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.
The keyring wasn't available for modification by the user.
The request was interrupted by a signal.
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.
Insufficient memory to create a key.
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.
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