2020-02-25 00:04:21

by Gustavo A. R. Silva

Subject: [PATCH][next] sfc: Replace zero-length array with flexible-array member

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 <[email protected]>
---
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;
};

/**
--
2.25.0

2020-02-25 13:53:53

by Martin Habets

Subject: Re: [PATCH][next] sfc: Replace zero-length array with flexible-array member

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 <[email protected]>

Acked-by: Martin Habets <[email protected]>

> ---
> 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;
> };
>
> /**
>

2020-02-25 14:02:23

by Gustavo A. R. Silva

Subject: Re: [PATCH][next] sfc: Replace zero-length array with flexible-array member

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 <[email protected]>
>
> Acked-by: Martin Habets <[email protected]>
>

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;
>> };
>>
>> /**
>>

2020-02-27 00:46:51

by David Miller

Subject: Re: [PATCH][next] sfc: Replace zero-length array with flexible-array member

From: "Gustavo A. R. Silva" <[email protected]>
Date: Mon, 24 Feb 2020 18:06:47 -0600

> 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 <[email protected]>

Applied.