e9a688bcb1
Until the very recent commits, many bounded random integers were calculated using `get_random_u32() % max_plus_one`, which not only incurs the price of a division -- indicating performance mostly was not a real issue -- but also does not result in a uniformly distributed output if max_plus_one is not a power of two. Recent commits moved to using `prandom_u32_max(max_plus_one)`, which replaces the division with a faster multiplication, but still does not solve the issue with non-uniform output. For some users, maybe this isn't a problem, and for others, maybe it is, but for the majority of users, probably the question has never been posed and analyzed, and nobody thought much about it, probably assuming random is random is random. In other words, the unthinking expectation of most users is likely that the resultant numbers are uniform. So we implement here an efficient way of generating uniform bounded random integers. Through use of compile-time evaluation, and avoiding divisions as much as possible, this commit introduces no measurable overhead. At least for hot-path uses tested, any potential difference was lost in the noise. On both clang and gcc, code generation is pretty small. The new function, get_random_u32_below(), lives in random.h, rather than prandom.h, and has a "get_random_xxx" function name, because it is suitable for all uses, including cryptography. In order to be efficient, we implement a kernel-specific variant of Daniel Lemire's algorithm from "Fast Random Integer Generation in an Interval", linked below. The kernel's variant takes advantage of constant folding to avoid divisions entirely in the vast majority of cases, works on both 32-bit and 64-bit architectures, and requests a minimal amount of bytes from the RNG. Link: https://arxiv.org/pdf/1805.10941.pdf Cc: stable@vger.kernel.org # to ease future backports that use this api Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> |
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| Documentation | ||
| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
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.