mirror of
https://github.com/qemu/qemu.git
synced 2024-12-10 20:23:35 +08:00
155 lines
7.1 KiB
C
155 lines
7.1 KiB
C
|
/*
|
||
|
* safe-syscall.h: prototypes for linux-user signal-race-safe syscalls
|
||
|
*
|
||
|
* This program is free software; you can redistribute it and/or modify
|
||
|
* it under the terms of the GNU General Public License as published by
|
||
|
* the Free Software Foundation; either version 2 of the License, or
|
||
|
* (at your option) any later version.
|
||
|
*
|
||
|
* This program 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 General Public License for more details.
|
||
|
*
|
||
|
* You should have received a copy of the GNU General Public License
|
||
|
* along with this program; if not, see <http://www.gnu.org/licenses/>.
|
||
|
*/
|
||
|
|
||
|
#ifndef LINUX_USER_SAFE_SYSCALL_H
|
||
|
#define LINUX_USER_SAFE_SYSCALL_H
|
||
|
|
||
|
/**
|
||
|
* 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.
|
||
|
*
|
||
|
* Some non-interruptible syscalls need to be handled using block_signals()
|
||
|
* to block signals for the duration of the syscall. This mainly applies
|
||
|
* to code which needs to modify the data structures used by the
|
||
|
* host_signal_handler() function and the functions it calls, including
|
||
|
* all syscalls which change the thread's signal mask.
|
||
|
*
|
||
|
* (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.)
|
||
|
*
|
||
|
* block_signals() cannot be used for interruptible syscalls.
|
||
|
*
|
||
|
*
|
||
|
* 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
|
||
|
|
||
|
#endif
|