IBM AIX Continuous Availability Features - IBM Redbooks

This is accomplished by exclusively dedicating hardware keys for use by applications. The
primary user key UKEY_PUBLIC is the default storage-key for user data. Access to this key
cannot be disabled in user-mode.
The UKEY values are an abstraction of storage keys. These key values are the same across
all applications. For example, if one process sets a shared page to UKEY_PRIVATE1, all
processes need UKEY_PRIVATE1 authority to access that page.
The sysconf() service can be used to determine if user keys are available without load time
dependencies. Applications must use ukey_enable() to enable user keys before user key
APIs can be used. All user memory pages are initialized to be in UKEY_PUBLIC. Applications
have the option to alter the user key for specific data pages that should not be publicly
accessible. User keys may not be altered on mapped files. The application must have write
authority to shared memory to alter the user key.
Other considerations for user keys application programming
Like kernel key support, user key support is intended as a Reliability and Serviceability
feature. It is not intended to provide a security function. A system call permits applications to
modify the AMR to allow and disallow access to user data regions, with no authority checking.
The kernel manages its own AMR when user keys are in use. When the kernel performs
loads or stores on behalf of an application, it respects the user mode AMR that was active
when the request was initiated. The user key values are shared among threads in a
multithreaded process, but a user mode AMR is maintained per thread. Kernel context
switches preserve the AMR. Threaded applications are prevented from running M:N mode
with user keys enabled by the ukey_enable() system call and pthread_create().
The user mode AMR is inherited by fork(), and it is reset to its default by exec(). The default
user mode value enables only UKEY_PUBLIC (read and write access). A system call,
ukeyset_activate() is available to modify the user mode AMR. Applications cannot disable
access to UKEY_PUBLIC. Preventing this key from being disabled allows memory that is
“unknown” to an application to always be accessible. For example, the TOC or data used by
an external key-unsafe library is normally set to UKEY_PUBLIC.
The ucontext_t structure is extended to allow the virtualized user mode AMR to be saved and
restored. The sigcontext structure is not changed. The jmp_buf structure is not extended to
contain an AMR, so callers of setjmp(), _setjmp(), and sig_setjmp() must perform explicit
AMR management. A ukey_setjmp() API is provided that is a front-end to setjmp() and
manages the user mode AMR.
The user mode AMR is reset to contain only UKEY_PUBLIC when signals are delivered and
the interrupted AMR is saved in the ucontext_t structure. Signal handlers that access storage
that is not mapped UKEY_PUBLIC are responsible for establishing their user mode AMR.
Hardware and operating system considerations
► AIX APIs for application user keys will be made available in AIX 5.3 TL6 running on
POWER6 hardware. AIX V5.3 does not support kernel keys.
► In AIX V5.3 TL6, application interfaces that exploit user keys only function with the 64-bit
kernel. When the 32-bit kernel is running, these user keys APIs fail.
► User keys are considered an optional platform feature. APIs are present to query if user
keys are supported at runtime, and how many user keys are available.
► User keys are available in 32-bit and 64-bit user-mode APIs.
► User keys can be used by threaded applications in 1:1 mode. They are not supported in
M:N mode.
Chapter 3. AIX advanced continuous availability tools and features 101