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sched/fair: Replace zero-length array with flexible-array

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The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200507192141.GA16183@embeddedor
This commit is contained in:
Gustavo A. R. Silva 2020-05-07 14:21:41 -05:00 committed by Peter Zijlstra
parent 95d685935a
commit 04f5c362ec
2 changed files with 2 additions and 2 deletions
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2
kernel/sched/fair.c
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@ -1094,7 +1094,7 @@ struct numa_group {
* more by CPU use than by memory faults.
*/
unsigned long *faults_cpu;
unsigned long faults[0];
unsigned long faults[];
};
/*

2
kernel/sched/sched.h
View File

@ -1462,7 +1462,7 @@ struct sched_group {
* by attaching extra space to the end of the structure,
* depending on how many CPUs the kernel has booted up with)
*/
unsigned long cpumask[0];
unsigned long cpumask[];
};
static inline struct cpumask *sched_group_span(struct sched_group *sg)