[PATCH][next] acpi: nfit.h: Replace zero-length array with flexible-array member

[PATCH][next] acpi: nfit.h: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

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>
---
 drivers/acpi/nfit/nfit.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/acpi/nfit/nfit.h b/drivers/acpi/nfit/nfit.h
index 24241941181c..af09143ce403 100644
--- a/drivers/acpi/nfit/nfit.h
+++ b/drivers/acpi/nfit/nfit.h
@@ -144,32 +144,32 @@ struct nfit_spa {
 	unsigned long ars_state;
 	u32 clear_err_unit;
 	u32 max_ars;
-	struct acpi_nfit_system_address spa[0];
+	struct acpi_nfit_system_address spa[];
 };
 
 struct nfit_dcr {
 	struct list_head list;
-	struct acpi_nfit_control_region dcr[0];
+	struct acpi_nfit_control_region dcr[];
 };
 
 struct nfit_bdw {
 	struct list_head list;
-	struct acpi_nfit_data_region bdw[0];
+	struct acpi_nfit_data_region bdw[];
 };
 
 struct nfit_idt {
 	struct list_head list;
-	struct acpi_nfit_interleave idt[0];
+	struct acpi_nfit_interleave idt[];
 };
 
 struct nfit_flush {
 	struct list_head list;
-	struct acpi_nfit_flush_address flush[0];
+	struct acpi_nfit_flush_address flush[];
 };
 
 struct nfit_memdev {
 	struct list_head list;
-	struct acpi_nfit_memory_map memdev[0];
+	struct acpi_nfit_memory_map memdev[];
 };
 
 enum nfit_mem_flags {
-- 
2.23.0

Re: [PATCH][next] acpi: nfit.h: Replace zero-length array with flexible-array member

[Rafael J. Wysocki]

On Thu, Mar 19, 2020 at 9:15 PM Gustavo A. R. Silva
<gustavo@embeddedor.com> 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]
>
> 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>

Dan,

I'm assuming that you will take care of this one or please let me know
otherwise.

Cheers!

> ---
>  drivers/acpi/nfit/nfit.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/acpi/nfit/nfit.h b/drivers/acpi/nfit/nfit.h
> index 24241941181c..af09143ce403 100644
> --- a/drivers/acpi/nfit/nfit.h
> +++ b/drivers/acpi/nfit/nfit.h
> @@ -144,32 +144,32 @@ struct nfit_spa {
>         unsigned long ars_state;
>         u32 clear_err_unit;
>         u32 max_ars;
> -       struct acpi_nfit_system_address spa[0];
> +       struct acpi_nfit_system_address spa[];
>  };
>
>  struct nfit_dcr {
>         struct list_head list;
> -       struct acpi_nfit_control_region dcr[0];
> +       struct acpi_nfit_control_region dcr[];
>  };
>
>  struct nfit_bdw {
>         struct list_head list;
> -       struct acpi_nfit_data_region bdw[0];
> +       struct acpi_nfit_data_region bdw[];
>  };
>
>  struct nfit_idt {
>         struct list_head list;
> -       struct acpi_nfit_interleave idt[0];
> +       struct acpi_nfit_interleave idt[];
>  };
>
>  struct nfit_flush {
>         struct list_head list;
> -       struct acpi_nfit_flush_address flush[0];
> +       struct acpi_nfit_flush_address flush[];
>  };
>
>  struct nfit_memdev {
>         struct list_head list;
> -       struct acpi_nfit_memory_map memdev[0];
> +       struct acpi_nfit_memory_map memdev[];
>  };
>
>  enum nfit_mem_flags {
> --
> 2.23.0
>

Re: [PATCH][next] acpi: nfit.h: Replace zero-length array with flexible-array member

[Dan Williams]

On Wed, Mar 25, 2020 at 3:06 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Thu, Mar 19, 2020 at 9:15 PM Gustavo A. R. Silva
> <gustavo@embeddedor.com> 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]
> >
> > 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>
>
> Dan,
>
> I'm assuming that you will take care of this one or please let me know
> otherwise.

Yes, this one and the other 2 related libnvdimm fixups are in my queue.