On arm64 the TCR_EL1.TBI0 bit has been always enabled on the arm64 kernel,
hence the userspace (EL0) is allowed to set a non-zero value in the top
byte but the resulting pointers are not allowed at the user-kernel syscall
ABI boundary.

This patchset proposes a relaxation of the ABI with which it is possible
to pass tagged tagged pointers to the syscalls, when these pointers are in
memory ranges obtained as described in tagged-address-abi.txt contained in
this patch series.

Since it is not desirable to relax the ABI to allow tagged user addresses
into the kernel indiscriminately, this patchset documents a new sysctl
interface (/proc/sys/abi/tagged_addr) that is used to prevent the applications
from enabling the relaxed ABI and a new prctl() interface that can be used to
enable or disable the relaxed ABI.

This patchset should be merged together with [1].

[1] https://patchwork.kernel.org/cover/10674351/

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
CC: Andrey Konovalov <redacted>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

Vincenzo Frascino (2):
  arm64: Define Documentation/arm64/tagged-address-abi.txt
  arm64: Relax Documentation/arm64/tagged-pointers.txt

 Documentation/arm64/tagged-address-abi.txt | 134 +++++++++++++++++++++
 Documentation/arm64/tagged-pointers.txt    |  23 ++--
 2 files changed, 150 insertions(+), 7 deletions(-)
 create mode 100644 Documentation/arm64/tagged-address-abi.txt

-- 
2.21.0


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