| Message ID | 20200513171556.GA21969@embeddedor (mailing list archive) |
|---|---|
| State | Accepted |
| Delegated to: | Marcel Holtmann |
| Headers | show |
| Series | Bluetooth: L2CAP: Replace zero-length array with flexible-array | expand |
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
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
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
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(-)