| Message ID | 20200228134758.GA29760@embeddedor (mailing list archive) |
|---|---|
| State | Accepted |
| Delegated to: | Marcel Holtmann |
| Headers | show |
| Series | [next] 6lowpan: Replace zero-length array with flexible-array member | 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] > > 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 <gustavo@embeddedor.com> > --- > include/net/6lowpan.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) patch has been applied to bluetooth-next tree. Regards Marcel
On 2/28/20 07:52, Marcel Holtmann wrote: >> >> [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 <gustavo@embeddedor.com> >> --- >> include/net/6lowpan.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) > > patch has been applied to bluetooth-next tree. > Thanks, Marcel. -- Gustavo
diff --git a/include/net/6lowpan.h b/include/net/6lowpan.h index a71378007e61..c80539be1542 100644 --- a/include/net/6lowpan.h +++ b/include/net/6lowpan.h @@ -138,7 +138,7 @@ struct lowpan_dev { struct lowpan_iphc_ctx_table ctx; /* must be last */ - u8 priv[0] __aligned(sizeof(void *)); + u8 priv[] __aligned(sizeof(void *)); }; struct lowpan_802154_neigh {
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] 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 <gustavo@embeddedor.com> --- include/net/6lowpan.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)