linux-kernel - Re: [PATCH] rsi: Properly initialize data in rsi_sdio_ta

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Date:   Thu, 2 May 2019 11:18:01 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Amitkumar Karwar <amitkarwar@...il.com>,
        Siva Rebbagondla <siva8118@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] rsi: Properly initialize data in rsi_sdio_ta_reset

On Thu, May 2, 2019 at 8:16 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> When building with -Wuninitialized, Clang warns:
>
> drivers/net/wireless/rsi/rsi_91x_sdio.c:940:43: warning: variable 'data'
> is uninitialized when used here [-Wuninitialized]
>         put_unaligned_le32(TA_HOLD_THREAD_VALUE, data);
>                                                  ^~~~
> drivers/net/wireless/rsi/rsi_91x_sdio.c:930:10: note: initialize the
> variable 'data' to silence this warning
>         u8 *data;
>                 ^
>                  = NULL
> 1 warning generated.
>
> Using Clang's suggestion of initializing data to NULL wouldn't work out
> because data will be dereferenced by put_unaligned_le32. Use kzalloc to
> properly initialize data, which matches a couple of other places in this
> driver.
>
> Fixes: e5a1ecc97e5f ("rsi: add firmware loading for 9116 device")
> Link: https://github.com/ClangBuiltLinux/linux/issues/464
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  drivers/net/wireless/rsi/rsi_91x_sdio.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/wireless/rsi/rsi_91x_sdio.c b/drivers/net/wireless/rsi/rsi_91x_sdio.c
> index f9c67ed473d1..b35728564c7b 100644
> --- a/drivers/net/wireless/rsi/rsi_91x_sdio.c
> +++ b/drivers/net/wireless/rsi/rsi_91x_sdio.c
> @@ -929,11 +929,15 @@ static int rsi_sdio_ta_reset(struct rsi_hw *adapter)
>         u32 addr;
>         u8 *data;
>
> +       data = kzalloc(sizeof(u32), GFP_KERNEL);

Something fishy is going on here.  We allocate 4 B but declare data as
a u8* (pointer to individual bytes)?  In general, dynamically
allocating that few bytes is a code smell; either you meant to just
use the stack, or this memory's lifetime extends past the lifetime of
this stackframe, at which point you probably just meant to stack
allocate space in a higher parent frame and pass this preallocated
memory down to the child frame to get filled in.

Reading through this code, I don't think that the memory is meant to
outlive the stack frame.  Is there a reason why we can't just declare
data as:

u8 data [4];

then use ARRAY_SIZE(data) or RSI_9116_REG_SIZE in rsi_reset_chip(),
getting rid of the kzalloc/kfree?

(Sorry, I hate when a simple fixup becomes a "hey let's rewrite all
this code" thus becoming "that guy.")
-- 
Thanks,
~Nick Desaulniers