linux-kernel - [PATCH 4/7] kdb: Replace double memcpy() with memmove() in kdb

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Message-Id: <20240416-kgdb_read_refactor-v1-4-b18c2d01076d@linaro.org>
Date: Tue, 16 Apr 2024 11:43:21 +0100
From: Daniel Thompson <daniel.thompson@...aro.org>
To: Jason Wessel <jason.wessel@...driver.com>, 
 Douglas Anderson <dianders@...omium.org>
Cc: kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: [PATCH 4/7] kdb: Replace double memcpy() with memmove() in
 kdb_read()

At several points in kdb_read() there are variants of the following
code pattern (with offsets slightly altered):

    memcpy(tmpbuffer, cp, lastchar - cp);
    memcpy(cp-1, tmpbuffer, lastchar - cp);
    *(--lastchar) = '\0';

There is no need to use tmpbuffer here, since we can use memmove() instead
so refactor in the obvious way. Additionally the strings that are being
copied are already properly terminated so let's also change the code so
that the library calls also move the terminator.

Changing how the terminators are managed has no functional effect for now
but might allow us to retire lastchar at a later point. lastchar, although
stored as a pointer, is functionally equivalent to caching strlen(buffer).

Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
---
 kernel/debug/kdb/kdb_io.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 69549fe42e87b..df7b5eb52d74f 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -250,12 +250,9 @@ static char *kdb_read(char *buffer, size_t bufsize)
 	switch (key) {
 	case 8: /* backspace */
 		if (cp > buffer) {
-			if (cp < lastchar) {
-				memcpy(tmpbuffer, cp, lastchar - cp);
-				memcpy(cp-1, tmpbuffer, lastchar - cp);
-			}
-			*(--lastchar) = '\0';
-			--cp;
+			memmove(cp-1, cp, lastchar - cp + 1);
+			lastchar--;
+			cp--;
 			kdb_printf("\b%s ", cp);
 			kdb_position_cursor(kdb_prompt_str, buffer, cp);
 		}
@@ -272,9 +269,8 @@ static char *kdb_read(char *buffer, size_t bufsize)
 		return buffer;
 	case 4: /* Del */
 		if (cp < lastchar) {
-			memcpy(tmpbuffer, cp+1, lastchar - cp - 1);
-			memcpy(cp, tmpbuffer, lastchar - cp - 1);
-			*(--lastchar) = '\0';
+			memmove(cp, cp+1, lastchar - cp);
+			lastchar--;
 			kdb_printf("%s ", cp);
 			kdb_position_cursor(kdb_prompt_str, buffer, cp);
 		}
@@ -389,9 +385,8 @@ static char *kdb_read(char *buffer, size_t bufsize)
 	default:
 		if (key >= 32 && lastchar < bufend) {
 			if (cp < lastchar) {
-				memcpy(tmpbuffer, cp, lastchar - cp);
-				memcpy(cp+1, tmpbuffer, lastchar - cp);
-				*++lastchar = '\0';
+				memmove(cp+1, cp, lastchar - cp + 1);
+				lastchar++;
 				*cp = key;
 				kdb_printf("%s", cp);
 				++cp;

-- 
2.43.0