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Dave Hansen found the "(long) addr >= 0" code in the x86-64 access_ok checks somewhat confusing, and suggested using a helper to clarify what the code is doing. So this does exactly that: clarifying what the sign bit check is all about, by adding a helper macro that makes it clear what it is testing. This also adds some explicit comments talking about how even with LAM enabled, any addresses with the sign bit will still GP-fault in the non-canonical region just above the sign bit. This is all what allows us to do the user address checks with just the sign bit, and furthermore be a bit cavalier about accesses that might be done with an additional offset even past that point. (And yes, this talks about 'positive' even though zero is also a valid user address and so technically we should call them 'non-negative'. But I don't think using 'non-negative' ends up being more understandable). Suggested-by: Dave Hansen <dave.hansen@intel.com> 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.