Messages in this thread | | | Subject | Re: [PATCH][next] sfc: Replace zero-length array with flexible-array member | From | "Gustavo A. R. Silva" <> | Date | Tue, 25 Feb 2020 08:01:56 -0600 |
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On 2/25/20 07:42, Martin Habets wrote: > 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. >
Correct.
>> 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> >
Thanks, Martin. -- Gustavo
>> --- >> 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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