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05f4216272
In any normal situation this really shouldn't matter, but in case the address passed in to masked_user_access_begin() were to be some complex expression, we should evaluate it fully before doing the 'stac' instruction. And even without that issue (which objdump would pick up on for any really bad case), just in general we should strive to minimize the amount of code we run with user accesses enabled. For example, even for the trivial pselect6() case, the code generation (obviously with a non-debug build) just diff with this ends up being - stac mov %rax,%rcx sar $0x3f,%rcx or %rax,%rcx + stac mov (%rcx),%r13 mov 0x8(%rcx),%r14 clac so the area delimeted by the 'stac / clac' pair is now literally just the two user access instructions, and the address generation has been moved out to before that code. This will be much more noticeable if we end up deciding that we can go back to just inlining "get_user()" using the new masked user access model. The get_user() pointers can often be more complex expressions involving kernel memory accesses or even function calls. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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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 reStructuredText 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.