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In the quest to remove all stack VLA usage from the kernel[1], this allocates a fixed size stack array to cover the range needed for bch. This was done instead of a preallocation on the SLAB due to performance reasons, shown by Ivan Djelic: little-endian, type sizes: int=4 long=8 longlong=8 cpu: Intel(R) Core(TM) i5 CPU 650 @ 3.20GHz calibration: iter=4.9143µs niter=2034 nsamples=200 m=13 t=4 Buffer allocation | Encoding throughput (Mbit/s) --------------------------------------------------- on-stack, VLA | 3988 on-stack, fixed | 4494 kmalloc | 1967 So this change actually improves performance too, it seems. The resulting stack allocation can get rather large; without CONFIG_BCH_CONST_PARAMS, it will allocate 4096 bytes, which trips the stack size checking: lib/bch.c: In function ‘encode_bch’: lib/bch.c:261:1: warning: the frame size of 4432 bytes is larger than 2048 bytes [-Wframe-larger-than=] Even the default case for "allmodconfig" (with CONFIG_BCH_CONST_M=14 and CONFIG_BCH_CONST_T=4) would have started throwing a warning: lib/bch.c: In function ‘encode_bch’: lib/bch.c:261:1: warning: the frame size of 2288 bytes is larger than 2048 bytes [-Wframe-larger-than=] But this is how large it's always been; it was just hidden from the checker because it was a VLA. So the Makefile has been adjusted to silence this warning for anything smaller than 4500 bytes, which should provide room for normal cases, but still low enough to catch any future pathological situations. [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com Signed-off-by: Kees Cook <keescook@chromium.org> Reviewed-by: Ivan Djelic <ivan.djelic@parrot.com> Tested-by: Ivan Djelic <ivan.djelic@parrot.com> Acked-by: Boris Brezillon <boris.brezillon@bootlin.com> Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.