[PATCH] lib/rsa: avoid -Wdiscarded-qualifiers

[Alex G.]

On 1/9/22 8:39 AM, Heinrich Schuchardt wrote:
> The return type of EVP_PKEY_get0_RSA() is const struct rsa_st *.
> Our code drops the const qualifier leading to
> 
> In file included from tools/lib/rsa/rsa-sign.c:1:
> ./tools/../lib/rsa/rsa-sign.c: In function ‘rsa_add_verify_data’:
> ./tools/../lib/rsa/rsa-sign.c:631:13: warning:
> assignment discards ‘const’ qualifier from pointer target type
> [-Wdiscarded-qualifiers]
>    631 |         rsa = EVP_PKEY_get0_RSA(pkey);
>        |             ^
> 
> Add a type conversion.
> 
> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> ---
>   lib/rsa/rsa-sign.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/rsa/rsa-sign.c b/lib/rsa/rsa-sign.c
> index 44f21416ce..3b6e5f0f86 100644
> --- a/lib/rsa/rsa-sign.c
> +++ b/lib/rsa/rsa-sign.c
> @@ -628,7 +628,7 @@ int rsa_add_verify_data(struct image_sign_info *info, void *keydest)
>   	if (ret)
>   		goto err_get_pub_key;
>   
> -	rsa = EVP_PKEY_get0_RSA(pkey);
> +	rsa = (RSA *)EVP_PKEY_get0_RSA(pkey);

I think it's the wrong path to discard const qualifiers, whether 
unwillingly or by type punning. I suggest making 'rsa' a "const RSA *" 
and fixing the downstream users to do the same.

Alex

>   	ret = rsa_get_params(rsa, &exponent, &n0_inv, &modulus, &r_squared);
>   	if (ret)
>   		goto err_get_params;
>