69 lines
2.3 KiB
ReStructuredText
69 lines
2.3 KiB
ReStructuredText
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=========================================
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Tagged virtual addresses in AArch64 Linux
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=========================================
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Author: Will Deacon <will.deacon@arm.com>
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Date : 12 June 2013
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This document briefly describes the provision of tagged virtual
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addresses in the AArch64 translation system and their potential uses
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in AArch64 Linux.
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The kernel configures the translation tables so that translations made
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via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
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the virtual address ignored by the translation hardware. This frees up
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this byte for application use.
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Passing tagged addresses to the kernel
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--------------------------------------
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All interpretation of userspace memory addresses by the kernel assumes
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an address tag of 0x00.
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This includes, but is not limited to, addresses found in:
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- pointer arguments to system calls, including pointers in structures
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passed to system calls,
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- the stack pointer (sp), e.g. when interpreting it to deliver a
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signal,
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- the frame pointer (x29) and frame records, e.g. when interpreting
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them to generate a backtrace or call graph.
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Using non-zero address tags in any of these locations may result in an
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error code being returned, a (fatal) signal being raised, or other modes
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of failure.
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For these reasons, passing non-zero address tags to the kernel via
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system calls is forbidden, and using a non-zero address tag for sp is
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strongly discouraged.
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Programs maintaining a frame pointer and frame records that use non-zero
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address tags may suffer impaired or inaccurate debug and profiling
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visibility.
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Preserving tags
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---------------
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Non-zero tags are not preserved when delivering signals. This means that
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signal handlers in applications making use of tags cannot rely on the
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tag information for user virtual addresses being maintained for fields
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inside siginfo_t. One exception to this rule is for signals raised in
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response to watchpoint debug exceptions, where the tag information will
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be preserved.
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The architecture prevents the use of a tagged PC, so the upper byte will
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be set to a sign-extension of bit 55 on exception return.
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Other considerations
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--------------------
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Special care should be taken when using tagged pointers, since it is
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likely that C compilers will not hazard two virtual addresses differing
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only in the upper byte.
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