/******************************************************************************

 *

 * Name:     cpl_userfaultfd.cpp

 * Project:  CPL - Common Portability Library

 * Purpose:  Use userfaultfd and VSIL to service page faults

 * Author:   James McClain, <james.mcclain@gmail.com>

 *

 ******************************************************************************

 * Copyright (c) 2018, Dr. James McClain <james.mcclain@gmail.com>

 *

 * SPDX-License-Identifier: MIT

 ****************************************************************************/

#ifdef ENABLE_UFFD

#include <cstdlib>

#include <cinttypes>

#include <cstring>

#include <string>

#include <errno.h>

#include <fcntl.h>

#include <poll.h>

#include <pthread.h>

#include <sched.h>

#include <signal.h>

#include <unistd.h>

#include <sys/ioctl.h>

#include <sys/mman.h>

#include <sys/stat.h>

#include <sys/syscall.h>

#include <sys/types.h>

#include <sys/utsname.h>

#include <linux/userfaultfd.h>

#include "cpl_conv.h"

#include "cpl_error.h"

#include "cpl_userfaultfd.h"

#include "cpl_string.h"

#include "cpl_vsi.h"

#include "cpl_multiproc.h"

#ifndef UFFD_USER_MODE_ONLY

// The UFFD_USER_MODE_ONLY flag got added in kernel 5.11 which is the one

// used by Ubuntu 20.04, but the linux-libc-dev package corresponds to 5.4

#define UFFD_USER_MODE_ONLY 1

#endif

#define BAD_MMAP (reinterpret_cast<void *>(-1))

#define MAX_MESSAGES (0x100)

static int64_t get_page_limit();

static void cpl_uffd_fault_handler(void *ptr);

static void signal_handler(int signal);

static void uffd_cleanup(void *ptr);

struct cpl_uffd_context

{

    bool keep_going = false;

    int uffd = -1;

    struct uffdio_register uffdio_register = {};

    struct uffd_msg uffd_msgs[MAX_MESSAGES];

    std::string filename = std::string("");

    int64_t page_limit = -1;

    int64_t pages_used = 0;

    size_t file_size = 0;

    size_t page_size = 0;

    void *page_ptr = nullptr;

    size_t vma_size = 0;

    void *vma_ptr = nullptr;

    CPLJoinableThread *thread = nullptr;

};

static void uffd_cleanup(void *ptr)

{

    struct cpl_uffd_context *ctx = static_cast<struct cpl_uffd_context *>(ptr);

    if (!ctx)

        return;

    // Signal shutdown

    ctx->keep_going = false;

    if (ctx->thread)

    {

        CPLJoinThread(ctx->thread);

        ctx->thread = nullptr;

    }

    if (ctx->uffd != -1)

    {

        ioctl(ctx->uffd, UFFDIO_UNREGISTER, &ctx->uffdio_register);

        close(ctx->uffd);

        ctx->uffd = -1;

    }

    if (ctx->page_ptr && ctx->page_size)

        munmap(ctx->page_ptr, ctx->page_size);

    if (ctx->vma_ptr && ctx->vma_size)

        munmap(ctx->vma_ptr, ctx->vma_size);

    ctx->page_ptr = nullptr;

    ctx->vma_ptr = nullptr;

    ctx->page_size = 0;

    ctx->vma_size = 0;

    ctx->pages_used = 0;

    ctx->page_limit = 0;

    delete ctx;

    return;

}

#ifdef HAVE_GCC_WARNING_ZERO_AS_NULL_POINTER_CONSTANT

#pragma GCC diagnostic push

#pragma GCC diagnostic ignored "-Wzero-as-null-pointer-constant"

#endif

static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

#ifdef HAVE_GCC_WARNING_ZERO_AS_NULL_POINTER_CONSTANT

#pragma GCC diagnostic pop

#endif

static int64_t get_page_limit()

{

    int64_t retval;

    const char *variable = CPLGetConfigOption(GDAL_UFFD_LIMIT, nullptr);

    if (variable && sscanf(variable, "%" PRId64, &retval) == 1)

        return retval;

    else

        return -1;

}

static void cpl_uffd_fault_handler(void *ptr)

{

    struct cpl_uffd_context *ctx = static_cast<struct cpl_uffd_context *>(ptr);

    struct uffdio_copy uffdio_copy;

    struct pollfd pollfd;

    // Setup pollfd structure

    pollfd.fd = ctx->uffd;

    pollfd.events = POLLIN;

    // Open asset for reading

    VSILFILE *file = VSIFOpenL(ctx->filename.c_str(), "rb");

    if (!file)

        return;

    // Loop until told to stop

    while (ctx->keep_going)

    {

        // Poll for event

        if (poll(&pollfd, 1, 16) == -1)

            break;  // 60Hz when no demand

        if ((pollfd.revents & POLLERR) || (pollfd.revents & POLLNVAL))

            break;

        if (!(pollfd.revents & POLLIN))

            continue;

        // Read page fault events

        ssize_t bytes_read = static_cast<ssize_t>(

            read(ctx->uffd, ctx->uffd_msgs, MAX_MESSAGES * sizeof(uffd_msg)));

        if (bytes_read < 1)

        {

            if (errno == EWOULDBLOCK)

                continue;

            else

                break;

        }

        // If too many pages are in use, evict all pages (evict them from

        // RAM and swap, not just to swap).  It is impossible to control

        // which/when threads access the VMA, so access to the VMA has to

        // forbidden while the activity is in progress.

        //

        // That is done by (1) installing special handlers for SIGSEGV and

        // SIGBUS, (2) mprotecting the VMA so that any threads accessing

        // it receive either SIGSEGV or SIGBUS (which one is apparently a

        // function of the C library, at least on one non-Linux GNU

        // system[1]), (3) unregistering the VMA from userfaultfd,

        // remapping the VMA to evict the pages, registering the VMA

        // again, (4) making the VMA accessible again, and finally (5)

        // restoring the previous signal-handling behavior.

        //

        // [1] https://lists.debian.org/debian-bsd/2011/05/msg00032.html

        if (ctx->page_limit > 0)

        {

            pthread_mutex_lock(&mutex);

            if (ctx->pages_used > ctx->page_limit)

            {

                struct sigaction segv;

                struct sigaction old_segv;

                struct sigaction bus;

                struct sigaction old_bus;

                memset(&segv, 0, sizeof(segv));

                memset(&old_segv, 0, sizeof(old_segv));

                memset(&bus, 0, sizeof(bus));

                memset(&old_bus, 0, sizeof(old_bus));

                // Step 1 from the block comment above

                segv.sa_handler = signal_handler;

                bus.sa_handler = signal_handler;

                if (sigaction(SIGSEGV, &segv, &old_segv) == -1)

                {

                    CPLError(

                        CE_Failure, CPLE_AppDefined,

                        "cpl_uffd_fault_handler: sigaction(SIGSEGV) failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                if (sigaction(SIGBUS, &bus, &old_bus) == -1)

                {

                    CPLError(

                        CE_Failure, CPLE_AppDefined,

                        "cpl_uffd_fault_handler: sigaction(SIGBUS) failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                // WARNING: LACK OF THREAD-SAFETY.

                //

                // For example, if a user program (or another part of the

                // library) installs a SIGSEGV or SIGBUS handler from another

                // thread after this one has installed its handlers but before

                // this one uninstalls its handlers, the intervening handler

                // will be eliminated.  There are other examples, as well, but

                // there can only be a problems with other threads because the

                // faulting thread is blocked here.

                //

                // This implies that one should not use cpl_virtualmem.h API

                // while other threads are actively generating faults that use

                // this mechanism.

                //

                // Having multiple active threads that use this mechanism but

                // with no changes to signal-handling in other threads is NOT a

                // problem.

                // Step 2

                if (mprotect(ctx->vma_ptr, ctx->vma_size, PROT_NONE) == -1)

                {

                    CPLError(CE_Failure, CPLE_AppDefined,

                             "cpl_uffd_fault_handler: mprotect() failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                // Step 3

                if (ioctl(ctx->uffd, UFFDIO_UNREGISTER, &ctx->uffdio_register))

                {

                    CPLError(CE_Failure, CPLE_AppDefined,

                             "cpl_uffd_fault_handler: ioctl(UFFDIO_UNREGISTER) "

                             "failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                ctx->vma_ptr =

                    mmap(ctx->vma_ptr, ctx->vma_size, PROT_NONE,

                         MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);

                if (ctx->vma_ptr == BAD_MMAP)

                {

                    CPLError(CE_Failure, CPLE_AppDefined,

                             "cpl_uffd_fault_handler: mmap() failed");

                    ctx->vma_ptr = nullptr;

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                ctx->pages_used = 0;

                if (ioctl(ctx->uffd, UFFDIO_REGISTER, &ctx->uffdio_register))

                {

                    CPLError(CE_Failure, CPLE_AppDefined,

                             "cpl_uffd_fault_handler: ioctl(UFFDIO_REGISTER) "

                             "failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                // Step 4.  Problem: A thread might attempt to read here (before

                // the mprotect) and receive a SIGSEGV or SIGBUS.

                if (mprotect(ctx->vma_ptr, ctx->vma_size, PROT_READ) == -1)

                {

                    CPLError(CE_Failure, CPLE_AppDefined,

                             "cpl_uffd_fault_handler: mprotect() failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                // Step 5.  Solution: Cannot unregister special handlers before

                // any such threads have been handled by them, so sleep for

                // 1/100th of a second.

                // Coverity complains about sleeping under a mutex

#ifndef __COVERITY__

                // coverity[sleep]

                usleep(10000);

#endif

                if (sigaction(SIGSEGV, &old_segv, nullptr) == -1)

                {

                    CPLError(

                        CE_Failure, CPLE_AppDefined,

                        "cpl_uffd_fault_handler: sigaction(SIGSEGV) failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

                if (sigaction(SIGBUS, &old_bus, nullptr) == -1)

                {

                    CPLError(

                        CE_Failure, CPLE_AppDefined,

                        "cpl_uffd_fault_handler: sigaction(SIGBUS) failed");

                    pthread_mutex_unlock(&mutex);

                    break;

                }

            }

            pthread_mutex_unlock(&mutex);

        }

        // Handle page fault events

        for (int i = 0; i < static_cast<int>(bytes_read / sizeof(uffd_msg));

             ++i)

        {

            const uintptr_t fault_addr =

                ctx->uffd_msgs[i].arg.pagefault.address & ~(ctx->page_size - 1);

            const uintptr_t offset =

                fault_addr - reinterpret_cast<uintptr_t>(ctx->vma_ptr);

            size_t bytes_needed = static_cast<size_t>(ctx->file_size - offset);

            if (bytes_needed > ctx->page_size)

                bytes_needed = ctx->page_size;

            // Copy data into page

            if (VSIFSeekL(file, offset, SEEK_SET) != 0 ||

                VSIFReadL(ctx->page_ptr, bytes_needed, 1, file) != 1)

            {

                CPLError(CE_Failure, CPLE_FileIO,

                         "Cannot get %d bytes at offset " CPL_FRMT_GUIB " of "

                         "file %s",

                         static_cast<int>(bytes_needed),

                         static_cast<GUIntBig>(offset), ctx->filename.c_str());

                memset(ctx->page_ptr, 0, bytes_needed);

            }

            ctx->pages_used++;

            // Use the page to fulfill the page fault

            uffdio_copy.src = reinterpret_cast<uintptr_t>(ctx->page_ptr);

            uffdio_copy.dst = fault_addr;

            uffdio_copy.len = static_cast<uintptr_t>(ctx->page_size);

            uffdio_copy.mode = 0;

            uffdio_copy.copy = 0;

            if (ioctl(ctx->uffd, UFFDIO_COPY, &uffdio_copy) == -1)

            {

                CPLError(CE_Failure, CPLE_AppDefined,

                         "ioctl(UFFDIO_COPY) failed");

                break;

            }

        }

    }  // end of while loop

    // Return resources

    VSIFCloseL(file);

}

static void signal_handler(int signal)

{

    if (signal == SIGSEGV || signal == SIGBUS)

        sched_yield();

    return;

}

bool CPLIsUserFaultMappingSupported()

{

    // Check the Linux kernel version.  Linux 4.3 or newer is needed for

    // userfaultfd.

    int major = 0, minor = 0;

    struct utsname utsname;

    if (uname(&utsname))

        return false;

    sscanf(utsname.release, "%d.%d", &major, &minor);

    if (major < 4)

        return false;

    if (major == 4 && minor < 3)

        return false;

    static int nEnableUserFaultFD = -1;

    if (nEnableUserFaultFD < 0)

    {

        nEnableUserFaultFD =

            CPLTestBool(CPLGetConfigOption("CPL_ENABLE_USERFAULTFD", "YES"));

    }

    if (!nEnableUserFaultFD)

        return false;

    // Since kernel 5.2, raw userfaultfd is disabled since if the fault

    // originates from the kernel, that could lead to easier exploitation of

    // kernel bugs. Since kernel 5.11, UFFD_USER_MODE_ONLY can be used to

    // restrict the mechanism to faults occurring only from user space, which is

    // likely to be our use case.

    int uffd = static_cast<int>(syscall(

        __NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY));

    if (uffd == -1 && errno == EINVAL)

        uffd =

            static_cast<int>(syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK));

    if (uffd == -1)

    {

        const int l_errno = errno;

        if (l_errno == EPERM)

        {

            // Since kernel 5.2

            CPLDebug(

                "GDAL",

                "CPLIsUserFaultMappingSupported(): syscall(__NR_userfaultfd) "

                "failed: "

                "insufficient permission. add CAP_SYS_PTRACE capability, or "

                "set /proc/sys/vm/unprivileged_userfaultfd to 1");

        }

        else

        {

            CPLDebug(

                "GDAL",

                "CPLIsUserFaultMappingSupported(): syscall(__NR_userfaultfd) "

                "failed: "

                "error = %d",

                l_errno);

        }

        nEnableUserFaultFD = false;

        return false;

    }

    close(uffd);

    nEnableUserFaultFD = true;

    return true;

}

/*

 * Returns nullptr on failure, a valid pointer on success.

 */

cpl_uffd_context *CPLCreateUserFaultMapping(const char *pszFilename,

                                            void **ppVma, uint64_t *pnVmaSize)

{

    VSIStatBufL statbuf;

    struct cpl_uffd_context *ctx = nullptr;

    if (!CPLIsUserFaultMappingSupported())

    {

        CPLError(

            CE_Failure, CPLE_NotSupported,

            "CPLCreateUserFaultMapping(): Linux kernel 4.3 or newer needed");

        return nullptr;

    }

    // Get the size of the asset

    if (VSIStatL(pszFilename, &statbuf))

        return nullptr;

    // Setup the `cpl_uffd_context` struct

    ctx = new cpl_uffd_context();

    ctx->keep_going = true;

    ctx->filename = std::string(pszFilename);

    ctx->page_limit = get_page_limit();

    ctx->pages_used = 0;

    ctx->file_size = static_cast<size_t>(statbuf.st_size);

    ctx->page_size = static_cast<size_t>(sysconf(_SC_PAGESIZE));

    ctx->vma_size = static_cast<size_t>(

        ((static_cast<vsi_l_offset>(statbuf.st_size) / ctx->page_size) + 1) *

        ctx->page_size);

    if (ctx->vma_size < static_cast<vsi_l_offset>(statbuf.st_size))

    {  // Check for overflow

        uffd_cleanup(ctx);

        CPLError(

            CE_Failure, CPLE_AppDefined,

            "CPLCreateUserFaultMapping(): File too large for architecture");

        return nullptr;

    }

    // If the mmap failed, free resources and return

    ctx->vma_ptr = mmap(nullptr, ctx->vma_size, PROT_READ,

                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    if (ctx->vma_ptr == BAD_MMAP)

    {

        ctx->vma_ptr = nullptr;

        uffd_cleanup(ctx);

        CPLError(CE_Failure, CPLE_AppDefined,

                 "CPLCreateUserFaultMapping(): mmap() failed");

        return nullptr;

    }

    // Attempt to acquire a scratch page to use to fulfill requests.

    ctx->page_ptr =

        mmap(nullptr, static_cast<size_t>(ctx->page_size),

             PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    if (ctx->page_ptr == BAD_MMAP)

    {

        ctx->page_ptr = nullptr;

        uffd_cleanup(ctx);

        CPLError(CE_Failure, CPLE_AppDefined,

                 "CPLCreateUserFaultMapping(): mmap() failed");

        return nullptr;

    }

    // Get userfaultfd

    // Since kernel 5.2, raw userfaultfd is disabled since if the fault

    // originates from the kernel, that could lead to easier exploitation of

    // kernel bugs. Since kernel 5.11, UFFD_USER_MODE_ONLY can be used to

    // restrict the mechanism to faults occurring only from user space, which is

    // likely to be our use case.

    ctx->uffd = static_cast<int>(syscall(

        __NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY));

    if (ctx->uffd == -1 && errno == EINVAL)

        ctx->uffd =

            static_cast<int>(syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK));

    if (ctx->uffd == -1)

    {

        const int l_errno = errno;

        ctx->uffd = -1;

        uffd_cleanup(ctx);

        if (l_errno == EPERM)

        {

            // Since kernel 5.2

            CPLError(

                CE_Failure, CPLE_AppDefined,

                "CPLCreateUserFaultMapping(): syscall(__NR_userfaultfd) "

                "failed: "

                "insufficient permission. add CAP_SYS_PTRACE capability, or "

                "set /proc/sys/vm/unprivileged_userfaultfd to 1");

        }

        else

        {

            CPLError(CE_Failure, CPLE_AppDefined,

                     "CPLCreateUserFaultMapping(): syscall(__NR_userfaultfd) "

                     "failed: "

                     "error = %d",

                     l_errno);

        }

        return nullptr;

    }

    // Query API

    {

        struct uffdio_api uffdio_api = {};

        uffdio_api.api = UFFD_API;

        uffdio_api.features = 0;

        if (ioctl(ctx->uffd, UFFDIO_API, &uffdio_api) == -1)

        {

            uffd_cleanup(ctx);

            CPLError(CE_Failure, CPLE_AppDefined,

                     "CPLCreateUserFaultMapping(): ioctl(UFFDIO_API) failed");

            return nullptr;

        }

    }

    // Register memory range

    ctx->uffdio_register.range.start =

        reinterpret_cast<uintptr_t>(ctx->vma_ptr);

    ctx->uffdio_register.range.len = ctx->vma_size;

    ctx->uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;

    if (ioctl(ctx->uffd, UFFDIO_REGISTER, &ctx->uffdio_register) == -1)

    {

        uffd_cleanup(ctx);

        CPLError(CE_Failure, CPLE_AppDefined,

                 "CPLCreateUserFaultMapping(): ioctl(UFFDIO_REGISTER) failed");

        return nullptr;

    }

    // Start handler thread

    ctx->thread = CPLCreateJoinableThread(cpl_uffd_fault_handler, ctx);

    if (ctx->thread == nullptr)

    {

        CPLError(

            CE_Failure, CPLE_AppDefined,

            "CPLCreateUserFaultMapping(): CPLCreateJoinableThread() failed");

        uffd_cleanup(ctx);

        return nullptr;

    }

    *ppVma = ctx->vma_ptr;

    *pnVmaSize = ctx->vma_size;

    return ctx;

}

void CPLDeleteUserFaultMapping(cpl_uffd_context *ctx)

{

    if (ctx)

    {

        uffd_cleanup(ctx);

    }

}

#endif  // ENABLE_UFFD