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Date:      Fri, 31 Jul 1998 20:05:19 -0400 (EDT)
From:      Brian Feldman <green@zone.baldcom.net>
To:        freebsd-current@FreeBSD.ORG
Subject:   flock(2) problem & fix
Message-ID:  <Pine.BSF.4.02.9807311954190.25645-200000@zone.baldcom.net>
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[-- Attachment #1 --]
Our flock(2) doesn't seem to do the right thing all the time. It seems
that any user with read access to a file is allowed exclusive locking of
it, which I think is wrong (does everyone agree?), and that a shared lock
should be used, because the user does not own the file, and has no write
permission, therefore no need for an exclusive lock.  I raise the question
as to whether it should matter if the lock is non-blocking or not, and
that should be considered I suppose, as to assure the Right Thing will
happen. Attached is a patch to sys/kern/kern_descrip.c which should fix
the problem but is untested.

Cheers,
Brian Feldman
green@unixhelp.org

--- kern_descrip.c.old  Fri Jul 31 18:28:30 1998
+++ kern_descrip.c      Fri Jul 31 19:31:26 1998
@@ -986,6 +986,9 @@
        register struct file *fp;
        struct vnode *vp;
        struct flock lf;
+       struct stat fst;
+       short gr;
+       boolean_t ok = 0;

        if ((unsigned)uap->fd >= fdp->fd_nfiles ||
            (fp = fdp->fd_ofiles[uap->fd]) == NULL)
@@ -1007,10 +1010,27 @@
                lf.l_type = F_RDLCK;
        else
                return (EBADF);
+       vn_stat((struct vnode *)fp->f_data, &fst, p);
+       if (uap->how & LOCK_EX && p->p_cred->pc_ucred->cr_uid != 0 &&
+               fp->f_cred->cr_uid != p->p_cred->pc_ucred->cr_uid &&
+               !(fst.st_mode & S_IWOTH) && !(fst.st_mode & S_IWGRP))
+               return (EPERM);
+       if (!(fst.st_mode & S_IWGRP))
+               ok = 1;
+       else {
+       for (gr = 0; gr < p->p_cred->pc_ucred->cr_ngroups; gr++)
+               if (p->p_cred->pc_ucred->cr_groups[gr - 1] == fst.st_gid)
{
+                       ok = 1;
+                       break;
+               }
+       }
+       if (ok) {
        fp->f_flag |= FHASLOCK;
        if (uap->how & LOCK_NB)
                return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf,
F_FLOCK));
        return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf,
F_FLOCK|F_WAIT));
+       } else
+               return (EPERM);
 }

 /*


[-- Attachment #2 --]
--- kern_descrip.c.old	Fri Jul 31 18:28:30 1998
+++ kern_descrip.c	Fri Jul 31 19:31:26 1998
@@ -986,6 +986,9 @@
 	register struct file *fp;
 	struct vnode *vp;
 	struct flock lf;
+	struct stat fst;
+	short gr;
+	boolean_t ok = 0;
 
 	if ((unsigned)uap->fd >= fdp->fd_nfiles ||
 	    (fp = fdp->fd_ofiles[uap->fd]) == NULL)
@@ -1007,10 +1010,27 @@
 		lf.l_type = F_RDLCK;
 	else
 		return (EBADF);
+	vn_stat((struct vnode *)fp->f_data, &fst, p);
+	if (uap->how & LOCK_EX && p->p_cred->pc_ucred->cr_uid != 0 &&
+		fp->f_cred->cr_uid != p->p_cred->pc_ucred->cr_uid &&
+		!(fst.st_mode & S_IWOTH) && !(fst.st_mode & S_IWGRP))
+		return (EPERM);
+	if (!(fst.st_mode & S_IWGRP))
+		ok = 1;
+	else {
+	for (gr = 0; gr < p->p_cred->pc_ucred->cr_ngroups; gr++)
+		if (p->p_cred->pc_ucred->cr_groups[gr - 1] == fst.st_gid) {
+			ok = 1;
+			break;
+		}	
+	}
+	if (ok) {
 	fp->f_flag |= FHASLOCK;
 	if (uap->how & LOCK_NB)
 		return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf, F_FLOCK));
 	return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf, F_FLOCK|F_WAIT));
+	} else
+		return (EPERM);
 }
 
 /*

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