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torvalds
GitHub Repository: torvalds/linux
 Path: blob/master/arch/arm64/kernel/efi-rt-wrapper.S
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/* SPDX-License-Identifier: GPL-2.0-only */

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/*

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 * Copyright (C) 2018 Linaro Ltd <[email protected]>

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 */

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#include <linux/linkage.h>

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#include <asm/assembler.h>

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SYM_FUNC_START(__efi_rt_asm_wrapper)

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	stp	x29, x30, [sp, #-112]!

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	mov	x29, sp

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	/*

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	 * Register x18 is designated as the 'platform' register by the AAPCS,

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	 * which means firmware running at the same exception level as the OS

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	 * (such as UEFI) should never touch it.

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	 */

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	stp	x1, x18, [sp, #16]

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	/*

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	 * Preserve all callee saved registers and preserve the stack pointer

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	 * value at the base of the EFI runtime stack so we can recover from

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	 * synchronous exceptions occurring while executing the firmware

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	 * routines.

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	 */

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	stp	x19, x20, [sp, #32]

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	stp	x21, x22, [sp, #48]

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	stp	x23, x24, [sp, #64]

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	stp	x25, x26, [sp, #80]

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	stp	x27, x28, [sp, #96]

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	ldr_l	x16, efi_rt_stack_top

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	mov	sp, x16

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	stp	x18, x29, [sp, #-16]!

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	/*

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	 * We are lucky enough that no EFI runtime services take more than

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	 * 5 arguments, so all are passed in registers rather than via the

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	 * stack.

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	 */

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	mov	x8, x0

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	mov	x0, x2

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	mov	x1, x3

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	mov	x2, x4

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	mov	x3, x5

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	mov	x4, x6

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	blr	x8

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	mov	x16, sp

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	mov	sp, x29

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	str	xzr, [x16, #8]			// clear recorded task SP value

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	ldp	x1, x2, [sp, #16]

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	cmp	x2, x18

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	ldp	x29, x30, [sp], #112

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	b.ne	0f

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	ret

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0:

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	/*

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	 * With CONFIG_SHADOW_CALL_STACK, the kernel uses x18 to store a

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	 * shadow stack pointer, which we need to restore before returning to

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	 * potentially instrumented code. This is safe because the wrapper is

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	 * called with preemption disabled and a separate shadow stack is used

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	 * for interrupts.

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	 */

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#ifdef CONFIG_SHADOW_CALL_STACK

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	ldr_l	x18, efi_rt_stack_top

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	ldr	x18, [x18, #-16]

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#endif

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	b	efi_handle_corrupted_x18	// tail call

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SYM_FUNC_END(__efi_rt_asm_wrapper)

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SYM_CODE_START(__efi_rt_asm_recover)

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	mov	sp, x30

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	ldr_l	x16, efi_rt_stack_top		// clear recorded task SP value

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	str	xzr, [x16, #-8]

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	ldp	x19, x20, [sp, #32]

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	ldp	x21, x22, [sp, #48]

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	ldp	x23, x24, [sp, #64]

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	ldp	x25, x26, [sp, #80]

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	ldp	x27, x28, [sp, #96]

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	ldp	x29, x30, [sp], #112

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	ret

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SYM_CODE_END(__efi_rt_asm_recover)

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