Re: [PATCH 3/3] block: print the server key type and fingerprint on failure

[Hanna Reitz <hreitz@redhat.com>]

On 18.11.21 15:35, Daniel P. Berrangé wrote:
> When validating the server key fingerprint fails, it is difficult for
> the user to know what they got wrong. The fingerprint accepted by QEMU
> is received in a different format than openssh displays. There can also
> be keys for multiple different ciphers in known_hosts. It may not be
> obvious which cipher QEMU will use and whether it will be the same
> as openssh. Address this by printing the server key type and its
> corresponding fingerprint in the format QEMU accepts.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>   block/ssh.c | 37 ++++++++++++++++++++++++++++++-------
>   1 file changed, 30 insertions(+), 7 deletions(-)

Nice!

Reviewed-by: Hanna Reitz <hreitz@redhat.com>

> diff --git a/block/ssh.c b/block/ssh.c
> index fcc0ab765a..967a2b971e 100644
> --- a/block/ssh.c
> +++ b/block/ssh.c
> @@ -386,14 +386,28 @@ static int compare_fingerprint(const unsigned char *fingerprint, size_t len,
>       return *host_key_check - '\0';
>   }
>   
> +static char *format_fingerprint(const unsigned char *fingerprint, size_t len)
> +{
> +    static const char *hex = "0123456789abcdef";
> +    char *ret = g_new0(char, (len * 2) + 1);
> +    for (size_t i = 0; i < len; i++) {
> +        ret[i * 2] = hex[((fingerprint[i] >> 4) & 0xf)];
> +        ret[(i * 2) + 1] = hex[(fingerprint[i] & 0xf)];

(I would have found an sn?printf() solution a bit simpler here
(snprintf(&ret[i * 2], 2, "%02x", fingerprint[i])),
but now you already wrote the code, so...)

> +    }
> +    ret[len * 2] = '\0';
> +    return ret;
> +}