fd_share.cc

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// Copyright (C) 2010-2019 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#include <config.h>

#include <cstring>
#include <cstdlib>

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h>
#include <errno.h>
#include <stdlib.h>             // for malloc and free
#include <unistd.h>
#include <util/io/fd_share.h>

namespace isc {
namespace util {
namespace io {

namespace {
// Not all OSes support advanced CMSG macros: CMSG_LEN and CMSG_SPACE.
// In order to ensure as much portability as possible, we provide wrapper
// functions of these macros.
// Note that cmsg_space() could run slow on OSes that do not have
// CMSG_SPACE.
inline socklen_t
cmsg_len(const socklen_t len) {
#ifdef CMSG_LEN
    return (CMSG_LEN(len));
#else
    // Cast NULL so that any pointer arithmetic performed by CMSG_DATA
    // is correct.
    const uintptr_t hdrlen = (uintptr_t)CMSG_DATA(((struct cmsghdr*)NULL));
    return (hdrlen + len);
#endif
}

inline socklen_t
cmsg_space(const socklen_t len) {
#ifdef CMSG_SPACE
    return (CMSG_SPACE(len));
#else
    struct msghdr msg;
    struct cmsghdr* cmsgp;
    // XXX: The buffer length is an ad hoc value, but should be enough
    // in a practical sense.
    char dummybuf[sizeof(struct cmsghdr) + 1024];

    memset(&msg, 0, sizeof(msg));
    msg.msg_control = dummybuf;
    msg.msg_controllen = sizeof(dummybuf);

    cmsgp = (struct cmsghdr*)dummybuf;
    cmsgp->cmsg_len = cmsg_len(len);

    cmsgp = CMSG_NXTHDR(&msg, cmsgp);
    if (cmsgp != NULL) {
        return ((char*)cmsgp - (char*)msg.msg_control);
    } else {
        return (0);
    }
#endif  // CMSG_SPACE
}
}

int
recv_fd(const int sock) {
    struct msghdr msghdr;
    struct iovec iov_dummy;
    unsigned char dummy_data;

    iov_dummy.iov_base = &dummy_data;
    iov_dummy.iov_len = sizeof(dummy_data);
    msghdr.msg_name = NULL;
    msghdr.msg_namelen = 0;
    msghdr.msg_iov = &iov_dummy;
    msghdr.msg_iovlen = 1;
    msghdr.msg_flags = 0;
    msghdr.msg_controllen = cmsg_space(sizeof(int));
    msghdr.msg_control = malloc(msghdr.msg_controllen);
    if (msghdr.msg_control == NULL) {
        return (FD_SYSTEM_ERROR);
    }

    const int cc = recvmsg(sock, &msghdr, 0);
    if (cc <= 0) {
        free(msghdr.msg_control);
        if (cc == 0) {
            errno = ECONNRESET;
        }
        return (FD_SYSTEM_ERROR);
    }
    const struct cmsghdr* cmsg = CMSG_FIRSTHDR(&msghdr);
    int fd = FD_OTHER_ERROR;
    if (cmsg != NULL && cmsg->cmsg_len == cmsg_len(sizeof(int)) &&
        cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
// Some systems (e.g. recent NetBSD) converted all CMSG access macros
// to static_cast when used in C++ code. As cmsg is declared const
// this makes the CMSG_DATA macro to not compile. But fortunately
// these systems provide a const alternative named CCMSG_DATA.
#ifdef CCMSG_DATA
        std::memcpy(&fd, CCMSG_DATA(cmsg), sizeof(int));
#else
        std::memcpy(&fd, CMSG_DATA(cmsg), sizeof(int));
#endif
    }
    free(msghdr.msg_control);
    int new_fd = -1;
    int close_error = -1;
    if (fd >= 0) {
        // It is strange, but the call can return the same file descriptor as
        // one returned previously, even if that one is not closed yet. So,
        // we just re-number every one we get, so they are unique.
        new_fd = dup(fd);
        close_error = close(fd);
    }
    if (close_error == -1 || new_fd == -1) {
        // We need to return an error, because something failed. But in case
        // it was the previous close, we at least try to close the duped FD.
        if (new_fd != -1) {
            close(new_fd); // If this fails, nothing but returning error can't
                           // be done and we are doing that anyway.
        }
        return (FD_SYSTEM_ERROR);
    }
    return (new_fd);
}

int
send_fd(const int sock, const int fd) {
    struct msghdr msghdr;
    struct iovec iov_dummy;
    unsigned char dummy_data = 0;

    iov_dummy.iov_base = &dummy_data;
    iov_dummy.iov_len = sizeof(dummy_data);
    msghdr.msg_name = NULL;
    msghdr.msg_namelen = 0;
    msghdr.msg_iov = &iov_dummy;
    msghdr.msg_iovlen = 1;
    msghdr.msg_flags = 0;
    msghdr.msg_controllen = cmsg_space(sizeof(int));
    msghdr.msg_control = malloc(msghdr.msg_controllen);
    if (msghdr.msg_control == NULL) {
        return (FD_OTHER_ERROR);
    }
    std::memset(msghdr.msg_control, 0, msghdr.msg_controllen);

    struct cmsghdr* cmsg = CMSG_FIRSTHDR(&msghdr);
    cmsg->cmsg_len = cmsg_len(sizeof(int));
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_RIGHTS;
    std::memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));

    const int ret = sendmsg(sock, &msghdr, 0);
    free(msghdr.msg_control);
    return (ret >= 0 ? 0 : FD_SYSTEM_ERROR);
}

} // End for namespace io
} // End for namespace util
} // End for namespace isc