89 lines
3.2 KiB
ReStructuredText
89 lines
3.2 KiB
ReStructuredText
=========================================
|
|
Tagged virtual addresses in AArch64 Linux
|
|
=========================================
|
|
|
|
Author: Will Deacon <will.deacon@arm.com>
|
|
|
|
Date : 12 June 2013
|
|
|
|
This document briefly describes the provision of tagged virtual
|
|
addresses in the AArch64 translation system and their potential uses
|
|
in AArch64 Linux.
|
|
|
|
The kernel configures the translation tables so that translations made
|
|
via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
|
|
the virtual address ignored by the translation hardware. This frees up
|
|
this byte for application use.
|
|
|
|
|
|
Passing tagged addresses to the kernel
|
|
--------------------------------------
|
|
|
|
All interpretation of userspace memory addresses by the kernel assumes
|
|
an address tag of 0x00, unless the application enables the AArch64
|
|
Tagged Address ABI explicitly
|
|
(Documentation/arm64/tagged-address-abi.rst).
|
|
|
|
This includes, but is not limited to, addresses found in:
|
|
|
|
- pointer arguments to system calls, including pointers in structures
|
|
passed to system calls,
|
|
|
|
- the stack pointer (sp), e.g. when interpreting it to deliver a
|
|
signal,
|
|
|
|
- the frame pointer (x29) and frame records, e.g. when interpreting
|
|
them to generate a backtrace or call graph.
|
|
|
|
Using non-zero address tags in any of these locations when the
|
|
userspace application did not enable the AArch64 Tagged Address ABI may
|
|
result in an error code being returned, a (fatal) signal being raised,
|
|
or other modes of failure.
|
|
|
|
For these reasons, when the AArch64 Tagged Address ABI is disabled,
|
|
passing non-zero address tags to the kernel via system calls is
|
|
forbidden, and using a non-zero address tag for sp is strongly
|
|
discouraged.
|
|
|
|
Programs maintaining a frame pointer and frame records that use non-zero
|
|
address tags may suffer impaired or inaccurate debug and profiling
|
|
visibility.
|
|
|
|
|
|
Preserving tags
|
|
---------------
|
|
|
|
When delivering signals, non-zero tags are not preserved in
|
|
siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in
|
|
sigaction.sa_flags when the signal handler was installed. This means
|
|
that signal handlers in applications making use of tags cannot rely
|
|
on the tag information for user virtual addresses being maintained
|
|
in these fields unless the flag was set.
|
|
|
|
Due to architecture limitations, bits 63:60 of the fault address
|
|
are not preserved in response to synchronous tag check faults
|
|
(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should
|
|
treat the values of these bits as undefined in order to accommodate
|
|
future architecture revisions which may preserve the bits.
|
|
|
|
For signals raised in response to watchpoint debug exceptions, the
|
|
tag information will be preserved regardless of the SA_EXPOSE_TAGBITS
|
|
flag setting.
|
|
|
|
Non-zero tags are never preserved in sigcontext.fault_address
|
|
regardless of the SA_EXPOSE_TAGBITS flag setting.
|
|
|
|
The architecture prevents the use of a tagged PC, so the upper byte will
|
|
be set to a sign-extension of bit 55 on exception return.
|
|
|
|
This behaviour is maintained when the AArch64 Tagged Address ABI is
|
|
enabled.
|
|
|
|
|
|
Other considerations
|
|
--------------------
|
|
|
|
Special care should be taken when using tagged pointers, since it is
|
|
likely that C compilers will not hazard two virtual addresses differing
|
|
only in the upper byte.
|