Adding upstream version 252.22.upstream/252.22

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 827 insertions, 0 deletions

diff --git a/src/basic/fd-util.c b/src/basic/fd-util.c
new file mode 100644
index 0000000..66bb756
--- /dev/null
+++ b/src/basic/fd-util.c
@@ -0,0 +1,827 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <errno.h>
+#include <fcntl.h>
+#if WANT_LINUX_FS_H
+#include <linux/fs.h>
+#endif
+#include <linux/magic.h>
+#include <sys/ioctl.h>
+#include <sys/resource.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "alloc-util.h"
+#include "dirent-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "fs-util.h"
+#include "io-util.h"
+#include "macro.h"
+#include "missing_fcntl.h"
+#include "missing_fs.h"
+#include "missing_syscall.h"
+#include "parse-util.h"
+#include "path-util.h"
+#include "process-util.h"
+#include "socket-util.h"
+#include "sort-util.h"
+#include "stat-util.h"
+#include "stdio-util.h"
+#include "tmpfile-util.h"
+#include "util.h"
+
+/* The maximum number of iterations in the loop to close descriptors in the fallback case
+ * when /proc/self/fd/ is inaccessible. */
+#define MAX_FD_LOOP_LIMIT (1024*1024)
+
+int close_nointr(int fd) {
+        assert(fd >= 0);
+
+        if (close(fd) >= 0)
+                return 0;
+
+        /*
+         * Just ignore EINTR; a retry loop is the wrong thing to do on
+         * Linux.
+         *
+         * http://lkml.indiana.edu/hypermail/linux/kernel/0509.1/0877.html
+         * https://bugzilla.gnome.org/show_bug.cgi?id=682819
+         * http://utcc.utoronto.ca/~cks/space/blog/unix/CloseEINTR
+         * https://sites.google.com/site/michaelsafyan/software-engineering/checkforeintrwheninvokingclosethinkagain
+         */
+        if (errno == EINTR)
+                return 0;
+
+        return -errno;
+}
+
+int safe_close(int fd) {
+
+        /*
+         * Like close_nointr() but cannot fail. Guarantees errno is
+         * unchanged. Is a NOP with negative fds passed, and returns
+         * -1, so that it can be used in this syntax:
+         *
+         * fd = safe_close(fd);
+         */
+
+        if (fd >= 0) {
+                PROTECT_ERRNO;
+
+                /* The kernel might return pretty much any error code
+                 * via close(), but the fd will be closed anyway. The
+                 * only condition we want to check for here is whether
+                 * the fd was invalid at all... */
+
+                assert_se(close_nointr(fd) != -EBADF);
+        }
+
+        return -1;
+}
+
+void safe_close_pair(int p[static 2]) {
+        assert(p);
+
+        if (p[0] == p[1]) {
+                /* Special case pairs which use the same fd in both
+                 * directions... */
+                p[0] = p[1] = safe_close(p[0]);
+                return;
+        }
+
+        p[0] = safe_close(p[0]);
+        p[1] = safe_close(p[1]);
+}
+
+void close_many(const int fds[], size_t n_fd) {
+        assert(fds || n_fd <= 0);
+
+        for (size_t i = 0; i < n_fd; i++)
+                safe_close(fds[i]);
+}
+
+int fclose_nointr(FILE *f) {
+        assert(f);
+
+        /* Same as close_nointr(), but for fclose() */
+
+        errno = 0; /* Extra safety: if the FILE* object is not encapsulating an fd, it might not set errno
+                    * correctly. Let's hence initialize it to zero first, so that we aren't confused by any
+                    * prior errno here */
+        if (fclose(f) == 0)
+                return 0;
+
+        if (errno == EINTR)
+                return 0;
+
+        return errno_or_else(EIO);
+}
+
+FILE* safe_fclose(FILE *f) {
+
+        /* Same as safe_close(), but for fclose() */
+
+        if (f) {
+                PROTECT_ERRNO;
+
+                assert_se(fclose_nointr(f) != -EBADF);
+        }
+
+        return NULL;
+}
+
+DIR* safe_closedir(DIR *d) {
+
+        if (d) {
+                PROTECT_ERRNO;
+
+                assert_se(closedir(d) >= 0 || errno != EBADF);
+        }
+
+        return NULL;
+}
+
+int fd_nonblock(int fd, bool nonblock) {
+        int flags, nflags;
+
+        assert(fd >= 0);
+
+        flags = fcntl(fd, F_GETFL, 0);
+        if (flags < 0)
+                return -errno;
+
+        nflags = UPDATE_FLAG(flags, O_NONBLOCK, nonblock);
+        if (nflags == flags)
+                return 0;
+
+        return RET_NERRNO(fcntl(fd, F_SETFL, nflags));
+}
+
+int fd_cloexec(int fd, bool cloexec) {
+        int flags, nflags;
+
+        assert(fd >= 0);
+
+        flags = fcntl(fd, F_GETFD, 0);
+        if (flags < 0)
+                return -errno;
+
+        nflags = UPDATE_FLAG(flags, FD_CLOEXEC, cloexec);
+        if (nflags == flags)
+                return 0;
+
+        return RET_NERRNO(fcntl(fd, F_SETFD, nflags));
+}
+
+int fd_cloexec_many(const int fds[], size_t n_fds, bool cloexec) {
+        int ret = 0, r;
+
+        assert(n_fds == 0 || fds);
+
+        for (size_t i = 0; i < n_fds; i++) {
+                if (fds[i] < 0) /* Skip gracefully over already invalidated fds */
+                        continue;
+
+                r = fd_cloexec(fds[i], cloexec);
+                if (r < 0 && ret >= 0) /* Continue going, but return first error */
+                        ret = r;
+                else
+                        ret = 1; /* report if we did anything */
+        }
+
+        return ret;
+}
+
+_pure_ static bool fd_in_set(int fd, const int fdset[], size_t n_fdset) {
+        assert(n_fdset == 0 || fdset);
+
+        for (size_t i = 0; i < n_fdset; i++) {
+                if (fdset[i] < 0)
+                        continue;
+
+                if (fdset[i] == fd)
+                        return true;
+        }
+
+        return false;
+}
+
+int get_max_fd(void) {
+        struct rlimit rl;
+        rlim_t m;
+
+        /* Return the highest possible fd, based RLIMIT_NOFILE, but enforcing FD_SETSIZE-1 as lower boundary
+         * and INT_MAX as upper boundary. */
+
+        if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
+                return -errno;
+
+        m = MAX(rl.rlim_cur, rl.rlim_max);
+        if (m < FD_SETSIZE) /* Let's always cover at least 1024 fds */
+                return FD_SETSIZE-1;
+
+        if (m == RLIM_INFINITY || m > INT_MAX) /* Saturate on overflow. After all fds are "int", hence can
+                                                * never be above INT_MAX */
+                return INT_MAX;
+
+        return (int) (m - 1);
+}
+
+static int close_all_fds_frugal(const int except[], size_t n_except) {
+        int max_fd, r = 0;
+
+        assert(n_except == 0 || except);
+
+        /* This is the inner fallback core of close_all_fds(). This never calls malloc() or opendir() or so
+         * and hence is safe to be called in signal handler context. Most users should call close_all_fds(),
+         * but when we assume we are called from signal handler context, then use this simpler call
+         * instead. */
+
+        max_fd = get_max_fd();
+        if (max_fd < 0)
+                return max_fd;
+
+        /* Refuse to do the loop over more too many elements. It's better to fail immediately than to
+         * spin the CPU for a long time. */
+        if (max_fd > MAX_FD_LOOP_LIMIT)
+                return log_debug_errno(SYNTHETIC_ERRNO(EPERM),
+                                       "Refusing to loop over %d potential fds.",
+                                       max_fd);
+
+        for (int fd = 3; fd >= 0; fd = fd < max_fd ? fd + 1 : -1) {
+                int q;
+
+                if (fd_in_set(fd, except, n_except))
+                        continue;
+
+                q = close_nointr(fd);
+                if (q < 0 && q != -EBADF && r >= 0)
+                        r = q;
+        }
+
+        return r;
+}
+
+static bool have_close_range = true; /* Assume we live in the future */
+
+static int close_all_fds_special_case(const int except[], size_t n_except) {
+        assert(n_except == 0 || except);
+
+        /* Handles a few common special cases separately, since they are common and can be optimized really
+         * nicely, since we won't need sorting for them. Returns > 0 if the special casing worked, 0
+         * otherwise. */
+
+        if (!have_close_range)
+                return 0;
+
+        if (n_except == 1 && except[0] < 0) /* Minor optimization: if we only got one fd, and it's invalid,
+                                             * we got none */
+                n_except = 0;
+
+        switch (n_except) {
+
+        case 0:
+                /* Close everything. Yay! */
+
+                if (close_range(3, -1, 0) >= 0)
+                        return 1;
+
+                if (ERRNO_IS_NOT_SUPPORTED(errno) || ERRNO_IS_PRIVILEGE(errno)) {
+                        have_close_range = false;
+                        return 0;
+                }
+
+                return -errno;
+
+        case 1:
+                /* Close all but exactly one, then we don't need no sorting. This is a pretty common
+                 * case, hence let's handle it specially. */
+
+                if ((except[0] <= 3 || close_range(3, except[0]-1, 0) >= 0) &&
+                    (except[0] >= INT_MAX || close_range(MAX(3, except[0]+1), -1, 0) >= 0))
+                        return 1;
+
+                if (ERRNO_IS_NOT_SUPPORTED(errno) || ERRNO_IS_PRIVILEGE(errno)) {
+                        have_close_range = false;
+                        return 0;
+                }
+
+                return -errno;
+
+        default:
+                return 0;
+        }
+}
+
+int close_all_fds_without_malloc(const int except[], size_t n_except) {
+        int r;
+
+        assert(n_except == 0 || except);
+
+        r = close_all_fds_special_case(except, n_except);
+        if (r < 0)
+                return r;
+        if (r > 0) /* special case worked! */
+                return 0;
+
+        return close_all_fds_frugal(except, n_except);
+}
+
+int close_all_fds(const int except[], size_t n_except) {
+        _cleanup_closedir_ DIR *d = NULL;
+        int r = 0;
+
+        assert(n_except == 0 || except);
+
+        r = close_all_fds_special_case(except, n_except);
+        if (r < 0)
+                return r;
+        if (r > 0) /* special case worked! */
+                return 0;
+
+        if (have_close_range) {
+                _cleanup_free_ int *sorted_malloc = NULL;
+                size_t n_sorted;
+                int *sorted;
+
+                /* In the best case we have close_range() to close all fds between a start and an end fd,
+                 * which we can use on the "inverted" exception array, i.e. all intervals between all
+                 * adjacent pairs from the sorted exception array. This changes loop complexity from O(n)
+                 * where n is number of open fds to O(m⋅log(m)) where m is the number of fds to keep
+                 * open. Given that we assume n ≫ m that's preferable to us. */
+
+                assert(n_except < SIZE_MAX);
+                n_sorted = n_except + 1;
+
+                if (n_sorted > 64) /* Use heap for large numbers of fds, stack otherwise */
+                        sorted = sorted_malloc = new(int, n_sorted);
+                else
+                        sorted = newa(int, n_sorted);
+
+                if (sorted) {
+                        memcpy(sorted, except, n_except * sizeof(int));
+
+                        /* Let's add fd 2 to the list of fds, to simplify the loop below, as this
+                         * allows us to cover the head of the array the same way as the body */
+                        sorted[n_sorted-1] = 2;
+
+                        typesafe_qsort(sorted, n_sorted, cmp_int);
+
+                        for (size_t i = 0; i < n_sorted-1; i++) {
+                                int start, end;
+
+                                start = MAX(sorted[i], 2); /* The first three fds shall always remain open */
+                                end = MAX(sorted[i+1], 2);
+
+                                assert(end >= start);
+
+                                if (end - start <= 1)
+                                        continue;
+
+                                /* Close everything between the start and end fds (both of which shall stay open) */
+                                if (close_range(start + 1, end - 1, 0) < 0) {
+                                        if (!ERRNO_IS_NOT_SUPPORTED(errno) && !ERRNO_IS_PRIVILEGE(errno))
+                                                return -errno;
+
+                                        have_close_range = false;
+                                        break;
+                                }
+                        }
+
+                        if (have_close_range) {
+                                /* The loop succeeded. Let's now close everything beyond the end */
+
+                                if (sorted[n_sorted-1] >= INT_MAX) /* Dont let the addition below overflow */
+                                        return 0;
+
+                                if (close_range(sorted[n_sorted-1] + 1, -1, 0) >= 0)
+                                        return 0;
+
+                                if (!ERRNO_IS_NOT_SUPPORTED(errno) && !ERRNO_IS_PRIVILEGE(errno))
+                                        return -errno;
+
+                                have_close_range = false;
+                        }
+                }
+
+                /* Fallback on OOM or if close_range() is not supported */
+        }
+
+        d = opendir("/proc/self/fd");
+        if (!d)
+                return close_all_fds_frugal(except, n_except); /* ultimate fallback if /proc/ is not available */
+
+        FOREACH_DIRENT(de, d, return -errno) {
+                int fd = -1, q;
+
+                if (!IN_SET(de->d_type, DT_LNK, DT_UNKNOWN))
+                        continue;
+
+                if (safe_atoi(de->d_name, &fd) < 0)
+                        /* Let's better ignore this, just in case */
+                        continue;
+
+                if (fd < 3)
+                        continue;
+
+                if (fd == dirfd(d))
+                        continue;
+
+                if (fd_in_set(fd, except, n_except))
+                        continue;
+
+                q = close_nointr(fd);
+                if (q < 0 && q != -EBADF && r >= 0) /* Valgrind has its own FD and doesn't want to have it closed */
+                        r = q;
+        }
+
+        return r;
+}
+
+int same_fd(int a, int b) {
+        struct stat sta, stb;
+        pid_t pid;
+        int r, fa, fb;
+
+        assert(a >= 0);
+        assert(b >= 0);
+
+        /* Compares two file descriptors. Note that semantics are quite different depending on whether we
+         * have kcmp() or we don't. If we have kcmp() this will only return true for dup()ed file
+         * descriptors, but not otherwise. If we don't have kcmp() this will also return true for two fds of
+         * the same file, created by separate open() calls. Since we use this call mostly for filtering out
+         * duplicates in the fd store this difference hopefully doesn't matter too much. */
+
+        if (a == b)
+                return true;
+
+        /* Try to use kcmp() if we have it. */
+        pid = getpid_cached();
+        r = kcmp(pid, pid, KCMP_FILE, a, b);
+        if (r == 0)
+                return true;
+        if (r > 0)
+                return false;
+        if (!ERRNO_IS_NOT_SUPPORTED(errno) && !ERRNO_IS_PRIVILEGE(errno))
+                return -errno;
+
+        /* We don't have kcmp(), use fstat() instead. */
+        if (fstat(a, &sta) < 0)
+                return -errno;
+
+        if (fstat(b, &stb) < 0)
+                return -errno;
+
+        if (!stat_inode_same(&sta, &stb))
+                return false;
+
+        /* We consider all device fds different, since two device fds might refer to quite different device
+         * contexts even though they share the same inode and backing dev_t. */
+
+        if (S_ISCHR(sta.st_mode) || S_ISBLK(sta.st_mode))
+                return false;
+
+        /* The fds refer to the same inode on disk, let's also check if they have the same fd flags. This is
+         * useful to distinguish the read and write side of a pipe created with pipe(). */
+        fa = fcntl(a, F_GETFL);
+        if (fa < 0)
+                return -errno;
+
+        fb = fcntl(b, F_GETFL);
+        if (fb < 0)
+                return -errno;
+
+        return fa == fb;
+}
+
+void cmsg_close_all(struct msghdr *mh) {
+        struct cmsghdr *cmsg;
+
+        assert(mh);
+
+        CMSG_FOREACH(cmsg, mh)
+                if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS)
+                        close_many((int*) CMSG_DATA(cmsg), (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int));
+}
+
+bool fdname_is_valid(const char *s) {
+        const char *p;
+
+        /* Validates a name for $LISTEN_FDNAMES. We basically allow
+         * everything ASCII that's not a control character. Also, as
+         * special exception the ":" character is not allowed, as we
+         * use that as field separator in $LISTEN_FDNAMES.
+         *
+         * Note that the empty string is explicitly allowed
+         * here. However, we limit the length of the names to 255
+         * characters. */
+
+        if (!s)
+                return false;
+
+        for (p = s; *p; p++) {
+                if (*p < ' ')
+                        return false;
+                if (*p >= 127)
+                        return false;
+                if (*p == ':')
+                        return false;
+        }
+
+        return p - s <= FDNAME_MAX;
+}
+
+int fd_get_path(int fd, char **ret) {
+        int r;
+
+        r = readlink_malloc(FORMAT_PROC_FD_PATH(fd), ret);
+        if (r == -ENOENT) {
+                /* ENOENT can mean two things: that the fd does not exist or that /proc is not mounted. Let's make
+                 * things debuggable and distinguish the two. */
+
+                if (proc_mounted() == 0)
+                        return -ENOSYS;  /* /proc is not available or not set up properly, we're most likely in some chroot
+                                          * environment. */
+                return -EBADF; /* The directory exists, hence it's the fd that doesn't. */
+        }
+
+        return r;
+}
+
+int move_fd(int from, int to, int cloexec) {
+        int r;
+
+        /* Move fd 'from' to 'to', make sure FD_CLOEXEC remains equal if requested, and release the old fd. If
+         * 'cloexec' is passed as -1, the original FD_CLOEXEC is inherited for the new fd. If it is 0, it is turned
+         * off, if it is > 0 it is turned on. */
+
+        if (from < 0)
+                return -EBADF;
+        if (to < 0)
+                return -EBADF;
+
+        if (from == to) {
+
+                if (cloexec >= 0) {
+                        r = fd_cloexec(to, cloexec);
+                        if (r < 0)
+                                return r;
+                }
+
+                return to;
+        }
+
+        if (cloexec < 0) {
+                int fl;
+
+                fl = fcntl(from, F_GETFD, 0);
+                if (fl < 0)
+                        return -errno;
+
+                cloexec = !!(fl & FD_CLOEXEC);
+        }
+
+        r = dup3(from, to, cloexec ? O_CLOEXEC : 0);
+        if (r < 0)
+                return -errno;
+
+        assert(r == to);
+
+        safe_close(from);
+
+        return to;
+}
+
+int fd_move_above_stdio(int fd) {
+        int flags, copy;
+        PROTECT_ERRNO;
+
+        /* Moves the specified file descriptor if possible out of the range [0…2], i.e. the range of
+         * stdin/stdout/stderr. If it can't be moved outside of this range the original file descriptor is
+         * returned. This call is supposed to be used for long-lasting file descriptors we allocate in our code that
+         * might get loaded into foreign code, and where we want ensure our fds are unlikely used accidentally as
+         * stdin/stdout/stderr of unrelated code.
+         *
+         * Note that this doesn't fix any real bugs, it just makes it less likely that our code will be affected by
+         * buggy code from others that mindlessly invokes 'fprintf(stderr, …' or similar in places where stderr has
+         * been closed before.
+         *
+         * This function is written in a "best-effort" and "least-impact" style. This means whenever we encounter an
+         * error we simply return the original file descriptor, and we do not touch errno. */
+
+        if (fd < 0 || fd > 2)
+                return fd;
+
+        flags = fcntl(fd, F_GETFD, 0);
+        if (flags < 0)
+                return fd;
+
+        if (flags & FD_CLOEXEC)
+                copy = fcntl(fd, F_DUPFD_CLOEXEC, 3);
+        else
+                copy = fcntl(fd, F_DUPFD, 3);
+        if (copy < 0)
+                return fd;
+
+        assert(copy > 2);
+
+        (void) close(fd);
+        return copy;
+}
+
+int rearrange_stdio(int original_input_fd, int original_output_fd, int original_error_fd) {
+
+        int fd[3] = { /* Put together an array of fds we work on */
+                original_input_fd,
+                original_output_fd,
+                original_error_fd
+        };
+
+        int r, i,
+                null_fd = -1,                /* if we open /dev/null, we store the fd to it here */
+                copy_fd[3] = { -1, -1, -1 }; /* This contains all fds we duplicate here temporarily, and hence need to close at the end */
+        bool null_readable, null_writable;
+
+        /* Sets up stdin, stdout, stderr with the three file descriptors passed in. If any of the descriptors is
+         * specified as -1 it will be connected with /dev/null instead. If any of the file descriptors is passed as
+         * itself (e.g. stdin as STDIN_FILENO) it is left unmodified, but the O_CLOEXEC bit is turned off should it be
+         * on.
+         *
+         * Note that if any of the passed file descriptors are > 2 they will be closed — both on success and on
+         * failure! Thus, callers should assume that when this function returns the input fds are invalidated.
+         *
+         * Note that when this function fails stdin/stdout/stderr might remain half set up!
+         *
+         * O_CLOEXEC is turned off for all three file descriptors (which is how it should be for
+         * stdin/stdout/stderr). */
+
+        null_readable = original_input_fd < 0;
+        null_writable = original_output_fd < 0 || original_error_fd < 0;
+
+        /* First step, open /dev/null once, if we need it */
+        if (null_readable || null_writable) {
+
+                /* Let's open this with O_CLOEXEC first, and convert it to non-O_CLOEXEC when we move the fd to the final position. */
+                null_fd = open("/dev/null", (null_readable && null_writable ? O_RDWR :
+                                             null_readable ? O_RDONLY : O_WRONLY) | O_CLOEXEC);
+                if (null_fd < 0) {
+                        r = -errno;
+                        goto finish;
+                }
+
+                /* If this fd is in the 0…2 range, let's move it out of it */
+                if (null_fd < 3) {
+                        int copy;
+
+                        copy = fcntl(null_fd, F_DUPFD_CLOEXEC, 3); /* Duplicate this with O_CLOEXEC set */
+                        if (copy < 0) {
+                                r = -errno;
+                                goto finish;
+                        }
+
+                        close_and_replace(null_fd, copy);
+                }
+        }
+
+        /* Let's assemble fd[] with the fds to install in place of stdin/stdout/stderr */
+        for (i = 0; i < 3; i++) {
+
+                if (fd[i] < 0)
+                        fd[i] = null_fd;        /* A negative parameter means: connect this one to /dev/null */
+                else if (fd[i] != i && fd[i] < 3) {
+                        /* This fd is in the 0…2 territory, but not at its intended place, move it out of there, so that we can work there. */
+                        copy_fd[i] = fcntl(fd[i], F_DUPFD_CLOEXEC, 3); /* Duplicate this with O_CLOEXEC set */
+                        if (copy_fd[i] < 0) {
+                                r = -errno;
+                                goto finish;
+                        }
+
+                        fd[i] = copy_fd[i];
+                }
+        }
+
+        /* At this point we now have the fds to use in fd[], and they are all above the stdio range, so that we
+         * have freedom to move them around. If the fds already were at the right places then the specific fds are
+         * -1. Let's now move them to the right places. This is the point of no return. */
+        for (i = 0; i < 3; i++) {
+
+                if (fd[i] == i) {
+
+                        /* fd is already in place, but let's make sure O_CLOEXEC is off */
+                        r = fd_cloexec(i, false);
+                        if (r < 0)
+                                goto finish;
+
+                } else {
+                        assert(fd[i] > 2);
+
+                        if (dup2(fd[i], i) < 0) { /* Turns off O_CLOEXEC on the new fd. */
+                                r = -errno;
+                                goto finish;
+                        }
+                }
+        }
+
+        r = 0;
+
+finish:
+        /* Close the original fds, but only if they were outside of the stdio range. Also, properly check for the same
+         * fd passed in multiple times. */
+        safe_close_above_stdio(original_input_fd);
+        if (original_output_fd != original_input_fd)
+                safe_close_above_stdio(original_output_fd);
+        if (original_error_fd != original_input_fd && original_error_fd != original_output_fd)
+                safe_close_above_stdio(original_error_fd);
+
+        /* Close the copies we moved > 2 */
+        for (i = 0; i < 3; i++)
+                safe_close(copy_fd[i]);
+
+        /* Close our null fd, if it's > 2 */
+        safe_close_above_stdio(null_fd);
+
+        return r;
+}
+
+int fd_reopen(int fd, int flags) {
+        int new_fd, r;
+
+        /* Reopens the specified fd with new flags. This is useful for convert an O_PATH fd into a regular one, or to
+         * turn O_RDWR fds into O_RDONLY fds.
+         *
+         * This doesn't work on sockets (since they cannot be open()ed, ever).
+         *
+         * This implicitly resets the file read index to 0. */
+
+        if (FLAGS_SET(flags, O_DIRECTORY)) {
+                /* If we shall reopen the fd as directory we can just go via "." and thus bypass the whole
+                 * magic /proc/ directory, and make ourselves independent of that being mounted. */
+                new_fd = openat(fd, ".", flags);
+                if (new_fd < 0)
+                        return -errno;
+
+                return new_fd;
+        }
+
+        new_fd = open(FORMAT_PROC_FD_PATH(fd), flags);
+        if (new_fd < 0) {
+                if (errno != ENOENT)
+                        return -errno;
+
+                r = proc_mounted();
+                if (r == 0)
+                        return -ENOSYS; /* if we have no /proc/, the concept is not implementable */
+
+                return r > 0 ? -EBADF : -ENOENT; /* If /proc/ is definitely around then this means the fd is
+                                                  * not valid, otherwise let's propagate the original
+                                                  * error */
+        }
+
+        return new_fd;
+}
+
+int read_nr_open(void) {
+        _cleanup_free_ char *nr_open = NULL;
+        int r;
+
+        /* Returns the kernel's current fd limit, either by reading it of /proc/sys if that works, or using the
+         * hard-coded default compiled-in value of current kernels (1M) if not. This call will never fail. */
+
+        r = read_one_line_file("/proc/sys/fs/nr_open", &nr_open);
+        if (r < 0)
+                log_debug_errno(r, "Failed to read /proc/sys/fs/nr_open, ignoring: %m");
+        else {
+                int v;
+
+                r = safe_atoi(nr_open, &v);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to parse /proc/sys/fs/nr_open value '%s', ignoring: %m", nr_open);
+                else
+                        return v;
+        }
+
+        /* If we fail, fall back to the hard-coded kernel limit of 1024 * 1024. */
+        return 1024 * 1024;
+}
+
+int fd_get_diskseq(int fd, uint64_t *ret) {
+        uint64_t diskseq;
+
+        assert(fd >= 0);
+        assert(ret);
+
+        if (ioctl(fd, BLKGETDISKSEQ, &diskseq) < 0) {
+                /* Note that the kernel is weird: non-existing ioctls currently return EINVAL
+                 * rather than ENOTTY on loopback block devices. They should fix that in the kernel,
+                 * but in the meantime we accept both here. */
+                if (!ERRNO_IS_NOT_SUPPORTED(errno) && errno != EINVAL)
+                        return -errno;
+
+                return -EOPNOTSUPP;
+        }
+
+        *ret = diskseq;
+
+        return 0;
+}