VOP_ACCESS(9)

NAME
VOP_ACCESS - check access permissions of a file or Unix domain socket

SYNOPSIS
#include <sys/vnode.h>
int
VOP_ACCESS(struct vnode *vp, int mode, struct ucred *cred, struct proc *p);

DESCRIPTION
This entry point checks the access permissions of the file against the given credentials.

Its arguments are:

vp  the vnode of the file to check

mode
    the type of access required

cred
    the user credentials to check

p   the process which is checking

The mode is a mask which can contain VREAD, VWRITE or VEXEC.

LOCKS
The vnode should be locked on entry.

RETURN VALUES
If the file is accessible in the specified way, then zero is returned, otherwise an appropriate error code is returned.

PSEUDOCODE

    int
    vop_access(struct vnode *vp, int mode, struct ucred *cred, struct proc *p)
    {
        int error;
        /*
         * Disallow write attempts on read-only file systems;
         * unless the file is a socket, fifo, or a block or
         * character device resident on the file system.
         */
        if (mode & VWRITE) {
    	switch (vp->v_type) {
    	case VDIR:
    	case VLNK:
    	case VREG:
    	    if (vp->v_mount->mnt_flag & MNT_RDONLY)
    		return EROFS;
    	    break;
    	}
        }
        /* If immutable bit set, nobody gets to write it. */
        if ((mode & VWRITE) && vp has immutable bit set)
    	return EPERM;
        /* Otherwise, user id 0 always gets access. */
        if (cred->cr_uid == 0)
    	return 0;
        mask = 0;
        /* Otherwise, check the owner. */
        if (cred->cr_uid == owner of vp) {
    	if (mode & VEXEC)
    	    mask |= S_IXUSR;
    	if (mode & VREAD)
    	    mask |= S_IRUSR;
    	if (mode & VWRITE)
    	    mask |= S_IWUSR;
    	return (((mode of vp) & mask) == mask ? 0 : EACCES);
        }
        /* Otherwise, check the groups. */
        for (i = 0, gp = cred->cr_groups; i < cred->cr_ngroups; i++, gp++)
    	if (group of vp == *gp) {
    	    if (mode & VEXEC)
    		mask |= S_IXGRP;
    	    if (mode & VREAD)
    		mask |= S_IRGRP;
    	    if (mode & VWRITE)
    		mask |= S_IWGRP;
    	    return (((mode of vp) & mask) == mask ? 0 : EACCES);
    	}

        /* Otherwise, check everyone else. */
        if (mode & VEXEC)
    	mask |= S_IXOTH;
        if (mode & VREAD)
    	mask |= S_IROTH;
        if (mode & VWRITE)
    	mask |= S_IWOTH;
        return (((mode of vp) & mask) == mask ? 0 : EACCES);
    }

ERRORS

[EPERM]
    An attempt was made to change an immutable file

[EACCES]
    Permission denied

SEE ALSO
vnode(9)

AUTHORS
This man page was written by Doug Rabson.