glibc/sysdeps/unix/sysv/linux/x86_64/swapcontext.S

/* Save current context and install the given one.
   Copyright (C) 2002-2020 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Andreas Jaeger <aj@suse.de>, 2002.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <sysdep.h>
#include <asm/prctl.h>

#include "ucontext_i.h"


/* int __swapcontext (ucontext_t *oucp, const ucontext_t *ucp);

  Saves the machine context in oucp such that when it is activated,
  it appears as if __swapcontextt() returned again, restores the
  machine context in ucp and thereby resumes execution in that
  context.

  This implementation is intended to be used for *synchronous* context
  switches only.  Therefore, it does not have to save anything
  other than the PRESERVED state.  */

ENTRY(__swapcontext)
	/* Save the preserved registers, the registers used for passing args,
	   and the return address.  */
	movq	%rbx, oRBX(%rdi)
	movq	%rbp, oRBP(%rdi)
	movq	%r12, oR12(%rdi)
	movq	%r13, oR13(%rdi)
	movq	%r14, oR14(%rdi)
	movq	%r15, oR15(%rdi)

	movq	%rdi, oRDI(%rdi)
	movq	%rsi, oRSI(%rdi)
	movq	%rdx, oRDX(%rdi)
	movq	%rcx, oRCX(%rdi)
	movq	%r8, oR8(%rdi)
	movq	%r9, oR9(%rdi)

	movq	(%rsp), %rcx
	movq	%rcx, oRIP(%rdi)
	leaq	8(%rsp), %rcx		/* Exclude the return address.  */
	movq	%rcx, oRSP(%rdi)

	/* We have separate floating-point register content memory on the
	   stack.  We use the __fpregs_mem block in the context.  Set the
	   links up correctly.  */
	leaq	oFPREGSMEM(%rdi), %rcx
	movq	%rcx, oFPREGS(%rdi)
	/* Save the floating-point environment.  */
	fnstenv	(%rcx)
	stmxcsr oMXCSR(%rdi)


	/* The syscall destroys some registers, save them.  */
	movq	%rsi, %r12
	movq	%rdi, %r9

	/* Save the current signal mask and install the new one with
	   rt_sigprocmask (SIG_BLOCK, newset, oldset,_NSIG/8).  */
	leaq	oSIGMASK(%rdi), %rdx
	leaq	oSIGMASK(%rsi), %rsi
	movl	$SIG_SETMASK, %edi
	movl	$_NSIG8,%r10d
	movl	$__NR_rt_sigprocmask, %eax
	syscall
	cmpq	$-4095, %rax		/* Check %rax for error.  */
	jae	SYSCALL_ERROR_LABEL	/* Jump to error handler if error.  */

	/* Restore destroyed register into RDX. The choice is arbitrary,
	   but leaving RDI and RSI available for use later can avoid
	   shuffling values.  */
	movq	%r12, %rdx

	/* Restore the floating-point context.  Not the registers, only the
	   rest.  */
	movq	oFPREGS(%rdx), %rcx
	fldenv	(%rcx)
	ldmxcsr oMXCSR(%rdx)

	/* Load the new stack pointer and the preserved registers.  */
	movq	oRSP(%rdx), %rsp
	movq	oRBX(%rdx), %rbx
	movq	oRBP(%rdx), %rbp
	movq	oR12(%rdx), %r12
	movq	oR13(%rdx), %r13
	movq	oR14(%rdx), %r14
	movq	oR15(%rdx), %r15

#if SHSTK_ENABLED
	/* Check if shadow stack is enabled.  */
	testl	$X86_FEATURE_1_SHSTK, %fs:FEATURE_1_OFFSET
	jz	L(no_shstk)

	xorl	%eax, %eax
	cmpq	%fs:SSP_BASE_OFFSET, %rax
	jnz	L(shadow_stack_bound_recorded)

	/* Get the base address and size of the default shadow stack
	   which must be the current shadow stack since nothing has
	   been recorded yet.  */
	sub	$24, %RSP_LP
	mov	%RSP_LP, %RSI_LP
	movl	$ARCH_CET_STATUS, %edi
	movl	$__NR_arch_prctl, %eax
	syscall
	testq	%rax, %rax
	jz	L(continue_no_err)

	/* This should never happen.  */
	hlt

L(continue_no_err):
	/* Record the base of the current shadow stack.  */
	movq	8(%rsp), %rax
	movq	%rax, %fs:SSP_BASE_OFFSET
	add	$24, %RSP_LP

L(shadow_stack_bound_recorded):
        /* If we unwind the stack, we can't undo stack unwinding.  Just
	   save the target shadow stack pointer as the current shadow
	   stack pointer.   */
	movq	oSSP(%rdx), %rcx
	movq	%rcx, oSSP(%r9)

	/* Save the base of the current shadow stack.  */
	movq	%fs:SSP_BASE_OFFSET, %rax
	movq	%rax, (oSSP + 8)(%r9)

	/* If the base of the target shadow stack is the same as the
	   base of the current shadow stack, we unwind the shadow
	   stack.  Otherwise it is a stack switch and we look for a
	   restore token.  */
	movq	oSSP(%rdx), %rsi
	movq	%rsi, %rdi

	/* Get the base of the target shadow stack.  */
	movq	(oSSP + 8)(%rdx), %rcx
	cmpq	%fs:SSP_BASE_OFFSET, %rcx
	je	L(unwind_shadow_stack)

L(find_restore_token_loop):
	/* Look for a restore token.  */
	movq	-8(%rsi), %rax
	andq	$-8, %rax
	cmpq	%rsi, %rax
	je	L(restore_shadow_stack)

	/* Try the next slot.  */
	subq	$8, %rsi
	jmp	L(find_restore_token_loop)

L(restore_shadow_stack):
        /* The target shadow stack will be restored.  Save the current
	   shadow stack pointer.  */
	rdsspq	%rcx
	movq	%rcx, oSSP(%r9)

	/* Restore the target shadow stack.  */
	rstorssp -8(%rsi)

	/* Save the restore token on the old shadow stack.  NB: This
	   restore token may be checked by setcontext or swapcontext
	   later.  */
	saveprevssp

	/* Record the new shadow stack base that was switched to.   */
	movq	(oSSP + 8)(%rdx), %rax
	movq	%rax, %fs:SSP_BASE_OFFSET

L(unwind_shadow_stack):
	rdsspq	%rcx
	subq	%rdi, %rcx
	je	L(skip_unwind_shadow_stack)
	negq	%rcx
	shrq	$3, %rcx
	movl	$255, %esi
L(loop):
	cmpq	%rsi, %rcx
	cmovb	%rcx, %rsi
	incsspq	%rsi
	subq	%rsi, %rcx
	ja	L(loop)

L(skip_unwind_shadow_stack):
	/* Setup registers used for passing args.  */
	movq	oRDI(%rdx), %rdi
	movq	oRSI(%rdx), %rsi
	movq	oRCX(%rdx), %rcx
	movq	oR8(%rdx), %r8
	movq	oR9(%rdx), %r9

	/* Get the return address set with getcontext.  */
	movq	oRIP(%rdx), %r10

	/* Setup finally %rdx.  */
	movq	oRDX(%rdx), %rdx

	/* Check if return address is valid for the case when setcontext
	   is invoked from __start_context with linked context.  */
	rdsspq	%rax
	cmpq	(%rax), %r10
	/* Clear rax to indicate success.  NB: Don't use xorl to keep
	   EFLAGS for jne.  */
	movl	$0, %eax
	jne	L(jmp)
	/* Return to the new context if return address valid.  */
	pushq	%r10
	ret

L(jmp):
	/* Jump to the new context directly.  */
	jmp	*%r10

L(no_shstk):
#endif
	/* The following ret should return to the address set with
	getcontext.  Therefore push the address on the stack.  */
	movq	oRIP(%rdx), %rcx
	pushq	%rcx

	/* Setup registers used for passing args.  */
	movq	oRDI(%rdx), %rdi
	movq	oRSI(%rdx), %rsi
	movq	oRCX(%rdx), %rcx
	movq	oR8(%rdx), %r8
	movq	oR9(%rdx), %r9

	/* Setup finally %rdx.  */
	movq	oRDX(%rdx), %rdx

	/* Clear rax to indicate success.  */
	xorl	%eax, %eax
	ret
PSEUDO_END(__swapcontext)

weak_alias (__swapcontext, swapcontext)