* [PATCH] Bluetooth: L2CAP: Replace zero-length array with flexible-array
@ 2020-05-13 17:15 Gustavo A. R. Silva
2020-05-18 7:58 ` Marcel Holtmann
0 siblings, 1 reply; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-13 17:15 UTC (permalink / raw
To: Marcel Holtmann, Johan Hedberg, David S. Miller, Jakub Kicinski
Cc: linux-bluetooth, netdev, linux-kernel, Gustavo A. R. Silva
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>
---
include/net/bluetooth/l2cap.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index dada14d0622c..8f1e6a7a2df8 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -499,7 +499,7 @@ struct l2cap_ecred_conn_req {
__le16 mtu;
__le16 mps;
__le16 credits;
- __le16 scid[0];
+ __le16 scid[];
} __packed;
struct l2cap_ecred_conn_rsp {
@@ -507,13 +507,13 @@ struct l2cap_ecred_conn_rsp {
__le16 mps;
__le16 credits;
__le16 result;
- __le16 dcid[0];
+ __le16 dcid[];
};
struct l2cap_ecred_reconf_req {
__le16 mtu;
__le16 mps;
- __le16 scid[0];
+ __le16 scid[];
} __packed;
#define L2CAP_RECONF_SUCCESS 0x0000
--
2.26.2
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] Bluetooth: L2CAP: Replace zero-length array with flexible-array
2020-05-13 17:15 [PATCH] Bluetooth: L2CAP: Replace zero-length array with flexible-array Gustavo A. R. Silva
@ 2020-05-18 7:58 ` Marcel Holtmann
2020-05-18 16:20 ` Gustavo A. R. Silva
0 siblings, 1 reply; 3+ messages in thread
From: Marcel Holtmann @ 2020-05-18 7:58 UTC (permalink / raw
To: Gustavo A. R. Silva
Cc: Johan Hedberg, David S. Miller, Jakub Kicinski, BlueZ, netdev,
linux-kernel, Gustavo A. R. Silva
Hi Gustavo,
> 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>
> ---
> include/net/bluetooth/l2cap.h | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
patch has been applied to bluetooth-next tree.
Regards
Marcel
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] Bluetooth: L2CAP: Replace zero-length array with flexible-array
2020-05-18 7:58 ` Marcel Holtmann
@ 2020-05-18 16:20 ` Gustavo A. R. Silva
0 siblings, 0 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-18 16:20 UTC (permalink / raw
To: Marcel Holtmann
Cc: Johan Hedberg, David S. Miller, Jakub Kicinski, BlueZ, netdev,
linux-kernel, Gustavo A. R. Silva
On Mon, May 18, 2020 at 09:58:22AM +0200, Marcel Holtmann wrote:
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > ---
> > include/net/bluetooth/l2cap.h | 6 +++---
> > 1 file changed, 3 insertions(+), 3 deletions(-)
>
> patch has been applied to bluetooth-next tree.
>
Thanks, Marcel.
--
Gustavo
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-05-13 17:15 [PATCH] Bluetooth: L2CAP: Replace zero-length array with flexible-array Gustavo A. R. Silva
2020-05-18 7:58 ` Marcel Holtmann
2020-05-18 16:20 ` Gustavo A. R. Silva
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