tree 384c11ebe2c2cb06fb26b891c74280a2644b3369
parent e817a7fb2f31c2fafc8e6e2dbf649bbd65b2f604
author Kristina Martsenko <kristina.martsenko@arm.com> 1493825868 +0100
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1495719885 +0200

arm64: documentation: document tagged pointer stack constraints

commit f0e421b1bf7af97f026e1bb8bfe4c5a7a8c08f42 upstream.

Some kernel features don't currently work if a task puts a non-zero
address tag in its stack pointer, frame pointer, or frame record entries
(FP, LR).

For example, with a tagged stack pointer, the kernel can't deliver
signals to the process, and the task is killed instead. As another
example, with a tagged frame pointer or frame records, perf fails to
generate call graphs or resolve symbols.

For now, just document these limitations, instead of finding and fixing
everything that doesn't work, as it's not known if anyone needs to use
tags in these places anyway.

In addition, as requested by Dave Martin, generalize the limitations
into a general kernel address tag policy, and refactor
tagged-pointers.txt to include it.

Fixes: d50240a5f6ce ("arm64: mm: permit use of tagged pointers at EL0")
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>