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SubjectRe: [PATCH] staging: most: don't access hdm_ch before checking it valid
On Mon, Sep 28, 2020 at 06:48:38PM +0800, Jing Xiangfeng wrote:
> In try_start_dim_transfer(), pointer hdm_ch is accessed before checking.
> This may lead to a potential null pointer dereference. Fix this by
> dereferencing hdm_ch after calling BUG_ON().
>
> Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
> ---
> drivers/staging/most/dim2/dim2.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/staging/most/dim2/dim2.c b/drivers/staging/most/dim2/dim2.c
> index 509c8012d20b..ccd7cc7545e4 100644
> --- a/drivers/staging/most/dim2/dim2.c
> +++ b/drivers/staging/most/dim2/dim2.c
> @@ -148,7 +148,7 @@ void dimcb_on_error(u8 error_id, const char *error_message)
> static int try_start_dim_transfer(struct hdm_channel *hdm_ch)
> {
> u16 buf_size;
> - struct list_head *head = &hdm_ch->pending_list;

This is not a dereference, it's just pointer math. In other words:

struct list_head *head = hdm_ch + offsetof(struct hdm_channel, pending_list);

So the commit message is wrong because this cannot lead to a NULL
dereference. It's better to just delete the BUG_ON(). We don't really
like BUG_ON(). Checkpatch will complain about them. An Oops gives
basically the same information as a BUG_ON() without completely killing
the kernel so just dereferencing a NULL is preferable. Finally, we can
see from the callers that "hdm_ch" is never NULL.

regards,
dan carpenter

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