[PATCH 17/28] MIPS: cmdline: Refactor bootcmdline_append() to use snprintf()

[Bill Wendling]

bootcmdline_append() passes "max" (the maximum length of the source
string "s") as the destination buffer size argument to strlcat(). Because
strlcat() expects the total size of the destination buffer
(COMMAND_LINE_SIZE), it fails to append anything whenever "max" is smaller
than the current length of boot_command_line, and could overflow
boot_command_line if "max" exceeds COMMAND_LINE_SIZE.

In preparation for removing the deprecated strlcat() API[1], replace the
strlcat() calls with snprintf() using the "%.*s" precision specifier. This
bounds the read from "s" to at most "max" bytes while properly respecting
the remaining space in boot_command_line.

Link: https://github.com/KSPP/linux/issues/370 [1]
Cc: codemender-patching+linux@xxxxxxxxxx
Assisted-by: SecurityEngineer:Gemini2.5Pro [editor_update_file, shell_command_run, make_kernel_build_single_file, test_runner_run_kunit_test]
Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
---
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---
 arch/mips/kernel/setup.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index 1ae6d0c0e1d6..0d9c2137d675 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -518,13 +518,28 @@ static void __init check_kernel_sections_mem(void)
 
 static void __init bootcmdline_append(const char *s, size_t max)
 {
+	size_t len;
+	int prec;
+
 	if (!s[0] || !max)
 		return;
 
-	if (boot_command_line[0])
-		strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
+	len = strlen(boot_command_line);
+	if (len >= COMMAND_LINE_SIZE - 1)
+		return;
 
-	strlcat(boot_command_line, s, max);
+	if (len) {
+		if (COMMAND_LINE_SIZE - len < 3)
+			return;
+
+		prec = min_t(size_t, max, COMMAND_LINE_SIZE - len - 2);
+		snprintf(boot_command_line + len, COMMAND_LINE_SIZE - len, " %.*s",
+			 prec, s);
+	} else {
+		prec = min_t(size_t, max, COMMAND_LINE_SIZE - 1);
+		snprintf(boot_command_line, COMMAND_LINE_SIZE, "%.*s",
+			 prec, s);
+	}
 }
 
 #ifdef CONFIG_OF_EARLY_FLATTREE
-- 
2.55.0.1032.g73a4cd73de-goog