basic/fs-util: skip fsync_directory_of_file() if /proc/self/fd/ is not available (#8386)

When systemd is running under lorax (in Fedora compose process), it'd think that
it failed to write /etc/machine-id, even though the write succeeded, because
fsync_directory_of_file() would fail, because /proc/self/fd/ is not available.
fsync_directory_of_file() is mostly an additional safety net, so I think it's best
to just silently ignore the error.

Strace of pid1:
35791 stat("/etc", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
35791 openat(AT_FDCWD, "/etc/machine-id", O_RDWR|O_CREAT|O_NOCTTY|O_CLOEXEC, 0444) = 3
35791 umask(022)                        = 000
35791 read(3, "", 38)                   = 0
35791 openat(AT_FDCWD, "/var/lib/dbus/machine-id", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file o
r directory)
35791 openat(AT_FDCWD, "/sys/class/dmi/id/product_name", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
35791 openat(AT_FDCWD, "/sys/class/dmi/id/sys_vendor", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
35791 openat(AT_FDCWD, "/sys/class/dmi/id/board_vendor", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
35791 openat(AT_FDCWD, "/sys/class/dmi/id/bios_vendor", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
35791 access("/proc/xen", F_OK)         = -1 ENOENT (No such file or directory)
35791 openat(AT_FDCWD, "/sys/hypervisor/type", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
35791 openat(AT_FDCWD, "/proc/cpuinfo", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
35791 getrandom("\xb8\x82\xed\xd4\x35\x11\xd0\xeb\xa6\x79\xd7\x31\x6e\x7b\x99\xce", 16, GRND_NONBLOCK) = 16
35791 writev(2, [{iov_base="Initializing machine ID from random generator.", iov_len=46}, {iov_base="\n", iov_len=1}],
 2) = 47
35791 lseek(3, 0, SEEK_SET)             = 0
35791 ftruncate(3, 0)                   = 0
35791 write(3, "b882edd4351140eba679d7316e7b99ce\n", 33) = 33
35791 fsync(3)                          = 0
35791 fstat(3, {st_mode=S_IFREG|0444, st_size=33, ...}) = 0
35791 readlinkat(AT_FDCWD, "/proc/self/fd/3", 0x564df8c694c0, 99) = -1 ENOENT (No such file or directory)
35791 close(3)                          = 0
35791 umask(022)                        = 022
35791 openat(AT_FDCWD, "/run/machine-id", O_WRONLY|O_CREAT|O_NOCTTY|O_TRUNC|O_CLOEXEC, 0444) = 3
35791 write(3, "b882edd4351140eba679d7316e7b99ce\n", 33) = 33
35791 close(3)                          = 0
35791 umask(022)                        = 022
35791 mount("/run/machine-id", "/etc/machine-id", NULL, MS_BIND, NULL) = 0
35791 writev(2, [{iov_base="Installed transient /etc/machine-id file.", iov_len=41}, {iov_base="\n", iov_len=1}], 2) = 42
35791 mount(NULL, "/etc/machine-id", NULL, MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = 0

https://bugzilla.redhat.com/show_bug.cgi?id=1552843

src/basic/fd-util.c

@@ -361,14 +361,21 @@ bool fdname_is_valid(const char *s) {
 }
 
 int fd_get_path(int fd, char **ret) {
-        char procfs_path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
+        _cleanup_close_ int dir = -1;
+        char fdname[DECIMAL_STR_MAX(int)];
         int r;
 
-        xsprintf(procfs_path, "/proc/self/fd/%i", fd);
+        dir = open("/proc/self/fd/", O_CLOEXEC | O_DIRECTORY | O_PATH);
+        if (dir < 0)
+                /* /proc is not available or not set up properly, we're most likely
+                 * in some chroot environment. */
+                return errno == ENOENT ? -EOPNOTSUPP : -errno;
 
-        r = readlink_malloc(procfs_path, ret);
+        xsprintf(fdname, "%i", fd);
 
-        if (r == -ENOENT) /* If the file doesn't exist the fd is invalid */
+        r = readlinkat_malloc(dir, fdname, ret);
+        if (r == -ENOENT)
+                /* If the file doesn't exist the fd is invalid */
                 return -EBADF;
 
         return r;

src/basic/fs-util.c

@@ -978,8 +978,19 @@ int fsync_directory_of_file(int fd) {
                 return r;
 
         r = fd_get_path(fd, &path);
-        if (r < 0)
+        if (r < 0) {
+                log_debug("Failed to query /proc/self/fd/%d%s: %m",
+                          fd,
+                          r == -EOPNOTSUPP ? ", ignoring" : "");
+
+                if (r == -EOPNOTSUPP)
+                        /* If /proc is not available, we're most likely running in some
+                         * chroot environment, and syncing the directory is not very
+                         * important in that case. Let's just silently do nothing. */
+                        return 0;
+
                 return r;
+        }
 
         if (!path_is_absolute(path))
                 return -EINVAL;