1/* Save current context and install the given one.
2 Copyright (C) 2002-2023 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
25/* int __swapcontext (ucontext_t *oucp, const ucontext_t *ucp);
26
27 Saves the machine context in oucp such that when it is activated,
28 it appears as if __swapcontextt() returned again, restores the
29 machine context in ucp and thereby resumes execution in that
30 context.
31
32 This implementation is intended to be used for *synchronous* context
33 switches only. Therefore, it does not have to save anything
34 other than the PRESERVED state. */
35
36ENTRY(__swapcontext)
37 /* Save the preserved registers, the registers used for passing args,
38 and the return address. */
39 movq %rbx, oRBX(%rdi)
40 movq %rbp, oRBP(%rdi)
41 movq %r12, oR12(%rdi)
42 movq %r13, oR13(%rdi)
43 movq %r14, oR14(%rdi)
44 movq %r15, oR15(%rdi)
45
46 movq %rdi, oRDI(%rdi)
47 movq %rsi, oRSI(%rdi)
48 movq %rdx, oRDX(%rdi)
49 movq %rcx, oRCX(%rdi)
50 movq %r8, oR8(%rdi)
51 movq %r9, oR9(%rdi)
52
53 movq (%rsp), %rcx
54 movq %rcx, oRIP(%rdi)
55 leaq 8(%rsp), %rcx /* Exclude the return address. */
56 movq %rcx, oRSP(%rdi)
57
58 /* We have separate floating-point register content memory on the
59 stack. We use the __fpregs_mem block in the context. Set the
60 links up correctly. */
61 leaq oFPREGSMEM(%rdi), %rcx
62 movq %rcx, oFPREGS(%rdi)
63 /* Save the floating-point environment. */
64 fnstenv (%rcx)
65 stmxcsr oMXCSR(%rdi)
66
67
68 /* The syscall destroys some registers, save them. */
69 movq %rsi, %r12
70 movq %rdi, %r9
71
72 /* Save the current signal mask and install the new one with
73 rt_sigprocmask (SIG_BLOCK, newset, oldset,_NSIG/8). */
74 leaq oSIGMASK(%rdi), %rdx
75 leaq oSIGMASK(%rsi), %rsi
76 movl $SIG_SETMASK, %edi
77 movl $_NSIG8,%r10d
78 movl $__NR_rt_sigprocmask, %eax
79 syscall
80 cmpq $-4095, %rax /* Check %rax for error. */
81 jae SYSCALL_ERROR_LABEL /* Jump to error handler if error. */
82
83 /* Restore destroyed register into RDX. The choice is arbitrary,
84 but leaving RDI and RSI available for use later can avoid
85 shuffling values. */
86 movq %r12, %rdx
87
88 /* Restore the floating-point context. Not the registers, only the
89 rest. */
90 movq oFPREGS(%rdx), %rcx
91 fldenv (%rcx)
92 ldmxcsr oMXCSR(%rdx)
93
94 /* Load the new stack pointer and the preserved registers. */
95 movq oRSP(%rdx), %rsp
96 movq oRBX(%rdx), %rbx
97 movq oRBP(%rdx), %rbp
98 movq oR12(%rdx), %r12
99 movq oR13(%rdx), %r13
100 movq oR14(%rdx), %r14
101 movq oR15(%rdx), %r15
102
103#if SHSTK_ENABLED
104 /* Check if shadow stack is enabled. */
105 testl $X86_FEATURE_1_SHSTK, %fs:FEATURE_1_OFFSET
106 jz L(no_shstk)
107
108 xorl %eax, %eax
109 cmpq %fs:SSP_BASE_OFFSET, %rax
110 jnz L(shadow_stack_bound_recorded)
111
112 /* Get the base address and size of the default shadow stack
113 which must be the current shadow stack since nothing has
114 been recorded yet. */
115 sub $24, %RSP_LP
116 mov %RSP_LP, %RSI_LP
117 movl $ARCH_CET_STATUS, %edi
118 movl $__NR_arch_prctl, %eax
119 syscall
120 testq %rax, %rax
121 jz L(continue_no_err)
122
123 /* This should never happen. */
124 hlt
125
126L(continue_no_err):
127 /* Record the base of the current shadow stack. */
128 movq 8(%rsp), %rax
129 movq %rax, %fs:SSP_BASE_OFFSET
130 add $24, %RSP_LP
131
132L(shadow_stack_bound_recorded):
133 /* If we unwind the stack, we can't undo stack unwinding. Just
134 save the target shadow stack pointer as the current shadow
135 stack pointer. */
136 movq oSSP(%rdx), %rcx
137 movq %rcx, oSSP(%r9)
138
139 /* Save the base of the current shadow stack. */
140 movq %fs:SSP_BASE_OFFSET, %rax
141 movq %rax, (oSSP + 8)(%r9)
142
143 /* If the base of the target shadow stack is the same as the
144 base of the current shadow stack, we unwind the shadow
145 stack. Otherwise it is a stack switch and we look for a
146 restore token. */
147 movq oSSP(%rdx), %rsi
148 movq %rsi, %rdi
149
150 /* Get the base of the target shadow stack. */
151 movq (oSSP + 8)(%rdx), %rcx
152 cmpq %fs:SSP_BASE_OFFSET, %rcx
153 je L(unwind_shadow_stack)
154
155L(find_restore_token_loop):
156 /* Look for a restore token. */
157 movq -8(%rsi), %rax
158 andq $-8, %rax
159 cmpq %rsi, %rax
160 je L(restore_shadow_stack)
161
162 /* Try the next slot. */
163 subq $8, %rsi
164 jmp L(find_restore_token_loop)
165
166L(restore_shadow_stack):
167 /* The target shadow stack will be restored. Save the current
168 shadow stack pointer. */
169 rdsspq %rcx
170 movq %rcx, oSSP(%r9)
171
172 /* Restore the target shadow stack. */
173 rstorssp -8(%rsi)
174
175 /* Save the restore token on the old shadow stack. NB: This
176 restore token may be checked by setcontext or swapcontext
177 later. */
178 saveprevssp
179
180 /* Record the new shadow stack base that was switched to. */
181 movq (oSSP + 8)(%rdx), %rax
182 movq %rax, %fs:SSP_BASE_OFFSET
183
184L(unwind_shadow_stack):
185 rdsspq %rcx
186 subq %rdi, %rcx
187 je L(skip_unwind_shadow_stack)
188 negq %rcx
189 shrq $3, %rcx
190 movl $255, %esi
191L(loop):
192 cmpq %rsi, %rcx
193 cmovb %rcx, %rsi
194 incsspq %rsi
195 subq %rsi, %rcx
196 ja L(loop)
197
198L(skip_unwind_shadow_stack):
199 /* Setup registers used for passing args. */
200 movq oRDI(%rdx), %rdi
201 movq oRSI(%rdx), %rsi
202 movq oRCX(%rdx), %rcx
203 movq oR8(%rdx), %r8
204 movq oR9(%rdx), %r9
205
206 /* Get the return address set with getcontext. */
207 movq oRIP(%rdx), %r10
208
209 /* Setup finally %rdx. */
210 movq oRDX(%rdx), %rdx
211
212 /* Check if return address is valid for the case when setcontext
213 is invoked from __start_context with linked context. */
214 rdsspq %rax
215 cmpq (%rax), %r10
216 /* Clear rax to indicate success. NB: Don't use xorl to keep
217 EFLAGS for jne. */
218 movl $0, %eax
219 jne L(jmp)
220 /* Return to the new context if return address valid. */
221 pushq %r10
222 ret
223
224L(jmp):
225 /* Jump to the new context directly. */
226 jmp *%r10
227
228L(no_shstk):
229#endif
230 /* The following ret should return to the address set with
231 getcontext. Therefore push the address on the stack. */
232 movq oRIP(%rdx), %rcx
233 pushq %rcx
234
235 /* Setup registers used for passing args. */
236 movq oRDI(%rdx), %rdi
237 movq oRSI(%rdx), %rsi
238 movq oRCX(%rdx), %rcx
239 movq oR8(%rdx), %r8
240 movq oR9(%rdx), %r9
241
242 /* Setup finally %rdx. */
243 movq oRDX(%rdx), %rdx
244
245 /* Clear rax to indicate success. */
246 xorl %eax, %eax
247 ret
248PSEUDO_END(__swapcontext)
249
250weak_alias (__swapcontext, swapcontext)
251