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2 Tagged virtual addresses in AArch64 Linux
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5 Author: Will Deacon <will.deacon@arm.com>
9 This document briefly describes the provision of tagged virtual
10 addresses in the AArch64 translation system and their potential uses
13 The kernel configures the translation tables so that translations made
14 via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
15 the virtual address ignored by the translation hardware. This frees up
16 this byte for application use.
19 Passing tagged addresses to the kernel
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22 All interpretation of userspace memory addresses by the kernel assumes
23 an address tag of 0x00.
25 This includes, but is not limited to, addresses found in:
27 - pointer arguments to system calls, including pointers in structures
28 passed to system calls,
30 - the stack pointer (sp), e.g. when interpreting it to deliver a
33 - the frame pointer (x29) and frame records, e.g. when interpreting
34 them to generate a backtrace or call graph.
36 Using non-zero address tags in any of these locations may result in an
37 error code being returned, a (fatal) signal being raised, or other modes
40 For these reasons, passing non-zero address tags to the kernel via
41 system calls is forbidden, and using a non-zero address tag for sp is
44 Programs maintaining a frame pointer and frame records that use non-zero
45 address tags may suffer impaired or inaccurate debug and profiling
52 Non-zero tags are not preserved when delivering signals. This means that
53 signal handlers in applications making use of tags cannot rely on the
54 tag information for user virtual addresses being maintained for fields
55 inside siginfo_t. One exception to this rule is for signals raised in
56 response to watchpoint debug exceptions, where the tag information will
59 The architecture prevents the use of a tagged PC, so the upper byte will
60 be set to a sign-extension of bit 55 on exception return.
66 Special care should be taken when using tagged pointers, since it is
67 likely that C compilers will not hazard two virtual addresses differing
68 only in the upper byte.