Re: [PATCH 12/11] man-pages: copy_file_range updates

Amir Goldstein
2019-05-21T05:53:04Z

On Mon, Dec 3, 2018 at 10:40 AM Dave Chinner <david@fromorbit.com> wrote:
>
> From: Dave Chinner <dchinner@redhat.com>
>
> Update with all the missing errors the syscall can return, the
> behaviour the syscall should have w.r.t. to copies within single
> files, etc.

Below are the changes I have made to V2 of this man-page update in accordance to
agreed change of behavior (i.e. short copy up to EOF).

This is a heads up before posting to verify my interpretation is correct.
I still have more testing to do before posting.

The main thing is adding:
.BR copy_file_range ()
will return the number of bytes copied between files.
This could be less than the length originally requested.
+If the file offset of
+.I fd_in
+is at or past the end of file, no bytes are copied, and
+.BR copy_file_range ()
+returns zero.

But see also other changes below...

>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> man2/copy_file_range.2 | 94 +++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 77 insertions(+), 17 deletions(-)
>
> diff --git a/man2/copy_file_range.2 b/man2/copy_file_range.2
> index 20374abb21f0..23b00c2f3fea 100644
> --- a/man2/copy_file_range.2
> +++ b/man2/copy_file_range.2
> @@ -42,9 +42,9 @@ without the additional cost of transferring data from the kernel to user space
> and then back into the kernel.
> It copies up to
> .I len
> -bytes of data from file descriptor
> +bytes of data from the source file descriptor
> .I fd_in
> -to file descriptor
> +to target file descriptor
> .IR fd_out ,
> overwriting any data that exists within the requested range of the target file.
> .PP
> @@ -74,6 +74,11 @@ is not changed, but
> .I off_in
> is adjusted appropriately.
> .PP
> +.I fd_in
> +and
> +.I fd_out
> +can refer to the same file. If they refer to the same file, then the source and
> +target ranges are not allowed to overlap.
> .PP
> The
> .I flags
> @@ -93,34 +98,73 @@ is set to indicate the error.
> .SH ERRORS
> .TP
> .B EBADF
> -One or more file descriptors are not valid; or
> +One or more file descriptors are not valid.
> +.TP
> +.B EBADF
> .I fd_in
> is not open for reading; or
> .I fd_out
> -is not open for writing; or
> -the
> +is not open for writing.
> +.TP
> +.B EBADF
> +The
> .B O_APPEND
> flag is set for the open file description referred to by
> .IR fd_out .
> .TP
> .B EFBIG
> -An attempt was made to write a file that exceeds the implementation-defined
> -maximum file size or the process's file size limit,
> -or to write at a position past the maximum allowed offset.
> +An attempt was made to write at a position past the maximum file offset the
> +kernel supports.

Updated to "...attempt made to read or write..."

> +.TP
> +.B EFBIG
> +An attempt was made to write a range that exceeds the allowed maximum file size.
> +The maximum file size differs between filesystem implemenations and can be
> +different to the maximum allowed file offset.
> +.TP
> +.B EFBIG
> +An attempt was made to write beyond the process's file size resource
> +limit. This may also result in the process receiving a
> +.I SIGXFSZ
> +signal.
> .TP
> .B EINVAL
> -Requested range extends beyond the end of the source file; or the

Removed this.

> -.I flags
> -argument is not 0.
> +.I (off_in + len)
> +spans the end of the source file.
> .TP
> -.B EIO
> -A low-level I/O error occurred while copying.
> +.B EINVAL
> +.I fd_in
> +and
> +.I fd_out
> +refer to the same file and the source and target ranges overlap.
> +.TP
> +.B EINVAL
> +.I fd_in
> +or
> +.I fd_out
> +is not a regular file.
> .TP
> .B EISDIR
> .I fd_in
> or
> .I fd_out
> refers to a directory.
> +.B EINVAL
> +The
> +.I flags
> +argument is not 0.
> +.TP
> +.B EINVAL
> +.I off_in
> +or
> +.I (off_in + len)
> +is beyond the maximum valid file offset.

Removed this. Updated entry for EFBIG with in offset.

> +.TP
> +.B EOVERFLOW
> +The requested source or destination range is too large to represent in the
> +specified data types.
> +.TP
> +.B EIO
> +A low-level I/O error occurred while copying.
> .TP
> .B ENOMEM
> Out of memory.
> @@ -128,16 +172,32 @@ Out of memory.
> .B ENOSPC
> There is not enough space on the target filesystem to complete the copy.
> .TP
> -.B EXDEV
> -The files referred to by
> -.IR file_in " and " file_out

Kept this one with added "(pre Linux 5.3)"

> -are not on the same mounted filesystem.
> +.B TXTBSY
> +.I fd_in
> +or
> +.I fd_out
> +refers to an active swap file.
> +.TP
> +.B EPERM
> +.I fd_out
> +refers to an immutable file.
> +.TP
> +.B EACCES
> +The user does not have write permissions for the destination file.
> .SH VERSIONS
> The
> .BR copy_file_range ()
> system call first appeared in Linux 4.5, but glibc 2.27 provides a user-space
> emulation when it is not available.
> .\" https://sourceware.org/git/?p=glibc.git;a=commit;f=posix/unistd.h;h=bad7a0c81f501fbbcc79af9eaa4b8254441c4a1f
> +.PP
> +A major rework of the kernel implementation occurred in 4.21. Areas of the API
> +that weren't clearly defined were clarified and the API bounds are much more
> +strictly checked than on earlier kernels. Applications should target the
> +behaviour and requirements of 4.21 kernels.
> +.PP
> +First support for cross-filesystem copies was introduced in Linux 4.21. Older
> +kernels will return -EXDEV when cross-filesystem copies are attempted.
> .SH CONFORMING TO
> The
> .BR copy_file_range ()

Updates example loop termination condition to:
len \-= ret;
- } while (len > 0);
+ } while (len > 0 && ret > 0);

WIP is available here:
https://github.com/amir73il/man-pages/commits/copy_file_range

Thanks,
Amir.