3085114ed7
Device service that provides support for bind files within the tree to loopback devices. Valid parameters are the `filename`, `offset` and `size`. This controls what part of the file to bind to the loop device. The unit for `size` and `offset` is sectors and the sector size can be configured via the `sector-size` parameter. The reason behind the sector unit is so that numbers can easily be compared with those specified in the partition table.
133 lines
3.3 KiB
Python
Executable file
133 lines
3.3 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.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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lo = loop.Loop(self.ctl.get_unbound())
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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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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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with open(path, "r+b") as fd:
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self.lo = self.make_loop(fd.fileno(), start, size)
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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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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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