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Subject[PATCH AUTOSEL 4.14 156/186] char: hpet: Use flexible-array member
Date
From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>

[ Upstream commit 987f028b8637cfa7658aa456ae73f8f21a7a7f6f ]

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
int length;
u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
int stuff;
u8 data[];
};

Also, 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
unadvertenly introduced[3] to the codebase from now on.

[1] https://github.com/KSPP/linux/issues/21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200120235326.GA29231@embeddedor.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/char/hpet.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 05ca269ddd05b..40ba552551324 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -113,7 +113,7 @@ struct hpets {
unsigned long hp_delta;
unsigned int hp_ntimer;
unsigned int hp_which;
- struct hpet_dev hp_dev[1];
+ struct hpet_dev hp_dev[];
};

static struct hpets *hpets;
--
2.20.1
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