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Subject: Re: [PATCH] dmaengine: qcom: bam_dma: Replace zero-length array with
 flexible-array
To:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Andy Gross <agross@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200507185016.GA13883@embeddedor>
From:   Jeffrey Hugo <jhugo@codeaurora.org>
Message-ID: <1b3cda25-5f3a-5359-4bf7-d16a8364f545@codeaurora.org>
Date:   Thu, 7 May 2020 13:24:57 -0600
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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.