doc/cap_get_file.3

.\"
.\" written by Andrew Main <zefram@dcs.warwick.ac.uk>
.\"
.TH CAP_GET_FILE 3 "2021-03-06" "" "Linux Programmer's Manual"
.SH NAME
cap_get_file, cap_set_file, cap_get_fd, cap_set_fd \- capability
manipulation on files
.SH SYNOPSIS
.nf
#include <sys/capability.h>

cap_t cap_get_file(const char *path_p);
int cap_set_file(const char *path_p, cap_t cap_p);
cap_t cap_get_fd(int fd);
int cap_set_fd(int fd, cap_t caps);
uid_t cap_get_nsowner(cap_t caps);
int cap_set_nsowner(cap_t caps, uid_t rootuid);
.fi
.sp
Link with \fI\-lcap\fP.
.SH DESCRIPTION
.BR cap_get_file ()
and
.BR cap_get_fd ()
allocate a capability state in working storage and set it to represent the
capability state of the pathname pointed to by
.I path_p
or the file open on descriptor
.IR fd .
These functions return a pointer to the newly created capability
state.  The effects of reading the capability state from any file
other than a regular file is undefined.  The caller should free any
releasable memory, when the capability state in working storage is no
longer required, by calling
.BR cap_free ()
with the used
.I cap_t
as an argument.
.PP
.BR cap_set_file ()
and
.BR cap_set_fd ()
set the values for all capability flags for all capabilities for the pathname
pointed to by
.I path_p
or the file open on descriptor
.IR fd ,
with the capability state identified by
.IR cap_p .
The new capability state of the file is completely determined by the
contents of
.IR cap_p .
A NULL value for
.IR cap_p
is used to indicate that capabilities for the file should be deleted.
For these functions to succeed, the calling process must have the
.BR CAP_SETFCAP
capability in its effective set
and either the effective user ID of the process must match the
file owner or the calling process must have the
.B CAP_FOWNER
capability in its effective capability set.  The effects of writing the
capability state to any file type other than a regular file are
undefined.
.PP
A capability set held in memory can be associated with the root user ID in
use in a specific user namespace. It is possible to get and set this value
(in the memory copy) with
.BR cap_get_nsowner ()
and
.BR cap_set_nsowner ()
respectively. The root user ID is ignored by the libcap library in all cases
other than when the capability is written to a file. Only if the value
is non-zero will the library attempt to include it in the written file
capability set.
.SH "RETURN VALUE"
.BR cap_get_file ()
and
.BR cap_get_fd ()
return a non-NULL value on success, and NULL on failure.
.PP
.BR cap_set_file ()
and
.BR cap_set_fd ()
return zero on success, and \-1 on failure.
.PP
On failure,
.I errno
is set to
.BR EACCES ,
.BR EBADFD ,
.BR ENAMETOOLONG ,
.BR ENOENT ,
.BR ENOMEM ,
.BR ENOTDIR ,
.BR EPERM ,
or
.BR EROFS .
.SH "CONFORMING TO"
These functions are specified by withdrawn POSIX.1e draft specification.
.SH NOTES
Support for file capabilities is provided on Linux since version 2.6.24.

On Linux, the file Effective set is a single bit.
If it is enabled, then all Permitted capabilities are enabled
in the Effective set of the calling process when the file is executed;
otherwise, no capabilities are enabled in the process's Effective set
following an
.BR execve (2).
Because the file Effective set is a single bit,
if any capability is enabled in the Effective set of the
.I cap_t
given to
.BR cap_set_file ()
or
.BR cap_set_fd (),
then all capabilities whose Permitted or Inheritable flag
is enabled must also have the Effective flag enabled.
Conversely, if the Effective bit is enabled on a file, then the
.I cap_t
returned by
.BR cap_get_file()
and
.BR cap_get_fd()
will have the Effective flag enabled for each capability that has the
Permitted or Inheritable flag enabled.
.SH "SEE ALSO"
.BR libcap (3),
.BR cap_clear (3),
.BR cap_copy_ext (3),
.BR cap_from_text (3),
.BR cap_get_proc (3),
.BR cap_init (3),
.BR capabilities (7),
.BR user_namespaces (7)