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

/* Install given context.
   Copyright (C) 2002-2023 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   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 __setcontext (const ucontext_t *ucp)

  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 restore anything
  other than the PRESERVED state.  */

ENTRY(__setcontext)
	/* Save argument since syscall will destroy it.  */
	pushq	%rdi
	cfi_adjust_cfa_offset(8)

	/* Set the signal mask with
	   rt_sigprocmask (SIG_SETMASK, mask, NULL, _NSIG/8).  */
	leaq	oSIGMASK(%rdi), %rsi
	xorl	%edx, %edx
	movl	$SIG_SETMASK, %edi
	movl	$_NSIG8,%r10d
	movl	$__NR_rt_sigprocmask, %eax
	syscall
	/* Pop the pointer into RDX. The choice is arbitrary, but
	   leaving RDI and RSI available for use later can avoid
	   shuffling values.  */
	popq	%rdx
	cfi_adjust_cfa_offset(-8)
	cmpq	$-4095, %rax		/* Check %rax for error.  */
	jae	SYSCALL_ERROR_LABEL	/* Jump to error handler if error.  */

	/* 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, the preserved registers and
	   registers used for passing args.  */
	cfi_def_cfa(%rdx, 0)
	cfi_offset(%rbx,oRBX)
	cfi_offset(%rbp,oRBP)
	cfi_offset(%r12,oR12)
	cfi_offset(%r13,oR13)
	cfi_offset(%r14,oR14)
	cfi_offset(%r15,oR15)
	cfi_offset(%rsp,oRSP)
	cfi_offset(%rip,oRIP)

	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)

	/* 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):
	/* Pop return address from the shadow stack since setcontext
	   will not return.  */
	movq	$1, %rax
	incsspq	%rax

	/* Use the restore stoken to 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):
	movq	oRSI(%rdx), %rsi
	movq	oRDI(%rdx), %rdi
	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

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

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

	/* End FDE here, we fall into another context.  */
	cfi_endproc
	cfi_startproc

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

weak_alias (__setcontext, setcontext)