avc_has_perm(3)

NAME
avc_has_perm, avc_has_perm_noaudit, avc_audit, avc_entry_ref_init - obtain and audit SELinux access decisions

SYNOPSIS
#include <selinux/selinux.h>
#include <selinux/avc.h>

void avc_entry_ref_init(struct avc_entry_ref *aeref);

int avc_has_perm(security_id_t ssid, security_id_t tsid,
security_class_t tclass, access_vector_t requested,
struct avc_entry_ref *aeref, void *auditdata);

int avc_has_perm_noaudit(security_id_t ssid, security_id_t tsid,
security_class_t tclass, access_vector_t requested,
struct avc_entry_ref *aeref, struct av_decision *avd);

void avc_audit(security_id_t ssid, security_id_t tsid,
security_class_t tclass, access_vector_t requested,
struct av_decision *avd, int result, void *auditdata);

DESCRIPTION

Direct use of these functions is generally discouraged in favor of the higher level interface selinux_check_access(3) since the latter automatically handles the dynamic mapping of class and permission names to their policy values and proper handling of allow_unknown.

When using any of the functions that take policy integer values for classes or permissions as inputs, use string_to_security_class(3) and string_to_av_perm(3) to map the class and permission names to their policy values. These values may change across a policy reload, so they should be re-acquired on every use or using a SELINUX_CB_POLICYLOAD callback set via selinux_set_callback(3).

An alternative approach is to use selinux_set_mapping(3) to create a mapping from class and permission index values used by the application to the policy values, thereby allowing the application to pass its own fixed constants for the classes and permissions to these functions and internally mapping them on demand. However, this also requires setting up a callback as above to address policy reloads.

avc_entry_ref_init() initializes an avc_entry_ref structure; see ENTRY REFERENCES below. This function may be implemented as a macro.

avc_has_perm() checks whether the requested permissions are granted for subject SID ssid and target SID tsid, interpreting the permissions based on tclass and updating aeref, if non-NULL, to refer to a cache entry with the resulting decision. The granting or denial of permissions is audited in accordance with the policy. The auditdata parameter is for supplemental auditing; see avc_audit() below.

avc_has_perm_noaudit() behaves as avc_has_perm() without producing an audit message. The access decision is returned in avd and can be passed to avc_audit() explicitly.

avc_audit() produces an audit message for the access query represented by ssid, tsid, tclass, and requested, with a decision represented by avd. Pass the value returned by avc_has_perm_noaudit() as result. The auditdata parameter is passed to the user-supplied func_audit callback and can be used to add supplemental information to the audit message; see avc_init(3).

ENTRY REFERENCES
Entry references can be used to speed cache performance for repeated queries on the same subject and target. The userspace AVC will check the aeref argument, if supplied, before searching the cache on a permission query. After a query is performed, aeref will be updated to reference the cache entry for that query. A subsequent query on the same subject and target will then have the decision at hand without having to walk the cache.

After declaring an avc_entry_ref structure, use avc_entry_ref_init() to initialize it before passing it to avc_has_perm() or avc_has_perm_noaudit() for the first time. Using an uninitialized structure will produce undefined behavior.

RETURN VALUE
If requested permissions are granted, zero is returned. If requested permissions are denied or an error occurred, -1 is returned and errno is set appropriately.

In permissive mode, zero will be returned and errno unchanged even if permissions were denied. avc_has_perm() will still produce an audit message in this case.

ERRORS

EACCES
    A requested permission was denied.

EINVAL
    The tclass and/or the security contexts referenced by ssid and tsid are not recognized by the currently loaded policy.

ENOMEM
    An attempt to allocate memory failed.

NOTES
Internal errors encountered by the userspace AVC may cause certain values of errno to be returned unexpectedly. For example, netlink socket errors may produce EACCES or EINVAL. Make sure that userspace object managers are granted appropriate access to netlink by the policy.

AUTHOR
Originally Eamon Walsh. Updated by Stephen Smalley <sds@tycho.nsa.gov>

SEE ALSO
selinux_check_access(3), string_to_security_class(3), string_to_av_perm(3), selinux_set_callback(3), selinux_set_mapping(3), avc_init(3), avc_context_to_sid(3), avc_cache_stats(3), avc_add_callback(3), security_compute_av(3), selinux(8)