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       request_key - request a key from the kernel's key management facility


       #include <keyutils.h>

       key_serial_t request_key(const char *type, const char *description,
                                const char *callout_info,
                                key_serial_t keyring);


       request_key()  asks  the kernel to find a key of the given type that matches the specified
       description and, if successful, to attach it to the nominated keyring and  to  return  its
       serial number.

       request_key()  first recursively searches all the keyrings attached to the calling process
       in the order thread-specific keyring, process-specific keyring and  then  session  keyring
       for a matching key.

       If request_key() is called from a program invoked by request_key() on behalf of some other
       process to generate a key, then the keyrings of that other process will be searched  next,
       using that other process's UID, GID, groups, and security context to control access.

       The  keys  in  each keyring searched are checked for a match before any child keyrings are
       recursed into.  Only keys that are searchable for  the  caller  may  be  found,  and  only
       searchable keyrings may be searched.

       If  the key is not found, then, if callout_info is set, this function will attempt to look
       further afield.  In such a case, the callout_info is passed to a user-space  service  such
       as /sbin/request-key to generate the key.

       If that is unsuccessful also, then an error will be returned, and a temporary negative key
       will be installed in the nominated keyring.  This will expire after  a  few  seconds,  but
       will cause subsequent calls to request_key() to fail until it does.

       The  keyring  serial  number  may be that of a valid keyring to which the caller has write
       permission, or it may be a special keyring ID:

              This specifies the caller's thread-specific keyring.

              This specifies the caller's process-specific keyring.

              This specifies the caller's session-specific keyring.

              This specifies the caller's UID-specific keyring.

              This specifies the caller's UID-session keyring.

       If a key is created, no matter whether it's a  valid  key  or  a  negative  key,  it  will
       displace any other key of the same type and description from the destination keyring.


       On  success  request_key()  returns  the serial number of the key it found.  On error, the
       value -1 will be returned and errno will have been set to an appropriate error.


       EACCES The keyring wasn't available for modification by the user.

       EINTR  The request was interrupted by a signal.

       EDQUOT The key quota for this user would be exceeded by creating this key or linking it to
              the keyring.

              An expired key was found, but no replacement could be obtained.

              The attempt to generate a new key was rejected.

              A revoked key was found, but no replacement could be obtained.

       ENOMEM Insufficient memory to create a key.

       ENOKEY No matching key was found.


       Although this is a Linux system call, it is not present in libc but can be found rather in
       libkeyutils.  When linking, -lkeyutils should be specified to the linker.


       keyctl(1), add_key(2), keyctl(2), keyctl(3), keyrings(7), request-key(8)


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