tagged-pointers.rst (3242B)
1========================================= 2Tagged virtual addresses in AArch64 Linux 3========================================= 4 5Author: Will Deacon <will.deacon@arm.com> 6 7Date : 12 June 2013 8 9This document briefly describes the provision of tagged virtual 10addresses in the AArch64 translation system and their potential uses 11in AArch64 Linux. 12 13The kernel configures the translation tables so that translations made 14via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of 15the virtual address ignored by the translation hardware. This frees up 16this byte for application use. 17 18 19Passing tagged addresses to the kernel 20-------------------------------------- 21 22All interpretation of userspace memory addresses by the kernel assumes 23an address tag of 0x00, unless the application enables the AArch64 24Tagged Address ABI explicitly 25(Documentation/arm64/tagged-address-abi.rst). 26 27This includes, but is not limited to, addresses found in: 28 29 - pointer arguments to system calls, including pointers in structures 30 passed to system calls, 31 32 - the stack pointer (sp), e.g. when interpreting it to deliver a 33 signal, 34 35 - the frame pointer (x29) and frame records, e.g. when interpreting 36 them to generate a backtrace or call graph. 37 38Using non-zero address tags in any of these locations when the 39userspace application did not enable the AArch64 Tagged Address ABI may 40result in an error code being returned, a (fatal) signal being raised, 41or other modes of failure. 42 43For these reasons, when the AArch64 Tagged Address ABI is disabled, 44passing non-zero address tags to the kernel via system calls is 45forbidden, and using a non-zero address tag for sp is strongly 46discouraged. 47 48Programs maintaining a frame pointer and frame records that use non-zero 49address tags may suffer impaired or inaccurate debug and profiling 50visibility. 51 52 53Preserving tags 54--------------- 55 56When delivering signals, non-zero tags are not preserved in 57siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in 58sigaction.sa_flags when the signal handler was installed. This means 59that signal handlers in applications making use of tags cannot rely 60on the tag information for user virtual addresses being maintained 61in these fields unless the flag was set. 62 63Due to architecture limitations, bits 63:60 of the fault address 64are not preserved in response to synchronous tag check faults 65(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should 66treat the values of these bits as undefined in order to accommodate 67future architecture revisions which may preserve the bits. 68 69For signals raised in response to watchpoint debug exceptions, the 70tag information will be preserved regardless of the SA_EXPOSE_TAGBITS 71flag setting. 72 73Non-zero tags are never preserved in sigcontext.fault_address 74regardless of the SA_EXPOSE_TAGBITS flag setting. 75 76The architecture prevents the use of a tagged PC, so the upper byte will 77be set to a sign-extension of bit 55 on exception return. 78 79This behaviour is maintained when the AArch64 Tagged Address ABI is 80enabled. 81 82 83Other considerations 84-------------------- 85 86Special care should be taken when using tagged pointers, since it is 87likely that C compilers will not hazard two virtual addresses differing 88only in the upper byte.