PR_SET_TAGGED_ADDR_CTRL(2const)

NAME
PR_SET_TAGGED_ADDR_CTRL - control support for passing tagged user-space addresses to the kernel

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>

int prctl(PR_SET_TAGGED_ADDR_CTRL, long mode, 0L, 0L, 0L);

DESCRIPTION
Controls support for passing tagged user-space addresses to the kernel (i.e., addresses where bits 56—63 are not all zero).

The level of support is selected by support, which can be one of the following:

0L  Addresses that are passed for the purpose of being dereferenced by the kernel must be untagged.

PR_TAGGED_ADDR_ENABLE
    Addresses that are passed for the purpose of being dereferenced by the kernel may be tagged, with the exceptions summarized below.

On success, the mode specified in mode is set for the calling thread.

If prctl(PR_SET_TAGGED_ADDR_CTRL, 0L, 0L, 0L, 0L) fails with EINVAL, then all addresses passed to the kernel must be untagged.

Irrespective of which mode is set, addresses passed to certain interfaces must always be untagged:

•   brk(2), mmap(2), shmat(2), shmdt(2), and the new_address argument of mremap(2).

    (Prior to Linux 5.6 these accepted tagged addresses, but the behaviour may not be what you expect. Don't rely on it.)

•   ‘polymorphic’ interfaces that accept pointers to arbitrary types cast to a void * or other generic type, specifically prctl(), ioctl(2), and in general setsockopt(2) (only certain specific setsockopt(2) options allow tagged addresses).

This list of exclusions may shrink when moving from one kernel version to a later kernel version. While the kernel may make some guarantees for backwards compatibility reasons, for the purposes of new software the effect of passing tagged addresses to these interfaces is unspecified.

The mode set by this call is inherited across fork(2) and clone(2). The mode is reset by execve(2) to 0 (i.e., tagged addresses not permitted in the user/kernel ABI).

RETURN VALUE
On success, 0 is returned. On error, -1 is returned, and errno is set to indicate the error.

ERRORS

EINVAL
    mode is invalid or unsupported.

EINVAL
    This feature is disabled via /proc/sys/abi/tagged_addr_disabled.

FILES

/proc/sys/abi/tagged_addr_disabled

STANDARDS
Linux. arm64 only.

HISTORY
Linux 5.4 (arm64).

CAVEATS
This call is primarily intended for use by the run-time environment. A successful PR_SET_TAGGED_ADDR_CTRL call elsewhere may crash the calling process. The conditions for using it safely are complex and system-dependent. Don't use it unless you know what you are doing.

SEE ALSO
prctl(2), PR_SET_TAGGED_ADDR_CTRL(2const)

For more information, see the kernel source file Documentation/arm64/tagged-address-abi.rst.