From 04f5c362ec6d3ff0e14f1c05230b550da7f528a4 Mon Sep 17 00:00:00 2001 From: "Gustavo A. R. Silva" Date: Thu, 7 May 2020 14:21:41 -0500 Subject: [PATCH] sched/fair: Replace zero-length array with flexible-array 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour") Signed-off-by: Gustavo A. R. Silva Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20200507192141.GA16183@embeddedor --- kernel/sched/fair.c | 2 +- kernel/sched/sched.h | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 44b0c8e..01f94cf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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[]; }; /* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 21416b3..2bd2a22 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -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) -- 2.7.4