The thing is that *ppos is loff_t, i.e. long long, whereas pos is long.
In turn, the assignment might lead to truncation. This means that the
user attempted to access a file beyond 2GB. Without the test, the file
system might access the wrong place of the file unnoticed.
This relates to the proposals for the 'large file system' spec. As
far as I can tell, the only thing that is wrong with that code is
the error code: it should not be EINVAL.
In turn, any other file system that does not support large files
should have similar code.
Regards,
Martin