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VACCESS(9) Kernel Developer's Manual VACCESS(9)


vaccessgenerate an access control decision using vnode parameters


#include <sys/param.h>
#include <sys/vnode.h>

vaccess(enum vtype type, mode_t file_mode, uid_t file_uid, gid_t file_gid, accmode_t accmode, struct ucred *cred, int *privused);


This call implements the logic for the UNIX discretionary file security model common to many file systems in FreeBSD. It accepts the vnodes type type, permissions via file_mode, owning UID file_uid, owning GID file_gid, desired access mode accmode, requesting credential cred, and an optional call-by-reference int pointer returning whether or not privilege was required for successful evaluation of the call; the privused pointer may be set to NULL by the caller in order not to be informed of privilege information, or it may point to an integer that will be set to 1 if privilege is used, and 0 otherwise.

This call is intended to support implementations of VOP_ACCESS(9), which will use their own access methods to retrieve the vnode properties, and then invoke () in order to perform the actual check. Implementations of VOP_ACCESS(9) may choose to implement additional security mechanisms whose results will be composed with the return value.

The algorithm used by () selects a component of the file permission bits based on comparing the passed credential, file owner, and file group. If the credential's effective UID matches the file owner, then the owner component of the permission bits is selected. If the UID does not match, then the credential's effective GID, followed by additional groups, are compared with the file group—if there is a match, then the group component of the permission bits is selected. If neither the credential UID or GIDs match the passed file owner and group, then the other component of the permission bits is selected.

Once appropriate protections are selected for the current credential, the requested access mode, in combination with the vnode type, will be compared with the discretionary rights available for the credential. If the rights granted by discretionary protections are insufficient, then super-user privilege, if available for the credential, will also be considered.


vaccess() will return 0 on success, or a non-zero error value on failure.


Permission denied. An attempt was made to access a file in a way forbidden by its file access permissions.
Operation not permitted. An attempt was made to perform an operation limited to processes with appropriate privileges or to the owner of a file or other resource.


vaccess_acl_nfs4(9), vaccess_acl_posix1e(9), vnode(9), VOP_ACCESS(9)


This manual page and the current implementation of vaccess() were written by Robert Watson.

September 18, 2009 Debian