Re: [PATCH libmnl 5/5] callback: avoid signed overflow on INT_MIN error code

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Fri, Nov 06, 2026 at 08:18:40PM +0100, Florian Stosse wrote:
> mnl_cb_error() negates the error field of an NLMSG_ERROR message to get
> a positive errno. For INT_MIN the negation overflows, which is undefined
> behaviour. No errno has that value, so report the message as malformed
> with EBADMSG instead.
> 
> Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
> Signed-off-by: Florian Stosse <florian.stosse@harvester.fr>
> ---
>  src/callback.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/src/callback.c b/src/callback.c
> index 703ae80..660c90a 100644
> --- a/src/callback.c
> +++ b/src/callback.c
> @@ -8,6 +8,7 @@
>   */
>  
>  #include <errno.h>
> +#include <limits.h>
>  #include <libmnl/libmnl.h>
>  #include "internal.h"
>  
> @@ -25,7 +26,9 @@ static int mnl_cb_error(const struct nlmsghdr *nlh, void *data)
>  		return MNL_CB_ERROR;
>  	}
>  	/* Netlink subsystems returns the errno value with different signess */
> -	if (err->error < 0)
> +	if (err->error == INT_MIN)
> +		errno = EBADMSG;

This is most likely processing an errno from the kernel.

IIRC, this is for historical reasons, I remember there was a very old
bug in netlink.

Probably this branch can just go away? 

> +	else if (err->error < 0)
>  		errno = -err->error;
>  	else
>  		errno = err->error;
> -- 
> 2.56.0.windows.4
>