Subject: Re: [PATCH] imap-send: handle NULL return of next_arg()

From: Junio C Hamano <[email-masked]>
Date: Thu, 02 Nov 2017 11:18:42 +0900

René Scharfe <[email-masked]> writes:

> @@ -807,6 +816,8 @@ static int get_cmd_result(struct imap_store *ctx, struct imap_cmd *tcmd)
>  			if (cmdp->cb.cont || cmdp->cb.data)
>  				imap->literal_pending = 0;
>  			arg = next_arg(&cmd);
> +			if (!arg)
> +				arg = "";

This is being cute and needs reading of the post-context a bit.

         arg = next_arg(&cmd);
+        if (!arg)
+                arg = "";
         if (!strcmp("OK", arg))
                 resp = DRV_OK;
         else {
                 if (!strcmp("NO", arg))
                         resp = RESP_NO;
                 else /*if (!strcmp("BAD", arg))*/
                         resp = RESP_BAD;
                 fprintf(stderr, "IMAP command '%s' returned response (%s) - %s\n",
                         !starts_with(cmdp->cmd, "LOGIN") ?
                                         cmdp->cmd : "LOGIN <user> <pass>",
                                         arg, cmd ? cmd : "");
         }
         if ((resp2 = parse_response_code(ctx, &cmdp->cb, cmd)) > resp)
                 resp = resp2;

Because the existing code does not explicitly check for "BAD", we
get RESP_BAD, which is what we want in the end, and the error
message we give has "returned response (%s)" which is filled with
this empty string.

I notice that when this change matters, i.e. we hit a premature end,
next_arg() that yielded NULL would have cleared cmd.  That is OK for
the fprintf() we see above, but wouldn't it hit parse_response_code()
that is shared between good and bad cases?  The function starts like
so:

    static int parse_response_code(struct imap_store *ctx, struct imap_cmd_cb *cb,
                                   char *s)
    {
            struct imap *imap = ctx->imap;
            char *arg, *p;

            if (*s != '[')
                    return RESP_OK;		/* no response code */

so we will get an immediate NULL dereference, it appears.

The fixes in other hunks looked OK (and not cute).