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There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].
For this particular case, it is important to notice that the cachelines
change from 7 to 6 after the flexible-array conversion:
$ pahole -C 'fifo' drivers/misc/hpilo.o
struct fifo {
u64 nrents; /* 0 8 */
u64 imask; /* 8 8 */
u64 merge; /* 16 8 */
u64 reset; /* 24 8 */
u8 pad_0[96]; /* 32 96 */
/* --- cacheline 2 boundary (128 bytes) --- */
u64 head; /* 128 8 */
u8 pad_1[120]; /* 136 120 */
/* --- cacheline 4 boundary (256 bytes) --- */
u64 tail; /* 256 8 */
u8 pad_2[120]; /* 264 120 */
/* --- cacheline 6 boundary (384 bytes) --- */
u64 fifobar[1]; /* 384 8 */
/* size: 392, cachelines: 7, members: 10 */
/* last cacheline: 8 bytes */
};
$ pahole -C 'fifo' drivers/misc/hpilo.o
struct fifo {
u64 nrents; /* 0 8 */
u64 imask; /* 8 8 */
u64 merge; /* 16 8 */
u64 reset; /* 24 8 */
u8 pad_0[96]; /* 32 96 */
/* --- cacheline 2 boundary (128 bytes) --- */
u64 head; /* 128 8 */
u8 pad_1[120]; /* 136 120 */
/* --- cacheline 4 boundary (256 bytes) --- */
u64 tail; /* 256 8 */
u8 pad_2[120]; /* 264 120 */
/* --- cacheline 6 boundary (384 bytes) --- */
u64 fifobar[]; /* 384 0 */
/* size: 384, cachelines: 6, members: 10 */
};
Lastly, remove unnecessary parentheses in fifo_sz() and fix the following
checkpatch.pl warning for the whole fifo structure:
WARNING: please, no spaces at the start of a line
[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://github.com/KSPP/linux/issues/79
Tested-by: kernel test robot <lkp@intel.com>
Link: https://github.com/GustavoARSilva/linux-hardening/blob/master/cii/kernel-ci/hpilo-20200714.md
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200714154449.GA26153@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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 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.