On Fri, Feb 12, 2021 at 12:38 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > Why are people trying to use copy_file_range on simple /proc and /sys > files in the first place? They can not seek (well most can not), so > that feels like a "oh look, a new syscall, let's use it everywhere!" > problem that userspace should not do. This may have been covered elsewhere, but it's not that people are saying "let's use copy_file_range on files in /proc." It's that the Go language standard library provides an interface to operating system files. When Go code uses the standard library function io.Copy to copy the contents of one open file to another open file, then on Linux kernels 5.3 and greater the Go standard library will use the copy_file_range system call. That seems to be exactly what copy_file_range is intended for. Unfortunately it appears that when people writing Go code open a file in /proc and use io.Copy the contents to another open file, copy_file_range does nothing and reports success. There isn't anything on the copy_file_range man page explaining this limitation, and there isn't any documented way to know that the Go standard library should not use copy_file_range on certain files. So ideally the kernel will report EOPNOTSUPP or EINVAL when using copy_file_range on a file in /proc or some other file system that fails (and, minor side note, the copy_file_range man page should document that it can return EOPNOTSUPP or EINVAL in some cases, which does already happen on at least some kernel versions using at least some file systems). Or, less ideally, there will be some documented way that the Go standard library can determine that copy_file_range will fail before trying to use it. If neither of those can be done, then I think the only option is for the Go standard library to never use copy_file_range, as even though it will almost always work and work well, in some unpredictable number of cases it will fail silently. Thanks. Ian
