Re: [PATCH] kdb: Cleanup math with KDB_CMD_HISTORY_COUNT

From: Sumit Garg <sumit.garg@linaro.org>
To: Douglas Anderson <dianders@chromium.org>
Cc: Jason Wessel <jason.wessel@windriver.com>,
	Daniel Thompson <daniel.thompson@linaro.org>,
	Chuhong Yuan <hslester96@gmail.com>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	kgdb-bugreport@lists.sourceforge.net,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] kdb: Cleanup math with KDB_CMD_HISTORY_COUNT
Date: Fri, 8 May 2020 17:32:24 +0530	[thread overview]
Message-ID: <CAFA6WYNbrhiagmqLGOGq+-WRdmbA0Y8FP9PYYsOoerqHeQcmoQ@mail.gmail.com> (raw)
In-Reply-To: <20200507161125.1.I2cce9ac66e141230c3644b8174b6c15d4e769232@changeid>

On Fri, 8 May 2020 at 04:42, Douglas Anderson <dianders@chromium.org> wrote:
>
> From code inspection the math in handle_ctrl_cmd() looks super sketchy
> because it subjects -1 from cmdptr and then does a "%
> KDB_CMD_HISTORY_COUNT".  It turns out that this code works because
> "cmdptr" is unsigned and KDB_CMD_HISTORY_COUNT is a nice power of 2.
> Let's make this a little less sketchy.
>
> This patch should be a no-op.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>

>  kernel/debug/kdb/kdb_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
> index 515379cbf209..6865a0f58d38 100644
> --- a/kernel/debug/kdb/kdb_main.c
> +++ b/kernel/debug/kdb/kdb_main.c
> @@ -1108,7 +1108,8 @@ static int handle_ctrl_cmd(char *cmd)
>         switch (*cmd) {
>         case CTRL_P:
>                 if (cmdptr != cmd_tail)
> -                       cmdptr = (cmdptr-1) % KDB_CMD_HISTORY_COUNT;
> +                       cmdptr = (cmdptr + KDB_CMD_HISTORY_COUNT - 1) %
> +                                KDB_CMD_HISTORY_COUNT;
>                 strscpy(cmd_cur, cmd_hist[cmdptr], CMD_BUFLEN);
>                 return 1;
>         case CTRL_N:
> --
> 2.26.2.645.ge9eca65c58-goog
>