Messages in this thread | | | Subject | Re: [PATCH][next] sfc: Replace zero-length array with flexible-array member | From | Martin Habets <> | Date | Tue, 25 Feb 2020 13:42:34 +0000 |
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On 25/02/2020 00:06, Gustavo A. R. Silva wrote: > 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]
These padding fields are only used to enforce alignment of the struct. But the patch is still ok.
> 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>
Acked-by: Martin Habets <mhabets@solarflare.com>
> --- > drivers/net/ethernet/sfc/falcon/net_driver.h | 2 +- > drivers/net/ethernet/sfc/net_driver.h | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/drivers/net/ethernet/sfc/falcon/net_driver.h b/drivers/net/ethernet/sfc/falcon/net_driver.h > index a49ea2e719b6..a529ff395ead 100644 > --- a/drivers/net/ethernet/sfc/falcon/net_driver.h > +++ b/drivers/net/ethernet/sfc/falcon/net_driver.h > @@ -288,7 +288,7 @@ struct ef4_rx_buffer { > struct ef4_rx_page_state { > dma_addr_t dma_addr; > > - unsigned int __pad[0] ____cacheline_aligned; > + unsigned int __pad[] ____cacheline_aligned; > }; > > /** > diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h > index 9f9886f222c8..392bd5b7017e 100644 > --- a/drivers/net/ethernet/sfc/net_driver.h > +++ b/drivers/net/ethernet/sfc/net_driver.h > @@ -336,7 +336,7 @@ struct efx_rx_buffer { > struct efx_rx_page_state { > dma_addr_t dma_addr; > > - unsigned int __pad[0] ____cacheline_aligned; > + unsigned int __pad[] ____cacheline_aligned; > }; > > /** >
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