1/* Install given context.
2   Copyright (C) 2001-2022 Free Software Foundation, Inc.
3   This file is part of the GNU C Library.
4
5   The GNU C Library is free software; you can redistribute it and/or
6   modify it under the terms of the GNU Lesser General Public
7   License as published by the Free Software Foundation; either
8   version 2.1 of the License, or (at your option) any later version.
9
10   The GNU C Library is distributed in the hope that it will be useful,
11   but WITHOUT ANY WARRANTY; without even the implied warranty of
12   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
13   Lesser General Public License for more details.
14
15   You should have received a copy of the GNU Lesser General Public
16   License along with the GNU C Library; if not, see
17   <https://www.gnu.org/licenses/>.  */
18
19#include <sysdep.h>
20#include <asm/prctl.h>
21
22#include "ucontext_i.h"
23
24
25ENTRY(__setcontext)
26	/* Load address of the context data structure.  */
27	movl	4(%esp), %eax
28
29	/* Get the current signal mask.  Note that we preserve EBX in case
30	   the system call fails and we return from the function with an
31	   error.  */
32	pushl	%ebx
33	cfi_adjust_cfa_offset (4)
34	xorl	%edx, %edx
35	leal	oSIGMASK(%eax), %ecx
36	movl	$SIG_SETMASK, %ebx
37	cfi_rel_offset (ebx, 0)
38	movl	$__NR_sigprocmask, %eax
39	ENTER_KERNEL
40	popl	%ebx
41	cfi_adjust_cfa_offset (-4)
42	cfi_restore (ebx)
43	cmpl	$-4095, %eax		/* Check %eax for error.  */
44	jae	SYSCALL_ERROR_LABEL	/* Jump to error handler if error.  */
45
46	/* EAX was modified, reload it.  */
47	movl	4(%esp), %eax
48
49	/* Restore the floating-point context.  Not the registers, only the
50	   rest.  */
51	movl	oFPREGS(%eax), %ecx
52	fldenv	(%ecx)
53
54	/* Restore the FS segment register.  We don't touch the GS register
55	   since it is used for threads.  */
56	movl	oFS(%eax), %ecx
57	movw	%cx, %fs
58
59	/* Load the new stack pointer.  */
60	cfi_def_cfa (eax, 0)
61	cfi_offset (edi, oEDI)
62	cfi_offset (esi, oESI)
63	cfi_offset (ebp, oEBP)
64	cfi_offset (ebx, oEBX)
65	movl	oESP(%eax), %esp
66
67#if SHSTK_ENABLED
68	/* Check if Shadow Stack is enabled.  */
69	testl	$X86_FEATURE_1_SHSTK, %gs:FEATURE_1_OFFSET
70	jz	L(no_shstk)
71
72	/* If the base of the target shadow stack is the same as the
73	   base of the current shadow stack, we unwind the shadow
74	   stack.  Otherwise it is a stack switch and we look for a
75	   restore token.  */
76	movl	oSSP(%eax), %esi
77	movl	%esi, %edi
78
79	/* Get the base of the target shadow stack.  */
80	movl	(oSSP + 4)(%eax), %ecx
81	cmpl	%gs:SSP_BASE_OFFSET, %ecx
82	je	L(unwind_shadow_stack)
83
84	/* Align the saved original shadow stack pointer to the next
85	   8 byte aligned boundary.  */
86	andl	$-8, %esi
87
88L(find_restore_token_loop):
89	/* Look for a restore token.  */
90	movl	-8(%esi), %ebx
91	andl	$-8, %ebx
92	cmpl	%esi, %ebx
93	je	L(restore_shadow_stack)
94
95	/* Try the next slot.  */
96	subl	$8, %esi
97	jmp	L(find_restore_token_loop)
98
99L(restore_shadow_stack):
100	/* Pop return address from the shadow stack since setcontext
101	   will not return.  */
102	movl	$1, %ebx
103	incsspd	%ebx
104
105	/* Use the restore stoken to restore the target shadow stack.  */
106	rstorssp -8(%esi)
107
108	/* Save the restore token on the old shadow stack.  NB: This
109	   restore token may be checked by setcontext or swapcontext
110	   later.  */
111	saveprevssp
112
113	/* Record the new shadow stack base that was switched to.  */
114	movl	(oSSP + 4)(%eax), %ebx
115	movl	%ebx, %gs:SSP_BASE_OFFSET
116
117L(unwind_shadow_stack):
118	rdsspd	%ebx
119	subl	%edi, %ebx
120	je	L(skip_unwind_shadow_stack)
121	negl	%ebx
122	shrl	$2, %ebx
123	movl	$255, %esi
124L(loop):
125	cmpl	%esi, %ebx
126	cmovb	%ebx, %esi
127	incsspd	%esi
128	subl	%esi, %ebx
129	ja	L(loop)
130
131L(skip_unwind_shadow_stack):
132
133	/* Load the values of all the preserved registers (except ESP).  */
134	movl	oEDI(%eax), %edi
135	movl	oESI(%eax), %esi
136	movl	oEBP(%eax), %ebp
137	movl	oEBX(%eax), %ebx
138
139	/* Get the return address set with getcontext.  */
140	movl	oEIP(%eax), %ecx
141
142	/* Check if return address is valid for the case when setcontext
143	   is invoked from L(exitcode) with linked context.  */
144	rdsspd	%eax
145	cmpl	(%eax), %ecx
146	/* Clear EAX to indicate success.  NB: Don't use xorl to keep
147	   EFLAGS for jne.  */
148	movl	$0, %eax
149	jne	L(jmp)
150	/* Return to the new context if return address valid.  */
151	pushl	%ecx
152	ret
153
154L(jmp):
155	/* Jump to the new context directly.  */
156	jmp	*%ecx
157
158L(no_shstk):
159#endif
160
161	/* Fetch the address to return to.  */
162	movl	oEIP(%eax), %ecx
163
164	/* Push the return address on the new stack so we can return there.  */
165	pushl	%ecx
166
167	/* Load the values of all the preserved registers (except ESP).  */
168	movl	oEDI(%eax), %edi
169	movl	oESI(%eax), %esi
170	movl	oEBP(%eax), %ebp
171	movl	oEBX(%eax), %ebx
172
173	/* All done, return 0 for success.  */
174	xorl	%eax, %eax
175
176	/* End FDE here, we fall into another context.  */
177	cfi_endproc
178	cfi_startproc
179
180	/* The following 'ret' will pop the address of the code and jump
181	   to it.  */
182
183	ret
184PSEUDO_END(__setcontext)
185libc_hidden_def (__setcontext)
186
187weak_alias (__setcontext, setcontext)
188