From: "Gustavo A. R. Silva" <gustavoars@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org
Subject: [PATCH] locking/lockdep: Replace zero-length array with flexible-array
Date: Thu, 7 May 2020 13:58:04 -0500 [thread overview]
Message-ID: <20200507185804.GA15036@embeddedor> (raw)
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 <gustavoars@kernel.org>
---
kernel/locking/lockdep.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ac10db66cc63..cfdff122905b 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -489,7 +489,7 @@ struct lock_trace {
struct hlist_node hash_entry;
u32 hash;
u32 nr_entries;
- unsigned long entries[0] __aligned(sizeof(unsigned long));
+ unsigned long entries[] __aligned(sizeof(unsigned long));
};
#define LOCK_TRACE_SIZE_IN_LONGS \
(sizeof(struct lock_trace) / sizeof(unsigned long))
next reply other threads:[~2020-05-07 18:53 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-07 18:58 Gustavo A. R. Silva [this message]
2020-05-19 18:44 ` [tip: locking/core] locking/lockdep: Replace zero-length array with flexible-array tip-bot2 for Gustavo A. R. Silva
2020-05-19 19:07 ` Gustavo A. R. Silva
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