coroutine-ucontext.c

/*
 * ucontext coroutine initialization code
 *
 * Copyright (C) 2006  Anthony Liguori <anthony@codemonkey.ws>
 * Copyright (C) 2011  Kevin Wolf <kwolf@redhat.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.0 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, see <http://www.gnu.org/licenses/>.
 */

/* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
#ifdef _FORTIFY_SOURCE
#undef _FORTIFY_SOURCE
#endif
#include "qemu/osdep.h"
#include <ucontext.h>
#include "qemu/coroutine_int.h"

#ifdef CONFIG_VALGRIND_H
#include <valgrind/valgrind.h>
#endif

#if defined(__SANITIZE_ADDRESS__) || __has_feature(address_sanitizer)
#ifdef CONFIG_ASAN_IFACE_FIBER
#define CONFIG_ASAN 1
#include <sanitizer/asan_interface.h>
#endif
#endif

#ifdef CONFIG_TSAN
#include <sanitizer/tsan_interface.h>
#endif

typedef struct {
    Coroutine base;
    void *stack;
    size_t stack_size;
#ifdef CONFIG_SAFESTACK
    /* Need an unsafe stack for each coroutine */
    void *unsafe_stack;
    size_t unsafe_stack_size;
#endif
    sigjmp_buf env;

#ifdef CONFIG_TSAN
    void *tsan_co_fiber;
    void *tsan_caller_fiber;
#endif

#ifdef CONFIG_VALGRIND_H
    unsigned int valgrind_stack_id;
#endif

} CoroutineUContext;

/**
 * Per-thread coroutine bookkeeping
 */
static __thread CoroutineUContext leader;
static __thread Coroutine *current;

/*
 * va_args to makecontext() must be type 'int', so passing
 * the pointer we need may require several int args. This
 * union is a quick hack to let us do that
 */
union cc_arg {
    void *p;
    int i[2];
};

/*
 * QEMU_ALWAYS_INLINE only does so if __OPTIMIZE__, so we cannot use it.
 * always_inline is required to avoid TSan runtime fatal errors.
 */
static inline __attribute__((always_inline))
void on_new_fiber(CoroutineUContext *co)
{
#ifdef CONFIG_TSAN
    co->tsan_co_fiber = __tsan_create_fiber(0); /* flags: sync on switch */
    co->tsan_caller_fiber = __tsan_get_current_fiber();
#endif
}

/* always_inline is required to avoid TSan runtime fatal errors. */
static inline __attribute__((always_inline))
void finish_switch_fiber(void *fake_stack_save)
{
#ifdef CONFIG_ASAN
    const void *bottom_old;
    size_t size_old;

    __sanitizer_finish_switch_fiber(fake_stack_save, &bottom_old, &size_old);

    if (!leader.stack) {
        leader.stack = (void *)bottom_old;
        leader.stack_size = size_old;
    }
#endif
#ifdef CONFIG_TSAN
    if (fake_stack_save) {
        __tsan_release(fake_stack_save);
        __tsan_switch_to_fiber(fake_stack_save, 0);  /* 0=synchronize */
    }
#endif
}

/* always_inline is required to avoid TSan runtime fatal errors. */
static inline __attribute__((always_inline))
void start_switch_fiber_asan(CoroutineAction action, void **fake_stack_save,
                             const void *bottom, size_t size)
{
#ifdef CONFIG_ASAN
    __sanitizer_start_switch_fiber(
            action == COROUTINE_TERMINATE ? NULL : fake_stack_save,
            bottom, size);
#endif
}

/* always_inline is required to avoid TSan runtime fatal errors. */
static inline __attribute__((always_inline))
void start_switch_fiber_tsan(void **fake_stack_save,
                             CoroutineUContext *co,
                             bool caller)
{
#ifdef CONFIG_TSAN
    void *new_fiber = caller ?
                      co->tsan_caller_fiber :
                      co->tsan_co_fiber;
    void *curr_fiber = __tsan_get_current_fiber();
    __tsan_acquire(curr_fiber);

    *fake_stack_save = curr_fiber;
    __tsan_switch_to_fiber(new_fiber, 0);  /* 0=synchronize */
#endif
}

static void coroutine_trampoline(int i0, int i1)
{
    union cc_arg arg;
    CoroutineUContext *self;
    Coroutine *co;
    void *fake_stack_save = NULL;

    finish_switch_fiber(NULL);

    arg.i[0] = i0;
    arg.i[1] = i1;
    self = arg.p;
    co = &self->base;

    /* Initialize longjmp environment and switch back the caller */
    if (!sigsetjmp(self->env, 0)) {
        start_switch_fiber_asan(COROUTINE_YIELD, &fake_stack_save, leader.stack,
                                leader.stack_size);
        start_switch_fiber_tsan(&fake_stack_save, self, true); /* true=caller */
        siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
    }

    finish_switch_fiber(fake_stack_save);

    while (true) {
        co->entry(co->entry_arg);
        qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
    }
}

Coroutine *qemu_coroutine_new(void)
{
    CoroutineUContext *co;
    ucontext_t old_uc, uc;
    sigjmp_buf old_env;
    union cc_arg arg = {0};
    void *fake_stack_save = NULL;

    /* The ucontext functions preserve signal masks which incurs a
     * system call overhead.  sigsetjmp(buf, 0)/siglongjmp() does not
     * preserve signal masks but only works on the current stack.
     * Since we need a way to create and switch to a new stack, use
     * the ucontext functions for that but sigsetjmp()/siglongjmp() for
     * everything else.
     */

    if (getcontext(&uc) == -1) {
        abort();
    }

    co = g_malloc0(sizeof(*co));
    co->stack_size = COROUTINE_STACK_SIZE;
    co->stack = qemu_alloc_stack(&co->stack_size);
#ifdef CONFIG_SAFESTACK
    co->unsafe_stack_size = COROUTINE_STACK_SIZE;
    co->unsafe_stack = qemu_alloc_stack(&co->unsafe_stack_size);
#endif
    co->base.entry_arg = &old_env; /* stash away our jmp_buf */

    uc.uc_link = &old_uc;
    uc.uc_stack.ss_sp = co->stack;
    uc.uc_stack.ss_size = co->stack_size;
    uc.uc_stack.ss_flags = 0;

#ifdef CONFIG_VALGRIND_H
    co->valgrind_stack_id =
        VALGRIND_STACK_REGISTER(co->stack, co->stack + co->stack_size);
#endif

    arg.p = co;

    on_new_fiber(co);
    makecontext(&uc, (void (*)(void))coroutine_trampoline,
                2, arg.i[0], arg.i[1]);

    /* swapcontext() in, siglongjmp() back out */
    if (!sigsetjmp(old_env, 0)) {
        start_switch_fiber_asan(COROUTINE_YIELD, &fake_stack_save, co->stack,
                                co->stack_size);
        start_switch_fiber_tsan(&fake_stack_save,
                                co, false); /* false=not caller */

#ifdef CONFIG_SAFESTACK
        /*
         * Before we swap the context, set the new unsafe stack
         * The unsafe stack grows just like the normal stack, so start from
         * the last usable location of the memory area.
         * NOTE: we don't have to re-set the usp afterwards because we are
         * coming back to this context through a siglongjmp.
         * The compiler already wrapped the corresponding sigsetjmp call with
         * code that saves the usp on the (safe) stack before the call, and
         * restores it right after (which is where we return with siglongjmp).
         */
        void *usp = co->unsafe_stack + co->unsafe_stack_size;
        __safestack_unsafe_stack_ptr = usp;
#endif

        swapcontext(&old_uc, &uc);
    }

    finish_switch_fiber(fake_stack_save);

    return &co->base;
}

#ifdef CONFIG_VALGRIND_H
/* Work around an unused variable in the valgrind.h macro... */
#if !defined(__clang__)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
#endif
static inline void valgrind_stack_deregister(CoroutineUContext *co)
{
    VALGRIND_STACK_DEREGISTER(co->valgrind_stack_id);
}
#if !defined(__clang__)
#pragma GCC diagnostic pop
#endif
#endif

void qemu_coroutine_delete(Coroutine *co_)
{
    CoroutineUContext *co = DO_UPCAST(CoroutineUContext, base, co_);

#ifdef CONFIG_VALGRIND_H
    valgrind_stack_deregister(co);
#endif

    qemu_free_stack(co->stack, co->stack_size);
#ifdef CONFIG_SAFESTACK
    qemu_free_stack(co->unsafe_stack, co->unsafe_stack_size);
#endif
    g_free(co);
}

/* This function is marked noinline to prevent GCC from inlining it
 * into coroutine_trampoline(). If we allow it to do that then it
 * hoists the code to get the address of the TLS variable "current"
 * out of the while() loop. This is an invalid transformation because
 * the sigsetjmp() call may be called when running thread A but
 * return in thread B, and so we might be in a different thread
 * context each time round the loop.
 */
CoroutineAction __attribute__((noinline))
qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
                      CoroutineAction action)
{
    CoroutineUContext *from = DO_UPCAST(CoroutineUContext, base, from_);
    CoroutineUContext *to = DO_UPCAST(CoroutineUContext, base, to_);
    int ret;
    void *fake_stack_save = NULL;

    current = to_;

    ret = sigsetjmp(from->env, 0);
    if (ret == 0) {
        start_switch_fiber_asan(action, &fake_stack_save, to->stack,
                                to->stack_size);
        start_switch_fiber_tsan(&fake_stack_save,
                                to, false); /* false=not caller */
        siglongjmp(to->env, action);
    }

    finish_switch_fiber(fake_stack_save);

    return ret;
}

Coroutine *qemu_coroutine_self(void)
{
    if (!current) {
        current = &leader.base;
    }
#ifdef CONFIG_TSAN
    if (!leader.tsan_co_fiber) {
        leader.tsan_co_fiber = __tsan_get_current_fiber();
    }
#endif
    return current;
}

bool qemu_in_coroutine(void)
{
    return current && current->caller;
}