1 /* 2 * safe-syscall.h: prototypes for linux-user signal-race-safe syscalls 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License as published by 6 * the Free Software Foundation; either version 2 of the License, or 7 * (at your option) any later version. 8 * 9 * This program is distributed in the hope that it will be useful, 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 * GNU General Public License for more details. 13 * 14 * You should have received a copy of the GNU General Public License 15 * along with this program; if not, see <http://www.gnu.org/licenses/>. 16 */ 17 18 #ifndef LINUX_USER_SAFE_SYSCALL_H 19 #define LINUX_USER_SAFE_SYSCALL_H 20 21 /** 22 * safe_syscall: 23 * @int number: number of system call to make 24 * ...: arguments to the system call 25 * 26 * Call a system call if guest signal not pending. 27 * This has the same API as the libc syscall() function, except that it 28 * may return -1 with errno == QEMU_ERESTARTSYS if a signal was pending. 29 * 30 * Returns: the system call result, or -1 with an error code in errno 31 * (Errnos are host errnos; we rely on QEMU_ERESTARTSYS not clashing 32 * with any of the host errno values.) 33 */ 34 35 /* 36 * A guide to using safe_syscall() to handle interactions between guest 37 * syscalls and guest signals: 38 * 39 * Guest syscalls come in two flavours: 40 * 41 * (1) Non-interruptible syscalls 42 * 43 * These are guest syscalls that never get interrupted by signals and 44 * so never return EINTR. They can be implemented straightforwardly in 45 * QEMU: just make sure that if the implementation code has to make any 46 * blocking calls that those calls are retried if they return EINTR. 47 * It's also OK to implement these with safe_syscall, though it will be 48 * a little less efficient if a signal is delivered at the 'wrong' moment. 49 * 50 * Some non-interruptible syscalls need to be handled using block_signals() 51 * to block signals for the duration of the syscall. This mainly applies 52 * to code which needs to modify the data structures used by the 53 * host_signal_handler() function and the functions it calls, including 54 * all syscalls which change the thread's signal mask. 55 * 56 * (2) Interruptible syscalls 57 * 58 * These are guest syscalls that can be interrupted by signals and 59 * for which we need to either return EINTR or arrange for the guest 60 * syscall to be restarted. This category includes both syscalls which 61 * always restart (and in the kernel return -ERESTARTNOINTR), ones 62 * which only restart if there is no handler (kernel returns -ERESTARTNOHAND 63 * or -ERESTART_RESTARTBLOCK), and the most common kind which restart 64 * if the handler was registered with SA_RESTART (kernel returns 65 * -ERESTARTSYS). System calls which are only interruptible in some 66 * situations (like 'open') also need to be handled this way. 67 * 68 * Here it is important that the host syscall is made 69 * via this safe_syscall() function, and *not* via the host libc. 70 * If the host libc is used then the implementation will appear to work 71 * most of the time, but there will be a race condition where a 72 * signal could arrive just before we make the host syscall inside libc, 73 * and then the guest syscall will not correctly be interrupted. 74 * Instead the implementation of the guest syscall can use the safe_syscall 75 * function but otherwise just return the result or errno in the usual 76 * way; the main loop code will take care of restarting the syscall 77 * if appropriate. 78 * 79 * (If the implementation needs to make multiple host syscalls this is 80 * OK; any which might really block must be via safe_syscall(); for those 81 * which are only technically blocking (ie which we know in practice won't 82 * stay in the host kernel indefinitely) it's OK to use libc if necessary. 83 * You must be able to cope with backing out correctly if some safe_syscall 84 * you make in the implementation returns either -QEMU_ERESTARTSYS or 85 * EINTR though.) 86 * 87 * block_signals() cannot be used for interruptible syscalls. 88 * 89 * 90 * How and why the safe_syscall implementation works: 91 * 92 * The basic setup is that we make the host syscall via a known 93 * section of host native assembly. If a signal occurs, our signal 94 * handler checks the interrupted host PC against the addresse of that 95 * known section. If the PC is before or at the address of the syscall 96 * instruction then we change the PC to point at a "return 97 * -QEMU_ERESTARTSYS" code path instead, and then exit the signal handler 98 * (causing the safe_syscall() call to immediately return that value). 99 * Then in the main.c loop if we see this magic return value we adjust 100 * the guest PC to wind it back to before the system call, and invoke 101 * the guest signal handler as usual. 102 * 103 * This winding-back will happen in two cases: 104 * (1) signal came in just before we took the host syscall (a race); 105 * in this case we'll take the guest signal and have another go 106 * at the syscall afterwards, and this is indistinguishable for the 107 * guest from the timing having been different such that the guest 108 * signal really did win the race 109 * (2) signal came in while the host syscall was blocking, and the 110 * host kernel decided the syscall should be restarted; 111 * in this case we want to restart the guest syscall also, and so 112 * rewinding is the right thing. (Note that "restart" semantics mean 113 * "first call the signal handler, then reattempt the syscall".) 114 * The other situation to consider is when a signal came in while the 115 * host syscall was blocking, and the host kernel decided that the syscall 116 * should not be restarted; in this case QEMU's host signal handler will 117 * be invoked with the PC pointing just after the syscall instruction, 118 * with registers indicating an EINTR return; the special code in the 119 * handler will not kick in, and we will return EINTR to the guest as 120 * we should. 121 * 122 * Notice that we can leave the host kernel to make the decision for 123 * us about whether to do a restart of the syscall or not; we do not 124 * need to check SA_RESTART flags in QEMU or distinguish the various 125 * kinds of restartability. 126 */ 127 128 /* The core part of this function is implemented in assembly */ 129 extern long safe_syscall_base(int *pending, long number, ...); 130 extern long safe_syscall_set_errno_tail(int value); 131 132 /* These are defined by the safe-syscall.inc.S file */ 133 extern char safe_syscall_start[]; 134 extern char safe_syscall_end[]; 135 136 #define safe_syscall(...) \ 137 safe_syscall_base(&((TaskState *)thread_cpu->opaque)->signal_pending, \ 138 __VA_ARGS__) 139 140 #endif 141