linux-kernel - Re: [PATCH AUTOSEL 4.4 080/100] char: hpet: Use flexible-array member

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Date:   Fri, 14 Feb 2020 09:43:14 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH AUTOSEL 4.4 080/100] char: hpet: Use flexible-array member

On Fri, Feb 14, 2020 at 11:24:04AM -0500, Sasha Levin wrote:
> From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
> 
> [ Upstream commit 987f028b8637cfa7658aa456ae73f8f21a7a7f6f ]
> 
> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> presence of a "variable length array":
> 
> struct something {
>     int length;
>     u8 data[1];
> };
> 
> struct something *instance;
> 
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
> 
> There is also 0-byte arrays. Both cases pose confusion for things like
> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
> to declare variable-length types such as the one above is a flexible array
> member[2] which need to be the last member of a structure and empty-sized:
> 
> struct something {
>         int stuff;
>         u8 data[];
> };
> 
> Also, 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
> unadvertenly introduced[3] to the codebase from now on.
> 
> [1] https://github.com/KSPP/linux/issues/21
> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> Link: https://lore.kernel.org/r/20200120235326.GA29231@embeddedor.com
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  drivers/char/hpet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> index 5b38d7a8202a1..38c2ae93ce492 100644
> --- a/drivers/char/hpet.c
> +++ b/drivers/char/hpet.c
> @@ -112,7 +112,7 @@ struct hpets {
>  	unsigned long hp_delta;
>  	unsigned int hp_ntimer;
>  	unsigned int hp_which;
> -	struct hpet_dev hp_dev[1];
> +	struct hpet_dev hp_dev[];
>  };
>  

Umm, why are you backporting this without the commit that fixes it?  Does your
AUTOSEL process really still not pay attention to Fixes tags?  They are there
for a reason.

And for that matter, why are you backporting it all, given that this is a
cleanup and not a fix?

- Eric