How useful should copy_file_range() be?

Posted Feb 19, 2021 20:29 UTC (Fri) by the8472 (guest, #144969)
In reply to: How useful should copy_file_range() be? by Deewiant
Parent article: How useful should copy_file_range() be?

You're looking at the linux implementation for io::copy which can copy arbitrary readers to writers but specializes to various syscalls when types wrapping file descriptors of unknown type are passed, hence the complexity.

There also is fs::copy which shares some code but has fewer cases to cover and does fewer checks before invoking copy_file_range since it's only meant to copy entire regular files.

The relevant parts are here and here

. Note that it doesn't use stat information to decide what to do in the fs::copy case, it just tries copy_file_range and then falls back in various cases, including 0 byte reads.

How useful should copy_file_range() be?

Posted Jul 10, 2021 12:57 UTC (Sat) by jaykrell (guest, #153190) [Link] (3 responses)

The bug is that /proc exists at all.
This data should be retrieved through strongly typed special purpose function calls.
Or at least, perhaps, allow open, for a hierarchical namespace, but not read, and reveal all information via ioctl.

Not everything is a file!

In fact, most things are not a file.
Is the screen a file?
Is the keyboard a file?
Are sockets files? They have read/write, but how about seek and mmap?

How useful should copy_file_range() be?

Posted Jul 10, 2021 13:54 UTC (Sat) by mpr22 (subscriber, #60784) [Link]

Those things are not files, but allowing userspace to treat them as file-like for the simplest common use case (sequential I/O) is one of the things that contributed to Unix eating most of the rest of the server operating system industry's breakfast, lunch, dinner, and face.

How useful should copy_file_range() be?

Posted Jul 10, 2021 21:48 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link]

> Or at least, perhaps, allow open, for a hierarchical namespace, but not read, and reveal all information via ioctl.
Windows NT tried that. It failed miserably.

How useful should copy_file_range() be?

Posted Jul 10, 2021 23:16 UTC (Sat) by flussence (guest, #85566) [Link]

>Is the screen a file?
>Is the keyboard a file?

You're being facetious, but it's occasionally very useful to be able to do things like check which port a monitor is plugged in on or framedump the console on a server using only ssh.