-pre2/mm/shmem.c C8-pre2-current/mm/shmem.c
--- C8-pre2/mm/shmem.c	Sat Apr  6 00:29:32 2002
+++ C8-pre2-current/mm/shmem.c	Sat Apr  6 12:43:50 2002
@@ -802,7 +802,7 @@
 
 	status	= 0;
 	if (count) {
-		remove_suid(inode);
+		remove_suid(file->f_dentry);
 		inode->i_ctime = inode->i_mtime = CURRENT_TIME;
 	}
 

Re: [WTF] ->setattr() locking changes

[Alan Cox]

> > Hmm...  While we are at it, why don't we remove suid/sgid on truncate(2)?
> 
> Are there any standards saying either way? But yes, it sounds logical.

SuS v2 specifically says they may be cleared