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This patch adds the bare minimum required to expose the ARMv8.5 Branch Target Identification feature to userspace. By itself, this does _not_ automatically enable BTI for any initial executable pages mapped by execve(). This will come later, but for now it should be possible to enable BTI manually on those pages by using mprotect() from within the target process. Other arches already using the generic mman.h are already using 0x10 for arch-specific prot flags, so we use that for PROT_BTI here. For consistency, signal handler entry points in BTI guarded pages are required to be annotated as such, just like any other function. This blocks a relatively minor attack vector, but comforming userspace will have the annotations anyway, so we may as well enforce them. Signed-off-by: Mark Brown <broonie@kernel.org> Signed-off-by: Dave Martin <Dave.Martin@arm.com> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
249 lines
6.5 KiB
ReStructuredText
249 lines
6.5 KiB
ReStructuredText
================
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ARM64 ELF hwcaps
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================
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This document describes the usage and semantics of the arm64 ELF hwcaps.
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1. Introduction
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---------------
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Some hardware or software features are only available on some CPU
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implementations, and/or with certain kernel configurations, but have no
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architected discovery mechanism available to userspace code at EL0. The
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kernel exposes the presence of these features to userspace through a set
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of flags called hwcaps, exposed in the auxilliary vector.
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Userspace software can test for features by acquiring the AT_HWCAP or
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AT_HWCAP2 entry of the auxiliary vector, and testing whether the relevant
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flags are set, e.g.::
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bool floating_point_is_present(void)
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{
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unsigned long hwcaps = getauxval(AT_HWCAP);
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if (hwcaps & HWCAP_FP)
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return true;
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return false;
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}
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Where software relies on a feature described by a hwcap, it should check
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the relevant hwcap flag to verify that the feature is present before
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attempting to make use of the feature.
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Features cannot be probed reliably through other means. When a feature
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is not available, attempting to use it may result in unpredictable
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behaviour, and is not guaranteed to result in any reliable indication
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that the feature is unavailable, such as a SIGILL.
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2. Interpretation of hwcaps
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---------------------------
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The majority of hwcaps are intended to indicate the presence of features
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which are described by architected ID registers inaccessible to
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userspace code at EL0. These hwcaps are defined in terms of ID register
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fields, and should be interpreted with reference to the definition of
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these fields in the ARM Architecture Reference Manual (ARM ARM).
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Such hwcaps are described below in the form::
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Functionality implied by idreg.field == val.
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Such hwcaps indicate the availability of functionality that the ARM ARM
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defines as being present when idreg.field has value val, but do not
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indicate that idreg.field is precisely equal to val, nor do they
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indicate the absence of functionality implied by other values of
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idreg.field.
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Other hwcaps may indicate the presence of features which cannot be
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described by ID registers alone. These may be described without
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reference to ID registers, and may refer to other documentation.
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3. The hwcaps exposed in AT_HWCAP
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---------------------------------
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HWCAP_FP
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Functionality implied by ID_AA64PFR0_EL1.FP == 0b0000.
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HWCAP_ASIMD
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Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0000.
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HWCAP_EVTSTRM
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The generic timer is configured to generate events at a frequency of
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approximately 100KHz.
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HWCAP_AES
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Functionality implied by ID_AA64ISAR0_EL1.AES == 0b0001.
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HWCAP_PMULL
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Functionality implied by ID_AA64ISAR0_EL1.AES == 0b0010.
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HWCAP_SHA1
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Functionality implied by ID_AA64ISAR0_EL1.SHA1 == 0b0001.
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HWCAP_SHA2
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Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0001.
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HWCAP_CRC32
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Functionality implied by ID_AA64ISAR0_EL1.CRC32 == 0b0001.
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HWCAP_ATOMICS
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Functionality implied by ID_AA64ISAR0_EL1.Atomic == 0b0010.
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HWCAP_FPHP
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Functionality implied by ID_AA64PFR0_EL1.FP == 0b0001.
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HWCAP_ASIMDHP
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Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0001.
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HWCAP_CPUID
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EL0 access to certain ID registers is available, to the extent
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described by Documentation/arm64/cpu-feature-registers.rst.
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These ID registers may imply the availability of features.
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HWCAP_ASIMDRDM
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Functionality implied by ID_AA64ISAR0_EL1.RDM == 0b0001.
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HWCAP_JSCVT
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Functionality implied by ID_AA64ISAR1_EL1.JSCVT == 0b0001.
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HWCAP_FCMA
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Functionality implied by ID_AA64ISAR1_EL1.FCMA == 0b0001.
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HWCAP_LRCPC
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Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0001.
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HWCAP_DCPOP
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Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0001.
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HWCAP_SHA3
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Functionality implied by ID_AA64ISAR0_EL1.SHA3 == 0b0001.
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HWCAP_SM3
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Functionality implied by ID_AA64ISAR0_EL1.SM3 == 0b0001.
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HWCAP_SM4
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Functionality implied by ID_AA64ISAR0_EL1.SM4 == 0b0001.
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HWCAP_ASIMDDP
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Functionality implied by ID_AA64ISAR0_EL1.DP == 0b0001.
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HWCAP_SHA512
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Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0010.
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HWCAP_SVE
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Functionality implied by ID_AA64PFR0_EL1.SVE == 0b0001.
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HWCAP_ASIMDFHM
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Functionality implied by ID_AA64ISAR0_EL1.FHM == 0b0001.
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HWCAP_DIT
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Functionality implied by ID_AA64PFR0_EL1.DIT == 0b0001.
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HWCAP_USCAT
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Functionality implied by ID_AA64MMFR2_EL1.AT == 0b0001.
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HWCAP_ILRCPC
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Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0010.
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HWCAP_FLAGM
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Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0001.
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HWCAP_SSBS
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Functionality implied by ID_AA64PFR1_EL1.SSBS == 0b0010.
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HWCAP_SB
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Functionality implied by ID_AA64ISAR1_EL1.SB == 0b0001.
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HWCAP_PACA
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Functionality implied by ID_AA64ISAR1_EL1.APA == 0b0001 or
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ID_AA64ISAR1_EL1.API == 0b0001, as described by
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Documentation/arm64/pointer-authentication.rst.
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HWCAP_PACG
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Functionality implied by ID_AA64ISAR1_EL1.GPA == 0b0001 or
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ID_AA64ISAR1_EL1.GPI == 0b0001, as described by
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Documentation/arm64/pointer-authentication.rst.
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HWCAP2_DCPODP
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Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0010.
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HWCAP2_SVE2
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Functionality implied by ID_AA64ZFR0_EL1.SVEVer == 0b0001.
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HWCAP2_SVEAES
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Functionality implied by ID_AA64ZFR0_EL1.AES == 0b0001.
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HWCAP2_SVEPMULL
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Functionality implied by ID_AA64ZFR0_EL1.AES == 0b0010.
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HWCAP2_SVEBITPERM
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Functionality implied by ID_AA64ZFR0_EL1.BitPerm == 0b0001.
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HWCAP2_SVESHA3
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Functionality implied by ID_AA64ZFR0_EL1.SHA3 == 0b0001.
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HWCAP2_SVESM4
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Functionality implied by ID_AA64ZFR0_EL1.SM4 == 0b0001.
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HWCAP2_FLAGM2
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Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0010.
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HWCAP2_FRINT
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Functionality implied by ID_AA64ISAR1_EL1.FRINTTS == 0b0001.
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HWCAP2_SVEI8MM
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Functionality implied by ID_AA64ZFR0_EL1.I8MM == 0b0001.
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HWCAP2_SVEF32MM
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Functionality implied by ID_AA64ZFR0_EL1.F32MM == 0b0001.
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HWCAP2_SVEF64MM
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Functionality implied by ID_AA64ZFR0_EL1.F64MM == 0b0001.
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HWCAP2_SVEBF16
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Functionality implied by ID_AA64ZFR0_EL1.BF16 == 0b0001.
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HWCAP2_I8MM
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Functionality implied by ID_AA64ISAR1_EL1.I8MM == 0b0001.
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HWCAP2_BF16
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Functionality implied by ID_AA64ISAR1_EL1.BF16 == 0b0001.
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HWCAP2_DGH
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Functionality implied by ID_AA64ISAR1_EL1.DGH == 0b0001.
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HWCAP2_RNG
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Functionality implied by ID_AA64ISAR0_EL1.RNDR == 0b0001.
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HWCAP2_BTI
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Functionality implied by ID_AA64PFR0_EL1.BT == 0b0001.
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4. Unused AT_HWCAP bits
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-----------------------
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For interoperation with userspace, the kernel guarantees that bits 62
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and 63 of AT_HWCAP will always be returned as 0.
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