From 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 27 Apr 2024 12:05:51 +0200 Subject: Adding upstream version 5.10.209. Signed-off-by: Daniel Baumann --- Documentation/arm64/tagged-pointers.rst | 75 +++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 Documentation/arm64/tagged-pointers.rst (limited to 'Documentation/arm64/tagged-pointers.rst') diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst new file mode 100644 index 000000000..eab432360 --- /dev/null +++ b/Documentation/arm64/tagged-pointers.rst @@ -0,0 +1,75 @@ +========================================= +Tagged virtual addresses in AArch64 Linux +========================================= + +Author: Will Deacon + +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 +--------------- + +Non-zero tags are not preserved when delivering signals. This means that +signal handlers in applications making use of tags cannot rely on the +tag information for user virtual addresses being maintained for fields +inside siginfo_t. One exception to this rule is for signals raised in +response to watchpoint debug exceptions, where the tag information will +be preserved. + +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. -- cgit v1.2.3