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From:   Douglas Anderson <dianders@chromium.org>
To:     Jason Wessel <jason.wessel@windriver.com>,
        Daniel Thompson <daniel.thompson@linaro.org>
Cc:     sumit.garg@linaro.org, Douglas Anderson <dianders@chromium.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@vger.kernel.org
Subject: [PATCH] kdb: Cleanup math with KDB_CMD_HISTORY_COUNT
Date:   Thu,  7 May 2020 16:11:46 -0700
Message-Id: <20200507161125.1.I2cce9ac66e141230c3644b8174b6c15d4e769232@changeid>
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---

 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