util-lib: split out image dissecting code and loopback code from nspawn

This adds two new APIs to systemd:

- loop-util.h is a simple internal API for allocating, setting up and releasing
  loopback block devices.

- dissect-image.h is an internal API for taking apart disk images and figuring
  out what the purpose of each partition is.

Both APIs are basically refactored versions of similar code in nspawn.  This
rework should permit us to reuse this in other places than just nspawn in the
future. Specifically: to implement RootImage= in the service image, similar to
RootDirectory=, but operating on a disk image; to unify the gpt-auto-discovery
generator code with the discovery logic in nspawn; to add new API to machined
for determining the OS version of a disk image (i.e. not just running
containers). This PR does not make any such changes however, it just provides
the new reworked API.

The reworked code is also slightly more powerful than the nspawn original one.
When pointing it to an image or block device with a naked file system (i.e. no
partition table) it will simply make it the root device.

1 files changed, 157 insertions, 0 deletions

diff --git a/src/shared/loop-util.c b/src/shared/loop-util.c
new file mode 100644
index 0000000000..8be4dbf938
--- /dev/null
+++ b/src/shared/loop-util.c
@@ -0,0 +1,157 @@
+/***
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <fcntl.h>
+#include <linux/loop.h>
+#include <sys/ioctl.h>
+#include <sys/stat.h>
+
+#include "alloc-util.h"
+#include "fd-util.h"
+#include "loop-util.h"
+
+int loop_device_make(int fd, int open_flags, LoopDevice **ret) {
+        const struct loop_info64 info = {
+                .lo_flags = LO_FLAGS_AUTOCLEAR|LO_FLAGS_PARTSCAN|(open_flags == O_RDONLY ? LO_FLAGS_READ_ONLY : 0),
+        };
+
+        _cleanup_close_ int control = -1, loop = -1;
+        _cleanup_free_ char *loopdev = NULL;
+        struct stat st;
+        LoopDevice *d;
+        int nr;
+
+        assert(fd >= 0);
+        assert(ret);
+        assert(IN_SET(open_flags, O_RDWR, O_RDONLY));
+
+        if (fstat(fd, &st) < 0)
+                return -errno;
+
+        if (S_ISBLK(st.st_mode)) {
+                int copy;
+
+                /* If this is already a block device, store a copy of the fd as it is */
+
+                copy = fcntl(fd, F_DUPFD_CLOEXEC, 3);
+                if (copy < 0)
+                        return -errno;
+
+                d = new0(LoopDevice, 1);
+                if (!d)
+                        return -ENOMEM;
+
+                *d = (LoopDevice) {
+                        .fd = copy,
+                        .nr = -1,
+                };
+
+                *ret = d;
+
+                return 0;
+        }
+
+        if (!S_ISREG(st.st_mode))
+                return -EINVAL;
+
+        control = open("/dev/loop-control", O_RDWR|O_CLOEXEC|O_NOCTTY|O_NONBLOCK);
+        if (control < 0)
+                return -errno;
+
+        nr = ioctl(control, LOOP_CTL_GET_FREE);
+        if (nr < 0)
+                return -errno;
+
+        if (asprintf(&loopdev, "/dev/loop%i", nr) < 0)
+                return -ENOMEM;
+
+        loop = open(loopdev, O_CLOEXEC|O_NONBLOCK|O_NOCTTY|open_flags);
+        if (loop < 0)
+                return -errno;
+
+        if (ioctl(loop, LOOP_SET_FD, fd) < 0)
+                return -errno;
+
+        if (ioctl(loop, LOOP_SET_STATUS64, &info) < 0)
+                return -errno;
+
+        d = new(LoopDevice, 1);
+        if (!d)
+                return -ENOMEM;
+
+        *d = (LoopDevice) {
+                .fd = loop,
+                .node = loopdev,
+                .nr = nr,
+        };
+
+        loop = -1;
+        loopdev = NULL;
+
+        *ret = d;
+
+        return (*ret)->fd;
+}
+
+int loop_device_make_by_path(const char *path, int open_flags, LoopDevice **ret) {
+        _cleanup_close_ int fd = -1;
+
+        assert(path);
+        assert(ret);
+        assert(IN_SET(open_flags, O_RDWR, O_RDONLY));
+
+        fd = open(path, O_CLOEXEC|O_NONBLOCK|O_NOCTTY|open_flags);
+        if (fd < 0)
+                return -errno;
+
+        return loop_device_make(fd, open_flags, ret);
+}
+
+LoopDevice* loop_device_unref(LoopDevice *d) {
+        if (!d)
+                return NULL;
+
+        if (d->fd >= 0) {
+
+                if (d->nr >= 0) {
+                        if (ioctl(d->fd, LOOP_CLR_FD) < 0)
+                                log_debug_errno(errno, "Failed to clear loop device: %m");
+
+                }
+
+                safe_close(d->fd);
+        }
+
+        if (d->nr >= 0) {
+                _cleanup_close_ int control = -1;
+
+                control = open("/dev/loop-control", O_RDWR|O_CLOEXEC|O_NOCTTY|O_NONBLOCK);
+                if (control < 0)
+                        log_debug_errno(errno, "Failed to open loop control device: %m");
+                else {
+                        if (ioctl(control, LOOP_CTL_REMOVE, d->nr) < 0)
+                                log_debug_errno(errno, "Failed to remove loop device: %m");
+                }
+        }
+
+        free(d->node);
+        free(d);
+
+        return NULL;
+}