[PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[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]

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/dma/qcom/bam_dma.c         |    2 +-
 drivers/firmware/qcom_scm-legacy.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
index ef73f65224b1..5a08dd0d3388 100644
--- a/drivers/dma/qcom/bam_dma.c
+++ b/drivers/dma/qcom/bam_dma.c
@@ -74,7 +74,7 @@ struct bam_async_desc {
 	struct list_head desc_node;
 	enum dma_transfer_direction dir;
 	size_t length;
-	struct bam_desc_hw desc[0];
+	struct bam_desc_hw desc[];
 };
 
 enum bam_reg {
diff --git a/drivers/firmware/qcom_scm-legacy.c b/drivers/firmware/qcom_scm-legacy.c
index 8532e7c78ef7..eba6b60bfb61 100644
--- a/drivers/firmware/qcom_scm-legacy.c
+++ b/drivers/firmware/qcom_scm-legacy.c
@@ -56,7 +56,7 @@ struct scm_legacy_command {
 	__le32 buf_offset;
 	__le32 resp_hdr_offset;
 	__le32 id;
-	__le32 buf[0];
+	__le32 buf[];
 };
 
 /**

Re: [PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[Jeffrey Hugo]

On 5/7/2020 12:50 PM, 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/dma/qcom/bam_dma.c         |    2 +-
>   drivers/firmware/qcom_scm-legacy.c |    2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 

Shouldn't these two files be two different patches?

-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

Re: [PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

On Thu, May 07, 2020 at 01:24:57PM -0600, Jeffrey Hugo wrote:
> >   drivers/dma/qcom/bam_dma.c         |    2 +-
> >   drivers/firmware/qcom_scm-legacy.c |    2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> 
> Shouldn't these two files be two different patches?
> 

I believe so... I'll split this patch up into two patches.

Thanks
--
Gustavo

Re: [PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[Jeffrey Hugo]

On 5/8/2020 11:02 AM, Gustavo A. R. Silva wrote:
> On Thu, May 07, 2020 at 01:24:57PM -0600, Jeffrey Hugo wrote:
>>>    drivers/dma/qcom/bam_dma.c         |    2 +-
>>>    drivers/firmware/qcom_scm-legacy.c |    2 +-
>>>    2 files changed, 2 insertions(+), 2 deletions(-)
>>>
>>
>> Shouldn't these two files be two different patches?
>>
> 
> I believe so... I'll split this patch up into two patches.
> 
> Thanks
> --
> Gustavo
> 

Sounds good to me.  When you do, you can add the following if you like

Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>

-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

Re: [PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

On Fri, May 08, 2020 at 11:20:16AM -0600, Jeffrey Hugo wrote:
> On 5/8/2020 11:02 AM, Gustavo A. R. Silva wrote:
> > On Thu, May 07, 2020 at 01:24:57PM -0600, Jeffrey Hugo wrote:
> > > >    drivers/dma/qcom/bam_dma.c         |    2 +-
> > > >    drivers/firmware/qcom_scm-legacy.c |    2 +-
> > > >    2 files changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > 
> > > Shouldn't these two files be two different patches?
> > > 
> > 
> > I believe so... I'll split this patch up into two patches.
> > 
> > Thanks
> > --
> > Gustavo
> > 
> 
> Sounds good to me.  When you do, you can add the following if you like
> 
> Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
> 

Awesome. :)

Thanks
--
Gustavo

[PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[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]

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")

Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 drivers/dma/qcom/bam_dma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
index ef73f65224b1..5a08dd0d3388 100644
--- a/drivers/dma/qcom/bam_dma.c
+++ b/drivers/dma/qcom/bam_dma.c
@@ -74,7 +74,7 @@ struct bam_async_desc {
 	struct list_head desc_node;
 	enum dma_transfer_direction dir;
 	size_t length;
-	struct bam_desc_hw desc[0];
+	struct bam_desc_hw desc[];
 };
 
 enum bam_reg {
-- 
2.26.2

Re: [PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with flexible-array

[Vinod Koul]

On 08-05-20, 16:07, 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")

Applied, thanks

-- 
~Vinod