[PATCH] locking/lockdep: Replace zero-length array with flexible-array

* [PATCH] locking/lockdep: Replace zero-length array with flexible-array
@ 2020-05-07 18:58 Gustavo A. R. Silva
  2020-05-19 18:44 ` [tip: locking/core] " tip-bot2 for Gustavo A. R. Silva
  0 siblings, 1 reply; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-07 18:58 UTC (permalink / raw)
  To: Peter Zijlstra, Ingo Molnar, Will Deacon; +Cc: linux-kernel

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))


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