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#------------------------------------------------------------------------------
#
# Copyright (c) Microsoft Corporation.
# SPDX-License-Identifier: BSD-2-Clause-Patent
#
# Module Name:
#
# DynamicCookie.S
#
# Abstract:
#
# Generates random number through the RNDR instruction on a 64-bit AARCH64 platform
# to store a random value in the GCC __stack_check_guard stack cookie.
# The first byte is 0'd to prevent string copy functions from clobbering
# the stack cookie.
#
# Notes:
#
# If RNDR fails, the build time static stack cookie value will be used instead.
#
#------------------------------------------------------------------------------
#include <AArch64/AArch64.h>
.text
.p2align 2
GCC_ASM_IMPORT(__stack_chk_guard)
GCC_ASM_IMPORT(_CModuleEntryPoint)
GCC_ASM_EXPORT(_ModuleEntryPoint)
#------------------------------------------------------------------------------
# VOID
# EFIAPI
# _ModuleEntryPoint (
# Parameters are passed through.
# )
#------------------------------------------------------------------------------
ASM_PFX(_ModuleEntryPoint):
AARCH64_BTI(c)
mrs x9, ID_AA64ISAR0_EL1 // Read the AArch64 Instruction Set Attribute Register 0
ubfx x9, x9, #60, #4 // Extract the RNDR bit field (bits 60-63)
cbz x9, c_entry // If RNDR is not supported, jump to c_entry
mrs x9, RNDR // Generate a random number
b.eq c_entry // RNDR sets NZCV to 0b0100 on failure
// So if the zero flag is set, use the static stack guard
and x9, x9, #0xFFFFFFFFFFFFFF00 // Zero the first byte of the random value
adrp x8, ASM_PFX(__stack_chk_guard) // Load the page address of __stack_chk_guard
str x9, [x8, :lo12:ASM_PFX(__stack_chk_guard)] // Store the random value in __stack_chk_guard
c_entry:
b ASM_PFX(_CModuleEntryPoint) // Jump to the C module entry point
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