c8427c392e
Getting unbound loop devices is racy, so we do it in a retry loop: in case of a recoverable error, like when the loop device signals it is busy, we close the it and try another one. Indeed the code, closed the loop device but did in fact not open a new one and we would therefore see file descriptor cannot be a negative integer errors when trying in `lo.set_fd(fd)` since `lo` is in fact closed now and thus indeed '-1'. Open a new loop device at the beginning of the retry-loop to fix this issue.
149 lines
3.9 KiB
Python
Executable file
149 lines
3.9 KiB
Python
Executable file
#!/usr/bin/python3
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"""
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Loopback device host service
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This service can be used to expose a file or a subset of it as a
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device node. The file is specified via the `filename`, and the
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subset can be specified via `offset` and `size`.
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The resulting device name is returned together with the device
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node numbers (`major`, `minor`). The device is closed when the
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service is shut down.
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A typical use case is formatting the file or a partition in the
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file with a file system or mounting a previously created file
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system contained in the file.
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"""
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import argparse
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import errno
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import os
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import sys
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from typing import Dict
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from osbuild import devices
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from osbuild import loop
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SCHEMA = """
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"additionalProperties": false,
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"required": ["filename"],
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"properties": {
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"filename": {
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"type": "string",
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"description": "File to associate with the loopback device"
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},
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"start": {
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"type": "number",
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"description": "Start of the data segment (in sectors)",
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"default": 0
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},
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"size": {
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"type": "number",
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"description": "Size limit of the data segment (in sectors)"
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},
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"sector-size": {
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"type": "number",
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"description": "Sector size (in bytes)",
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"default": 512
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}
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}
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"""
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class LoopbackService(devices.DeviceService):
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def __init__(self, args: argparse.Namespace):
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super().__init__(args)
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self.fd = None
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self.lo = None
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self.ctl = loop.LoopControl()
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def make_loop(self, fd: int, offset, sizelimit):
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if not sizelimit:
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stat = os.fstat(fd)
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sizelimit = stat.st_size - offset
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else:
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sizelimit *= self.sector_size
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while True:
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# Getting an unbound loopback device and attaching a backing
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# file descriptor to it is racy, so we must use a retry loop
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lo = loop.Loop(self.ctl.get_unbound())
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try:
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lo.set_fd(fd)
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except OSError as e:
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lo.close()
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if e.errno == errno.EBUSY:
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continue
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raise e
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# `set_status` returns EBUSY when the pages from the previously
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# bound file have not been fully cleared yet.
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try:
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lo.set_status(offset=offset,
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sizelimit=sizelimit,
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autoclear=True)
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except BlockingIOError:
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lo.clear_fd()
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lo.close()
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continue
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break
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return lo
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def open(self, devpath: str, tree: str, options: Dict):
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filename = options["filename"]
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self.sector_size = options.get("sector-size", 512)
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start = options.get("start", 0) * self.sector_size
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size = options.get("size")
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path = os.path.join(tree, filename.lstrip("/"))
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self.fd = os.open(path, os.O_RDWR | os.O_CLOEXEC)
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print(f"file '{filename}'' opened as {self.fd}")
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try:
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self.lo = self.make_loop(self.fd, start, size)
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except Exception as error: # pylint: disable: broad-except
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self.close()
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raise error from None
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dir_fd = -1
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try:
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dir_fd = os.open(devpath, os.O_CLOEXEC | os.O_PATH)
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self.lo.mknod(dir_fd)
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finally:
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if dir_fd > -1:
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os.close(dir_fd)
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res = {
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"path": self.lo.devname,
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"node": {
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"major": self.lo.LOOP_MAJOR,
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"minor": self.lo.minor,
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}
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}
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return res
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def close(self):
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if self.lo:
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self.lo.close()
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self.lo = None
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if self.fd is not None:
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fd = self.fd
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self.fd = None
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try:
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os.fsync(fd)
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finally:
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os.close(fd)
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def main():
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service = LoopbackService.from_args(sys.argv[1:])
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service.main()
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if __name__ == '__main__':
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main()
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