Re: [PATCH] Bluetooth: L2CAP: Elide a string overflow warning

[Siddh Raman Pant]

On Fri, 12 Aug 2022 11:22:49 +0530  Palmer Dabbelt  wrote:
> From: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> 
> Without this I get a string op warning related to copying from a
> possibly NULL pointer.  I think the warning is spurious, but it's
> tripping up allmodconfig.

I think it is not spurious, and is due to the following commit:
d0be8347c623 ("Bluetooth: L2CAP: Fix use-after-free caused by l2cap_chan_put")

The following commit fixes a similar problem (added the NULL check on line 1996):
332f1795ca20 ("Bluetooth: L2CAP: Fix l2cap_global_chan_by_psm regression")

> In file included from /scratch/merges/ko-linux-next/linux/include/linux/string.h:253,
>                  from /scratch/merges/ko-linux-next/linux/include/linux/bitmap.h:11,
>                  from /scratch/merges/ko-linux-next/linux/include/linux/cpumask.h:12,
>                  from /scratch/merges/ko-linux-next/linux/include/linux/mm_types_task.h:14,
>                  from /scratch/merges/ko-linux-next/linux/include/linux/mm_types.h:5,
>                  from /scratch/merges/ko-linux-next/linux/include/linux/buildid.h:5,
>                  from /scratch/merges/ko-linux-next/linux/include/linux/module.h:14,
>                  from /scratch/merges/ko-linux-next/linux/net/bluetooth/l2cap_core.c:31:
> In function 'memcmp',
>     inlined from 'bacmp' at /scratch/merges/ko-linux-next/linux/include/net/bluetooth/bluetooth.h:347:9,
>     inlined from 'l2cap_global_chan_by_psm' at /scratch/merges/ko-linux-next/linux/net/bluetooth/l2cap_core.c:2003:15:
> /scratch/merges/ko-linux-next/linux/include/linux/fortify-string.h:44:33: error: '__builtin_memcmp' specified bound 6 exceeds source size 0 [-Werror=stringop-overread]
>    44 | #define __underlying_memcmp     __builtin_memcmp
>       |                                 ^
> /scratch/merges/ko-linux-next/linux/include/linux/fortify-string.h:420:16: note: in expansion of macro '__underlying_memcmp'
>   420 |         return __underlying_memcmp(p, q, size);
>       |                ^~~~~~~~~~~~~~~~~~~
> In function 'memcmp',
>     inlined from 'bacmp' at /scratch/merges/ko-linux-next/linux/include/net/bluetooth/bluetooth.h:347:9,
>     inlined from 'l2cap_global_chan_by_psm' at /scratch/merges/ko-linux-next/linux/net/bluetooth/l2cap_core.c:2004:15:
> /scratch/merges/ko-linux-next/linux/include/linux/fortify-string.h:44:33: error: '__builtin_memcmp' specified bound 6 exceeds source size 0 [-Werror=stringop-overread]
>    44 | #define __underlying_memcmp     __builtin_memcmp
>       |                                 ^
> /scratch/merges/ko-linux-next/linux/include/linux/fortify-string.h:420:16: note: in expansion of macro '__underlying_memcmp'
>   420 |         return __underlying_memcmp(p, q, size);
>       |                ^~~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
> 
> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>

Tested-by: Siddh Raman Pant <code@xxxxxxxx>

> ---
>  net/bluetooth/l2cap_core.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index cbe0cae73434..be7f47e52119 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -2000,11 +2000,13 @@ static struct l2cap_chan *l2cap_global_chan_by_psm(int state, __le16 psm,
>  			}
>  
>  			/* Closest match */
> -			src_any = !bacmp(&c->src, BDADDR_ANY);
> -			dst_any = !bacmp(&c->dst, BDADDR_ANY);
> -			if ((src_match && dst_any) || (src_any && dst_match) ||
> -			    (src_any && dst_any))
> -				c1 = c;
> +			if (c) {
> +				src_any = !bacmp(&c->src, BDADDR_ANY);
> +				dst_any = !bacmp(&c->dst, BDADDR_ANY);
> +				if ((src_match && dst_any) || (src_any && dst_match) ||
> +				    (src_any && dst_any))
> +					c1 = c;
> +			}
>  		}
>  	}
>