copy_file_range(2): Fix for small values of input file offset and len

[rmacklem]

Summary

copy_file_range(2): Fix for small values of input file offset and len

r366302 broke copy_file_range(2) for small values of 
input file offset and len.

It was possible for rem to be greater than len and then
"len - rem" was a large value, since both variables are
unsigned.

Reported by: koobs, Pablo <pablogsal gmail com> (Python)

Test Plan

Tested via a small C program that did the same test
as the python test that had failed.

[asomers]

What sort of function arguments would lead to this problem? It would be good to turn this into an automated test.

[rmacklem]

It was found with an automated test. I just don't know
the details, except it is python code. I'll forward the
email and koobs@ can fill you in.

The test did the call with *inoffp = 3 and len = 6.

[asomers]

In D28981#648853, @rmacklem wrote:

It was found with an automated test. I just don't know
the details, except it is python code. I'll forward the
email and koobs@ can fill you in.

The test did the call with *inoffp = 3 and len = 6.

I downloaded the Python script and reproduced the problem. But what should we do about it? The only thing in /usr/tests that exercises copy_file_range is the fusefs test suite. A regression test for this bug certainly doesn't belong there. There's pjdfstest, though I find its TAP-based design cumbersome. I suppose I could extract fusefs's copy_file_range tests into a standalone ATF test that exercises TMPDIR. What did you use for testing when you developed copy_file_range?

[rmacklem]

I have a few C programs, which could be cleaned up.
But I know nothing about automated testing nor
what FreeBSD currently does.

I recall someone did not like the fact it used
random number generators for arguments for
part of the tests. The rest were "corner cases"
although I don't recall if I had a "small offset+small len"
case.

I probably did not do a full test exercise for the
commit that caused this breakage.

[koobs]

Thanks @rmacklem :)

[asomers]

Let's commit this then, and not wait for a proper test suite. We'll keep discussing that in the mail thread I included you on.