diff mbox series

fpga: dfl: Replace zero-length array with flexible-array

Message ID 20200507185256.GA14313@embeddedor (mailing list archive)
State Mainlined, archived
Headers show
Series fpga: dfl: Replace zero-length array with flexible-array | expand

Commit Message

Gustavo A. R. Silva May 7, 2020, 6:52 p.m. UTC
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>
---
 drivers/fpga/dfl.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

matthew.gerlach@linux.intel.com May 8, 2020, 6:20 p.m. UTC | #1
This looks like a a good change to me.

Tested-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>

On Thu, 7 May 2020, 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]
>
> 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>
> ---
> drivers/fpga/dfl.h |    2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/fpga/dfl.h b/drivers/fpga/dfl.h
> index 9f0e656de720..1cd51ea52ce1 100644
> --- a/drivers/fpga/dfl.h
> +++ b/drivers/fpga/dfl.h
> @@ -235,7 +235,7 @@ struct dfl_feature_platform_data {
> 	unsigned long dev_status;
> 	void *private;
> 	int num;
> -	struct dfl_feature features[0];
> +	struct dfl_feature features[];
> };
>
> static inline
>
>
Wu, Hao May 9, 2020, 3:57 a.m. UTC | #2
I think Moritz had already applied it to his tree per last submission.
https://lkml.org/lkml/2020/3/21/373

Thanks
Hao

> -----Original Message-----
> From: matthew.gerlach@linux.intel.com <matthew.gerlach@linux.intel.com>
> Sent: Saturday, May 9, 2020 2:21 AM
> To: Gustavo A. R. Silva <gustavoars@kernel.org>
> Cc: Wu, Hao <hao.wu@intel.com>; linux-fpga@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH] fpga: dfl: Replace zero-length array with flexible-array
> 
> 
> 
> This looks like a a good change to me.
> 
> Tested-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
> 
> On Thu, 7 May 2020, 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]
> >
> > 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>
> > ---
> > drivers/fpga/dfl.h |    2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/fpga/dfl.h b/drivers/fpga/dfl.h
> > index 9f0e656de720..1cd51ea52ce1 100644
> > --- a/drivers/fpga/dfl.h
> > +++ b/drivers/fpga/dfl.h
> > @@ -235,7 +235,7 @@ struct dfl_feature_platform_data {
> > 	unsigned long dev_status;
> > 	void *private;
> > 	int num;
> > -	struct dfl_feature features[0];
> > +	struct dfl_feature features[];
> > };
> >
> > static inline
> >
> >
diff mbox series

Patch

diff --git a/drivers/fpga/dfl.h b/drivers/fpga/dfl.h
index 9f0e656de720..1cd51ea52ce1 100644
--- a/drivers/fpga/dfl.h
+++ b/drivers/fpga/dfl.h
@@ -235,7 +235,7 @@  struct dfl_feature_platform_data {
 	unsigned long dev_status;
 	void *private;
 	int num;
-	struct dfl_feature features[0];
+	struct dfl_feature features[];
 };
 
 static inline