diff options
Diffstat (limited to 'linux-user/qemu.h')
| -rw-r--r-- | linux-user/qemu.h | 127 |
1 files changed, 126 insertions, 1 deletions
diff --git a/linux-user/qemu.h b/linux-user/qemu.h index 208c63eb2a..f09b750bbf 100644 --- a/linux-user/qemu.h +++ b/linux-user/qemu.h @@ -1,7 +1,7 @@ #ifndef QEMU_H #define QEMU_H - +#include "hostdep.h" #include "cpu.h" #include "exec/exec-all.h" #include "exec/cpu_ldst.h" @@ -205,6 +205,131 @@ unsigned long init_guest_space(unsigned long host_start, #include "qemu/log.h" +/* safe_syscall.S */ + +/** + * safe_syscall: + * @int number: number of system call to make + * ...: arguments to the system call + * + * Call a system call if guest signal not pending. + * This has the same API as the libc syscall() function, except that it + * may return -1 with errno == TARGET_ERESTARTSYS if a signal was pending. + * + * Returns: the system call result, or -1 with an error code in errno + * (Errnos are host errnos; we rely on TARGET_ERESTARTSYS not clashing + * with any of the host errno values.) + */ + +/* A guide to using safe_syscall() to handle interactions between guest + * syscalls and guest signals: + * + * Guest syscalls come in two flavours: + * + * (1) Non-interruptible syscalls + * + * These are guest syscalls that never get interrupted by signals and + * so never return EINTR. They can be implemented straightforwardly in + * QEMU: just make sure that if the implementation code has to make any + * blocking calls that those calls are retried if they return EINTR. + * It's also OK to implement these with safe_syscall, though it will be + * a little less efficient if a signal is delivered at the 'wrong' moment. + * + * (2) Interruptible syscalls + * + * These are guest syscalls that can be interrupted by signals and + * for which we need to either return EINTR or arrange for the guest + * syscall to be restarted. This category includes both syscalls which + * always restart (and in the kernel return -ERESTARTNOINTR), ones + * which only restart if there is no handler (kernel returns -ERESTARTNOHAND + * or -ERESTART_RESTARTBLOCK), and the most common kind which restart + * if the handler was registered with SA_RESTART (kernel returns + * -ERESTARTSYS). System calls which are only interruptible in some + * situations (like 'open') also need to be handled this way. + * + * Here it is important that the host syscall is made + * via this safe_syscall() function, and *not* via the host libc. + * If the host libc is used then the implementation will appear to work + * most of the time, but there will be a race condition where a + * signal could arrive just before we make the host syscall inside libc, + * and then then guest syscall will not correctly be interrupted. + * Instead the implementation of the guest syscall can use the safe_syscall + * function but otherwise just return the result or errno in the usual + * way; the main loop code will take care of restarting the syscall + * if appropriate. + * + * (If the implementation needs to make multiple host syscalls this is + * OK; any which might really block must be via safe_syscall(); for those + * which are only technically blocking (ie which we know in practice won't + * stay in the host kernel indefinitely) it's OK to use libc if necessary. + * You must be able to cope with backing out correctly if some safe_syscall + * you make in the implementation returns either -TARGET_ERESTARTSYS or + * EINTR though.) + * + * + * How and why the safe_syscall implementation works: + * + * The basic setup is that we make the host syscall via a known + * section of host native assembly. If a signal occurs, our signal + * handler checks the interrupted host PC against the addresse of that + * known section. If the PC is before or at the address of the syscall + * instruction then we change the PC to point at a "return + * -TARGET_ERESTARTSYS" code path instead, and then exit the signal handler + * (causing the safe_syscall() call to immediately return that value). + * Then in the main.c loop if we see this magic return value we adjust + * the guest PC to wind it back to before the system call, and invoke + * the guest signal handler as usual. + * + * This winding-back will happen in two cases: + * (1) signal came in just before we took the host syscall (a race); + * in this case we'll take the guest signal and have another go + * at the syscall afterwards, and this is indistinguishable for the + * guest from the timing having been different such that the guest + * signal really did win the race + * (2) signal came in while the host syscall was blocking, and the + * host kernel decided the syscall should be restarted; + * in this case we want to restart the guest syscall also, and so + * rewinding is the right thing. (Note that "restart" semantics mean + * "first call the signal handler, then reattempt the syscall".) + * The other situation to consider is when a signal came in while the + * host syscall was blocking, and the host kernel decided that the syscall + * should not be restarted; in this case QEMU's host signal handler will + * be invoked with the PC pointing just after the syscall instruction, + * with registers indicating an EINTR return; the special code in the + * handler will not kick in, and we will return EINTR to the guest as + * we should. + * + * Notice that we can leave the host kernel to make the decision for + * us about whether to do a restart of the syscall or not; we do not + * need to check SA_RESTART flags in QEMU or distinguish the various + * kinds of restartability. + */ +#ifdef HAVE_SAFE_SYSCALL +/* The core part of this function is implemented in assembly */ +extern long safe_syscall_base(int *pending, long number, ...); + +#define safe_syscall(...) \ + ({ \ + long ret_; \ + int *psp_ = &((TaskState *)thread_cpu->opaque)->signal_pending; \ + ret_ = safe_syscall_base(psp_, __VA_ARGS__); \ + if (is_error(ret_)) { \ + errno = -ret_; \ + ret_ = -1; \ + } \ + ret_; \ + }) + +#else + +/* Fallback for architectures which don't yet provide a safe-syscall assembly + * fragment; note that this is racy! + * This should go away when all host architectures have been updated. + */ +#define safe_syscall syscall + +#endif + /* syscall.c */ int host_to_target_waitstatus(int status); |