sigaltstack.h

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#ifndef ZTH_CONTEXT_SIGALTSTACK_H
#define ZTH_CONTEXT_SIGALTSTACK_H
/*
 * SPDX-FileCopyrightText: 2019-2026 Jochem Rutgers
 *
 * SPDX-License-Identifier: MPL-2.0
 */
 
#ifndef ZTH_CONTEXT_CONTEXT_H
#  error This file must be included by libzth/context/context.h.
#endif
 
#ifdef __cplusplus
 
#  ifdef ZTH_ENABLE_ASAN
#    error Invalid configuration combination sigaltstack with asan.
#  endif
 
#  include <csetjmp>
#  include <csignal>
#  include <sys/types.h>
 
#  ifdef ZTH_HAVE_PTHREAD
#    include <pthread.h>
#    ifndef ZTH_OS_MAC
#      define pthread_yield_np() pthread_yield()
#    endif
#  else
#    define pthread_sigmask(...) sigprocmask(__VA_ARGS__)
#    define pthread_kill(...)    kill(__VA_ARGS__)
#    define pthread_self()       getpid()
#    define pthread_yield_np()   sched_yield()
#  endif
 
namespace zth {
namespace impl {
 
static void context_global_init()
{
        // By default, block SIGUSR1 for the main/all thread(s).
        int res = 0;
 
        sigset_t sigs;
        sigemptyset(&sigs);
        sigaddset(&sigs, SIGUSR1);
        if((res = pthread_sigmask(SIG_BLOCK, &sigs, nullptr)))
                goto error;
 
        return;
error:
        zth_abort("Cannot initialize signals; %s", err(res).c_str());
}
ZTH_INIT_CALL(context_global_init)
 
} // namespace impl
 
class Context : public impl::ContextArch<Context> {
        ZTH_CLASS_NEW_DELETE(Context)
public:
        typedef impl::ContextArch<Context> base;
 
        constexpr explicit Context(ContextAttr const& attr) noexcept
                : base(attr)
                , m_trampoline_env()
                , m_mask()
                , m_did_trampoline()
        {}
 
private:
        static void context_trampoline(int sig)
        {
                // At this point, we are in the signal handler.
 
                if(unlikely(sig != SIGUSR1))
                        // Huh?
                        return;
 
                // Put the context on the stack, as trampoline_context is about to change.
                Context* context = ZTH_TLS_GET(trampoline_context);
 
                if(unlikely(!context || context->m_did_trampoline))
                        // Huh?
                        return;
 
                if(setjmp(context->m_trampoline_env) == 0) {
                        // Return immediately to complete the signal handler.
                        context->m_did_trampoline = 1;
                        return;
                }
 
                // If we get here, we are not in the signal handler anymore, but in the actual fiber
                // context.
                zth_dbg(context, "Bootstrapping %p", context);
 
                if(Config::ContextSignals) {
                        int res = pthread_sigmask(SIG_SETMASK, &context->m_mask, nullptr);
                        if(unlikely(res))
                                zth_abort("Cannot set signal mask");
                }
 
                // However, we still need to setup the context for the first time.
                if(sigsetjmp(context->m_env, Config::ContextSignals) == 0) {
                        // Now, we are ready to be context switched to this fiber.
                        // Go back to our parent, as it was not our schedule turn to continue with
                        // actual execution.
                        siglongjmp(*context->m_parent, 1);
                }
 
                // This is actually the first time we enter the fiber by the normal scheduler.
                context_entry(context);
        }
 
        ZTH_TLS_MEMBER(Context*, trampoline_context)
 
public:
        static int init() noexcept
        {
                int res = base::init();
                if(res)
                        return res;
 
                // Claim SIGUSR1 for context_create().
                struct sigaction sa;
                sa.sa_handler = &context_trampoline;
                sigemptyset(&sa.sa_mask);
                sa.sa_flags = SA_ONSTACK;
                if(sigaction(SIGUSR1, &sa, nullptr))
                        return errno;
 
                // Let SIGUSR1 remain blocked.
 
                // All set.
                return 0;
        }
 
        static void deinit() noexcept
        {
                // Release SIGUSR1.
                struct sigaction sa;
                sa.sa_handler = SIG_DFL;
                sigemptyset(&sa.sa_mask);
                sa.sa_flags = 0;
                sigaction(SIGUSR1, &sa, nullptr);
 
                base::deinit();
        }
 
        int create() noexcept
        {
                int res = base::create();
                if(unlikely(res))
                        return res;
 
                Stack const& stack_ = stackUsable();
                if(unlikely(!stack_))
                        // Stackless fiber only saves current context; nothing to do.
                        return 0;
 
                m_did_trampoline = 0;
                stack_t ss = {};
                stack_t oss = {};
 
                // Let the new context inherit our signal mask.
                if(Config::ContextSignals)
                        if(unlikely((res = pthread_sigmask(0, nullptr, &m_mask))))
                                goto rollback;
 
                ss.ss_sp = stack_.p;
                ss.ss_size = stack_.size;
 
#  ifdef ZTH_USE_VALGRIND
                // Disable checking during bootstrap.
                VALGRIND_DISABLE_ADDR_ERROR_REPORTING_IN_RANGE(ss.ss_sp, ss.ss_size);
#  endif
 
#  ifdef SS_AUTODISARM
                ss.ss_flags |= SS_AUTODISARM;
#  endif
                if(unlikely(sigaltstack(&ss, &oss))) {
                        res = errno;
                        goto rollback;
                }
 
                ZTH_TLS_SET(trampoline_context, this);
 
                // Ready to do the signal.
                sigset_t sigs;
                sigemptyset(&sigs);
                sigaddset(&sigs, SIGUSR1);
                if(unlikely((res = pthread_sigmask(SIG_UNBLOCK, &sigs, nullptr))))
                        goto rollback_altstack;
 
                // As we are not blocking SIGUSR1, we will get the signal.
                if(unlikely((res = pthread_kill(pthread_self(), SIGUSR1))))
                        goto rollback_altstack;
 
                // Shouldn't take long...
                while(!m_did_trampoline)
                        pthread_yield_np();
 
                // Block again.
                if(unlikely((res = pthread_sigmask(SIG_BLOCK, &sigs, nullptr))))
                        goto rollback_altstack;
 
                if(Config::Debug)
                        ZTH_TLS_SET(trampoline_context, nullptr);
 
                if(Config::EnableAssert) {
                        if(unlikely(sigaltstack(nullptr, &ss))) {
                                res = errno;
                                goto rollback_altstack;
                        }
                        zth_assert(!(ss.ss_flags & SS_ONSTACK));
                }
 
#  ifndef SS_AUTODISARM
                // Reset sigaltstack.
                ss.ss_flags = SS_DISABLE;
                if(unlikely(sigaltstack(&ss, nullptr))) {
                        res = errno;
                        goto rollback_altstack;
                }
#  endif
 
                if(unlikely(!(oss.ss_flags & SS_DISABLE))) {
                        if(sigaltstack(&oss, nullptr)) {
                                res = errno;
                                goto rollback;
                        }
                }
#  ifdef ZTH_HAVE_VALGRIND
                else if(RUNNING_ON_VALGRIND) {
                        // Valgrind has a hard time tracking when we left the signal handler
                        // and are not using the altstack anymore. Reset the altstack, such
                        // that successive sigaltstack() do not fail with EPERM.
                        stack_t dss;
                        dss.ss_sp = dummyAltStack;
                        dss.ss_size = sizeof(dummyAltStack);
                        dss.ss_flags = 0;
                        if(unlikely(sigaltstack(&dss, nullptr))) {
                                res = errno;
                                goto rollback;
                        }
 
                        // Although the previous stack was disabled, it is still valid.
                        VALGRIND_MAKE_MEM_DEFINED(ss.ss_sp, ss.ss_size);
                }
#  endif
 
                // Ok, we have returned from the signal handler.
                // Now go back again and save a proper env.
 
                sigjmp_buf me;
                m_parent = &me;
                if(sigsetjmp(me, Config::ContextSignals) == 0)
                        longjmp(m_trampoline_env, 1);
 
#  ifdef ZTH_USE_VALGRIND
                // Disabled checking during bootstrap.
                VALGRIND_ENABLE_ADDR_ERROR_REPORTING_IN_RANGE(ss.ss_sp, ss.ss_size);
#  endif
 
                if(Config::EnableAssert) {
                        if(unlikely(sigaltstack(nullptr, &oss))) {
                                res = errno;
                                goto rollback;
                        }
                        zth_assert(!(oss.ss_flags & SS_ONSTACK));
                }
 
                // context is setup properly and is ready for normal scheduling.
                if(Config::Debug)
                        m_parent = nullptr;
 
                return 0;
 
rollback_altstack:
#  ifdef ZTH_USE_VALGRIND
                VALGRIND_ENABLE_ADDR_ERROR_REPORTING_IN_RANGE(ss.ss_sp, ss.ss_size);
#  endif
                if(!(oss.ss_flags & SS_DISABLE))
                        sigaltstack(&oss, nullptr);
rollback:
                base::destroy();
                return res ? res : EINVAL;
        }
 
        void context_switch(Context& to) noexcept
        {
                if(sigsetjmp(m_env, Config::ContextSignals) == 0) {
                        // Go switch away from here.
                        siglongjmp(to.m_env, 1);
                }
        }
 
private:
        union {
                jmp_buf m_trampoline_env;
                sigjmp_buf m_env;
        };
        sigset_t m_mask;
        union {
                sig_atomic_t volatile m_did_trampoline;
                sigjmp_buf* volatile m_parent;
        };
#  ifdef ZTH_HAVE_VALGRIND
        static char dummyAltStack[MINSIGSTKSZ];
#  endif
};
 
ZTH_TLS_DEFINE(Context*, Context::trampoline_context, nullptr)
 
#  ifdef ZTH_HAVE_VALGRIND
char Context::dummyAltStack[MINSIGSTKSZ];
#  endif
 
} // namespace zth
#endif // __cplusplus
#endif // ZTH_CONTEXT_SIGALTSTACK_H