fix copy_file_range(2) so that it handles the case where input offset + len would wrap around

[rmacklem]

Summary

Without this patch, if a call to copy_file_range(2) specifies an input file
outset + len that would wrap around, EINVAL is returned.
I thought that was the Linux behaviour, but recent testing showed that
Linux accepts this case and does the copy_file_range() to EOF.

This patch changes the FreeBSD code to exhibit the same behaviour as
Linux for this case.

Test Plan

Tested with a little file copy program and a patched variant
of "cp", where the len argument for copy_file_range() is
SSIZE_MAX.

[asomers]

sys/kern/vfs_vnops.c

2808

Why do you still return EINVAL if the output offset + len would wrap around? I would think that the common case would be copy_file_range(infd, some_potentially_nonzero_offset, outfd, some_potentially_nonzero_offset, SIZE_MAX, 0) to copy to the EOF of the input file. In that case, you should allow outoffp to wrap, too.

[rmacklem]

sys/kern/vfs_vnops.c

2808

If outoffp wraps around it goes negative.
I can't see VOP_WRITE() with a negative
file offset being a good idea.
I agree that, usually, the copy will end before
the wrap around happens (is there a real file
system that can handle a sparse file of size
INT64_MAX?), but I think a wraparound is
conceivable?

[rmacklem]

Change handling of a wrap around of *outoffp + len
to clip "len" instead of return EINVAL.

[rmacklem]

sys/kern/vfs_vnops.c

2818

Although allowing it to wrap around doesn't
seem like a good idea, doing a further clip
of "len" seems ok.
This will allow SSIZE_MAX to be used for
"copy to EOF" when the output offset > input offset.

[rmacklem]

Add a check for len being clipped down to 0.
No sense in doing the VOP call for this case.

[kib]

sys/kern/vfs_vnops.c

2816

What if uval + len overflow so that the sum is less then either of addends ?

[rmacklem]

sys/kern/vfs_vnops.c

2816

I don't think this can happen.
*inoffp is off_t (signed 64bit) and has been already checked
for non-negative, so it must be <= INT64_MAX.
len has been clipped to SSIZE_MAX, so it can't be
greater than INT64_MAX.
--> The sum of these can't exceed UINT64_MAX.
Sound right?

[asomers]

Treating overflow of inoffp and outoffp identically makes sense to me.