e1f2750edc
Every caller passes in zero, meaning they don't want any partial copy to zero the remainder of the destination buffer. Which is just as well, because the implementation of that function didn't actually even look at that argument, and wasn't even aware it existed, although some misleading comments did mention it still. The 'zerorest' thing is a historical artifact of how "copy_from_user()" worked, in that it would zero the rest of the kernel buffer that it copied into. That zeroing still exists, but it's long since been moved to generic code, and the raw architecture-specific code doesn't do it. See _copy_from_user() in lib/usercopy.c for this all. However, while __copy_user_nocache() shares some history and superficial other similarities with copy_from_user(), it is in many ways also very different. In particular, while the code makes it *look* similar to the generic user copy functions that can copy both to and from user space, and take faults on both reads and writes as a result, __copy_user_nocache() does no such thing at all. __copy_user_nocache() always copies to kernel space, and will never take a page fault on the destination. What *can* happen, though, is that the non-temporal stores take a machine check because one of the use cases is for writing to stable memory, and any memory errors would then take synchronous faults. So __copy_user_nocache() does look a lot like copy_from_user(), but has faulting behavior that is more akin to our old copy_in_user() (which no longer exists, but copied from user space to user space and could fault on both source and destination). And it very much does not have the "zero the end of the destination buffer", since a problem with the destination buffer is very possibly the very source of the partial copy. So this whole thing was just a confusing historical artifact from having shared some code with a completely different function with completely different use cases. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.