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qemu/linux-user/safe-syscall.S

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linux-user: Provide safe_syscall for fixing races between signals and syscalls If a signal is delivered immediately before a blocking system call the handler will only be called after the system call returns, which may be a long time later or never. This is fixed by using a function (safe_syscall) that checks if a guest signal is pending prior to making a system call, and if so does not call the system call and returns -TARGET_ERESTARTSYS. If a signal is received between the check and the system call host_signal_handler() rewinds execution to before the check. This rewinding has the effect of closing the race window so that safe_syscall will reliably either (a) go into the host syscall with no unprocessed guest signals pending or or (b) return -TARGET_ERESTARTSYS so that the caller can deal with the signals. Implementing this requires a per-host-architecture assembly language fragment. This will also resolve the mishandling of the SA_RESTART flag where we would restart a host system call and not call the guest signal handler until the syscall finally completed -- syscall restarting now always happens at the guest syscall level so the guest signal handler will run. (The host syscall will never be restarted because if the host kernel rewinds the PC to point at the syscall insn for a restart then our host_signal_handler() will see this and arrange the guest PC rewind.) This commit contains the infrastructure for implementing safe_syscall and the assembly language fragment for x86-64, but does not change any syscalls to use it. Signed-off-by: Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk> Message-id: 1441497448-32489-14-git-send-email-T.E.Baldwin99@members.leeds.ac.uk [PMM: * Avoid having an architecture if-ladder in configure by putting linux-user/host/$(ARCH) on the include path and including safe-syscall.inc.S from it * Avoid ifdef ladder in signal.c by creating new hostdep.h to hold host-architecture-specific things * Added copyright/license header to safe-syscall.inc.S * Rewrote commit message * Added comments to safe-syscall.inc.S * Changed calling convention of safe_syscall() to match syscall() (returns -1 and host error in errno on failure) * Added a long comment in qemu.h about how to use safe_syscall() to implement guest syscalls. ] RV: squashed Peters "fixup! linux-user: compile on non-x86-64 hosts" patch Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
2016-05-12 18:47:46 +01:00
/*
* safe-syscall.S : include the host-specific assembly fragment
* to handle signals occurring at the same time as system calls.
*
* Written by Peter Maydell <peter.maydell@linaro.org>
*
* Copyright (C) 2016 Linaro Limited
*
* This work is licensed under the terms of the GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*/
#include "hostdep.h"
#include "errno_defs.h"
/* We have the correct host directory on our include path
* so that this will pull in the right fragment for the architecture.
*/
#ifdef HAVE_SAFE_SYSCALL
#include "safe-syscall.inc.S"
#endif
/* We must specifically say that we're happy for the stack to not be
* executable, otherwise the toolchain will default to assuming our
* assembly needs an executable stack and the whole QEMU binary will
* needlessly end up with one. This should be the last thing in this file.
*/
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack, "", %progbits
#endif
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