Userspace selinux labeling interface and configuration file format for the file contexts backend
#include <selinux/label.h>
int selabel_lookup(struct selabel_handle *hnd, char **context,
const char *path, int mode);
int selabel_lookup_raw(struct selabel_handle *hnd, char **context,
const char *path, int mode);
The file contexts backend maps from pathname/mode combinations into security contexts. It is used to find the appropriate context for each file when relabeling a file system. The returned context must be freed using freecon(3).
selabel_lookup(3) describes the function with its return and error codes, however the following errno is clarified further for the file contexts backend:
ENOENT
No context corresponding to the path and mode was found - This will also be returned when the file contexts series of files have a context of <<none>> against the path (see the FILE FORMAT section).
The path argument should be set to the full pathname of the file whose assigned context is being checked. The mode argument should be set to the mode bits of the file, as determined by lstat(2). mode may be zero, however full matching may not occur.
Any messages generated by selabel_lookup(3) are sent to stderr by default, although this can be changed by selinux_set_callback(3).
selabel_lookup_raw(3) behaves identically to selabel_lookup(3) but does not perform context translation.
The FILES section details the configuration files used to determine a file context.
In addition to the global options described in selabel_open(3), this backend recognizes the following options:
SELABEL_OPT_PATH
A non-null value for this option specifies a path to a file that will be opened in lieu of the standard file contexts file. This value is also used as the base name for determining the names of local customization files.
SELABEL_OPT_BASEONLY
A non-null value for this option indicates that any local customizations to the file contexts mapping should be ignored.
SELABEL_OPT_SUBSET
A non-null value for this option is interpreted as a path prefix, for example "/etc". Only file context specifications starting with the given prefix are loaded. This may increase lookup performance, however any attempt to look up a path not starting with the given prefix will fail.
The file context files used to retrieve the default context depends on the SELABEL_OPT_PATH parameter passed to selabel_open(3). If NULL, then the SELABEL_OPT_PATH value will default to the active policy file contexts location (as returned by selinux_file_context_path(3)), otherwise the actual SELABEL_OPT_PATH value specified is used.
If SELABEL_OPT_BASEONLY is set, then the following files will be processed:
The mandatory file contexts file that is either the fully qualified file name from SELABEL_OPT_PATH.value or if NULL, then the path returned by selinux_file_context_path(3).
The optional local and distribution substitution files that perform path aliasing on the 'in memory' version of the file contexts file.
These files have the same name as the mandatory file contexts file with the extensions .subs and .subs_dist added.
If the SELABEL_OPT_BASEONLY is not set, then the following files will be processed:
The mandatory file contexts file that is either the fully qualified file name from SELABEL_OPT_PATH.value or if NULL, then the path returned by selinux_file_context_path(3).
The optional local customizations file that has the same name as the mandatory file contexts file with the extension .local added.
selinux_file_context_local_path(3) will return the default path to this file.
The optional user home directory customizations file that has the same name as the mandatory file contexts file with the extension .homedirs added.
selinux_file_context_homedir_path(3) will return the default path to this file.
The optional local and distribution substitution files that perform any path aliasing on the 'in memory' version of the file contexts file (and the .local and/or .homedirs if present). These files have the same name as the mandatory file contexts file with the extensions .subs and .subs_dist added.
selinux_file_context_subs_path(3) and selinux_file_context_subs_dist_path(3) will return the default paths to these files.
The default file context series of files are:
/etc/selinux/{SELINUXTYPE}/contexts/files/file_contexts
/etc/selinux/{SELINUXTYPE}/contexts/files/file_contexts.local
/etc/selinux/{SELINUXTYPE}/contexts/files/file_contexts.homedirs
/etc/selinux/{SELINUXTYPE}/contexts/files/file_contexts.subs
/etc/selinux/{SELINUXTYPE}/contexts/files/file_contexts.subs_dist
Where {SELINUXTYPE} is the entry from the selinux configuration file config (see selinux_config(5)).
Only the file_contexts file is mandatory, the remainder are optional.
The entries within the file contexts series of files are shown in the FILE FORMAT section.
Each line within the file_contexts and the two customization files (.local and .homedirs) is as follows:
pathname [file_type] context
Where:
pathname
An entry that defines the pathname that may be in the form of a regular expression.
file_type
An optional file type consisting of:
-b - Block Device -c - Character Device
-d - Directory -p - Named Pipe
-l - Symbolic Link -s - Socket
-- - Ordinary file
context
This entry can be either:
The security context that will be assigned to the file (i.e. returned as context).
A value of <<none>> can be used to indicate that the matching files should not be re-labeled and causes selabel_lookup(3) to return -1 with errno set to ENOENT.
Example:
# ./contexts/files/file_contexts
# pathname file_type context
/.* system_u:object_r:default_t:s0
/[^/]+ -- system_u:object_r:etc_runtime_t:s0
/tmp/.* <<none>>
Each line within the substitution files (.subs and .subs_dist) has the form:
subs_pathname pathname
Where:
pathname
A path that matches an entry in one or more of the file contexts policy configuration file.
subs_pathname
The path that will be aliased (considered equivalent) with pathname by the look up process.
Example:
# ./contexts/files/file_contexts.subs
# pathname subs_pathname
/myweb /var/www
/myspool /var/spool/mail
Using the above example, when selabel_lookup(3) is passed a path of /myweb/index.html the function will substitute the /myweb component with /var/www, therefore the path used is:
/var/www/index.html
If contexts are to be validated, then the global option SELABEL_OPT_VALIDATE must be set before calling selabel_open(3). If this is not set, then it is possible for an invalid context to be returned.
If the size of file contexts series of files contain many entries, then selabel_open(3) may have a delay as it reads in the files, and if requested validates the entries. If possible use the SELABEL_OPT_SUBSET option to reduce the number of entries processed.
Depending on the version of SELinux it is possible that a file_contexts.template file may also be present, however this is now deprecated.
The template file has the same format as the file_contexts file and may also contain the keywords HOME_ROOT, HOME_DIR, ROLE and USER. This functionality has now been moved to the policy store and managed by semodule(8) and genhomedircon(8).
selinux(8), selabel_open(3), selabel_lookup(3), selabel_stats(3), selabel_close(3), selinux_set_callback(3), selinux_file_context_path(3), freecon(3), selinux_config(5), lstat(2), selinux_file_context_subs_path(3), selinux_file_context_subs_dist_path(3), selinux_file_context_homedir_path(3), selinux_file_context_local_path(3), semodule(8), genhomedircon(8)