141 lines
6.5 KiB
C
141 lines
6.5 KiB
C
/*
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* safe-syscall.h: prototypes for linux-user signal-race-safe syscalls
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef LINUX_USER_SAFE_SYSCALL_H
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#define LINUX_USER_SAFE_SYSCALL_H
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/**
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* safe_syscall:
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* @int number: number of system call to make
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* ...: arguments to the system call
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*
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* Call a system call if guest signal not pending.
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* This has the same API as the libc syscall() function, except that it
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* may return -1 with errno == QEMU_ERESTARTSYS if a signal was pending.
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*
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* Returns: the system call result, or -1 with an error code in errno
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* (Errnos are host errnos; we rely on QEMU_ERESTARTSYS not clashing
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* with any of the host errno values.)
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*/
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/*
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* A guide to using safe_syscall() to handle interactions between guest
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* syscalls and guest signals:
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*
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* Guest syscalls come in two flavours:
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*
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* (1) Non-interruptible syscalls
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*
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* These are guest syscalls that never get interrupted by signals and
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* so never return EINTR. They can be implemented straightforwardly in
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* QEMU: just make sure that if the implementation code has to make any
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* blocking calls that those calls are retried if they return EINTR.
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* It's also OK to implement these with safe_syscall, though it will be
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* a little less efficient if a signal is delivered at the 'wrong' moment.
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*
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* Some non-interruptible syscalls need to be handled using block_signals()
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* to block signals for the duration of the syscall. This mainly applies
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* to code which needs to modify the data structures used by the
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* host_signal_handler() function and the functions it calls, including
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* all syscalls which change the thread's signal mask.
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*
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* (2) Interruptible syscalls
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*
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* These are guest syscalls that can be interrupted by signals and
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* for which we need to either return EINTR or arrange for the guest
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* syscall to be restarted. This category includes both syscalls which
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* always restart (and in the kernel return -ERESTARTNOINTR), ones
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* which only restart if there is no handler (kernel returns -ERESTARTNOHAND
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* or -ERESTART_RESTARTBLOCK), and the most common kind which restart
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* if the handler was registered with SA_RESTART (kernel returns
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* -ERESTARTSYS). System calls which are only interruptible in some
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* situations (like 'open') also need to be handled this way.
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*
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* Here it is important that the host syscall is made
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* via this safe_syscall() function, and *not* via the host libc.
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* If the host libc is used then the implementation will appear to work
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* most of the time, but there will be a race condition where a
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* signal could arrive just before we make the host syscall inside libc,
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* and then then guest syscall will not correctly be interrupted.
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* Instead the implementation of the guest syscall can use the safe_syscall
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* function but otherwise just return the result or errno in the usual
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* way; the main loop code will take care of restarting the syscall
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* if appropriate.
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*
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* (If the implementation needs to make multiple host syscalls this is
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* OK; any which might really block must be via safe_syscall(); for those
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* which are only technically blocking (ie which we know in practice won't
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* stay in the host kernel indefinitely) it's OK to use libc if necessary.
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* You must be able to cope with backing out correctly if some safe_syscall
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* you make in the implementation returns either -QEMU_ERESTARTSYS or
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* EINTR though.)
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*
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* block_signals() cannot be used for interruptible syscalls.
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*
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*
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* How and why the safe_syscall implementation works:
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*
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* The basic setup is that we make the host syscall via a known
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* section of host native assembly. If a signal occurs, our signal
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* handler checks the interrupted host PC against the addresse of that
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* known section. If the PC is before or at the address of the syscall
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* instruction then we change the PC to point at a "return
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* -QEMU_ERESTARTSYS" code path instead, and then exit the signal handler
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* (causing the safe_syscall() call to immediately return that value).
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* Then in the main.c loop if we see this magic return value we adjust
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* the guest PC to wind it back to before the system call, and invoke
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* the guest signal handler as usual.
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*
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* This winding-back will happen in two cases:
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* (1) signal came in just before we took the host syscall (a race);
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* in this case we'll take the guest signal and have another go
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* at the syscall afterwards, and this is indistinguishable for the
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* guest from the timing having been different such that the guest
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* signal really did win the race
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* (2) signal came in while the host syscall was blocking, and the
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* host kernel decided the syscall should be restarted;
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* in this case we want to restart the guest syscall also, and so
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* rewinding is the right thing. (Note that "restart" semantics mean
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* "first call the signal handler, then reattempt the syscall".)
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* The other situation to consider is when a signal came in while the
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* host syscall was blocking, and the host kernel decided that the syscall
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* should not be restarted; in this case QEMU's host signal handler will
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* be invoked with the PC pointing just after the syscall instruction,
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* with registers indicating an EINTR return; the special code in the
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* handler will not kick in, and we will return EINTR to the guest as
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* we should.
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*
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* Notice that we can leave the host kernel to make the decision for
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* us about whether to do a restart of the syscall or not; we do not
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* need to check SA_RESTART flags in QEMU or distinguish the various
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* kinds of restartability.
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*/
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/* The core part of this function is implemented in assembly */
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extern long safe_syscall_base(int *pending, long number, ...);
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extern long safe_syscall_set_errno_tail(int value);
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/* These are defined by the safe-syscall.inc.S file */
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extern char safe_syscall_start[];
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extern char safe_syscall_end[];
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#define safe_syscall(...) \
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safe_syscall_base(&((TaskState *)thread_cpu->opaque)->signal_pending, \
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__VA_ARGS__)
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#endif
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