about summary refs log tree commit diff
path: root/sysdeps/unix/sysv/linux/i386/setcontext.S
blob: f5a6495549c6cd528c2236e53b07f978c60b1176 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
/* Install given context.
   Copyright (C) 2001-2021 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"


ENTRY(__setcontext)
	/* Load address of the context data structure.  */
	movl	4(%esp), %eax

	/* Get the current signal mask.  Note that we preserve EBX in case
	   the system call fails and we return from the function with an
	   error.  */
	pushl	%ebx
	cfi_adjust_cfa_offset (4)
	xorl	%edx, %edx
	leal	oSIGMASK(%eax), %ecx
	movl	$SIG_SETMASK, %ebx
	cfi_rel_offset (ebx, 0)
	movl	$__NR_sigprocmask, %eax
	ENTER_KERNEL
	popl	%ebx
	cfi_adjust_cfa_offset (-4)
	cfi_restore (ebx)
	cmpl	$-4095, %eax		/* Check %eax for error.  */
	jae	SYSCALL_ERROR_LABEL	/* Jump to error handler if error.  */

	/* EAX was modified, reload it.  */
	movl	4(%esp), %eax

	/* Restore the floating-point context.  Not the registers, only the
	   rest.  */
	movl	oFPREGS(%eax), %ecx
	fldenv	(%ecx)

	/* Restore the FS segment register.  We don't touch the GS register
	   since it is used for threads.  */
	movl	oFS(%eax), %ecx
	movw	%cx, %fs

	/* Load the new stack pointer.  */
	cfi_def_cfa (eax, 0)
	cfi_offset (edi, oEDI)
	cfi_offset (esi, oESI)
	cfi_offset (ebp, oEBP)
	cfi_offset (ebx, oEBX)
	movl	oESP(%eax), %esp

#if SHSTK_ENABLED
	/* Check if Shadow Stack is enabled.  */
	testl	$X86_FEATURE_1_SHSTK, %gs: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.  */
	movl	oSSP(%eax), %esi
	movl	%esi, %edi

	/* Get the base of the target shadow stack.  */
	movl	(oSSP + 4)(%eax), %ecx
	cmpl	%gs:SSP_BASE_OFFSET, %ecx
	je	L(unwind_shadow_stack)

	/* Align the saved original shadow stack pointer to the next
	   8 byte aligned boundary.  */
	andl	$-8, %esi

L(find_restore_token_loop):
	/* Look for a restore token.  */
	movl	-8(%esi), %ebx
	andl	$-8, %ebx
	cmpl	%esi, %ebx
	je	L(restore_shadow_stack)

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

L(restore_shadow_stack):
	/* Pop return address from the shadow stack since setcontext
	   will not return.  */
	movl	$1, %ebx
	incsspd	%ebx

	/* Use the restore stoken to restore the target shadow stack.  */
	rstorssp -8(%esi)

	/* 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.  */
	movl	(oSSP + 4)(%eax), %ebx
	movl	%ebx, %gs:SSP_BASE_OFFSET

L(unwind_shadow_stack):
	rdsspd	%ebx
	subl	%edi, %ebx
	je	L(skip_unwind_shadow_stack)
	negl	%ebx
	shrl	$2, %ebx
	movl	$255, %esi
L(loop):
	cmpl	%esi, %ebx
	cmovb	%ebx, %esi
	incsspd	%esi
	subl	%esi, %ebx
	ja	L(loop)

L(skip_unwind_shadow_stack):

	/* Load the values of all the preserved registers (except ESP).  */
	movl	oEDI(%eax), %edi
	movl	oESI(%eax), %esi
	movl	oEBP(%eax), %ebp
	movl	oEBX(%eax), %ebx

	/* Get the return address set with getcontext.  */
	movl	oEIP(%eax), %ecx

	/* Check if return address is valid for the case when setcontext
	   is invoked from L(exitcode) with linked context.  */
	rdsspd	%eax
	cmpl	(%eax), %ecx
	/* Clear EAX 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.  */
	pushl	%ecx
	ret

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

L(no_shstk):
#endif

	/* Fetch the address to return to.  */
	movl	oEIP(%eax), %ecx

	/* Push the return address on the new stack so we can return there.  */
	pushl	%ecx

	/* Load the values of all the preserved registers (except ESP).  */
	movl	oEDI(%eax), %edi
	movl	oESI(%eax), %esi
	movl	oEBP(%eax), %ebp
	movl	oEBX(%eax), %ebx

	/* All done, return 0 for success.  */
	xorl	%eax, %eax

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

	/* The following 'ret' will pop the address of the code and jump
	   to it.  */

	ret
PSEUDO_END(__setcontext)
libc_hidden_def (__setcontext)

weak_alias (__setcontext, setcontext)