[PATCH v7 15/33] arm64: idreg-override: Avoid sprintf() for simple string concatenation

[Ard Biesheuvel]

Instead of using sprintf() with the "%s.%s=" format, where the first
string argument is always the same in the inner loop of match_options(),
use simple memcpy() for string concatenation, and move the first copy to
the outer loop. This removes the dependency on sprintf(), which will be
difficult to fulfil when we move this code into the early mini C
runtime.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/idreg-override.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index 97ec832d87d44f64..a5299aa1d1711adc 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -166,14 +166,15 @@ static const struct {
 	{ "nokaslr",			"arm64_sw.nokaslr=1" },
 };
 
-static int __init find_field(const char *cmdline,
+static int __init find_field(const char *cmdline, char *opt, int len,
 			     const struct ftr_set_desc *reg, int f, u64 *v)
 {
-	char opt[FTR_DESC_NAME_LEN + FTR_DESC_FIELD_LEN + 2];
-	int len;
+	int flen = strlen(reg->fields[f].name);
 
-	len = snprintf(opt, ARRAY_SIZE(opt), "%s.%s=",
-		       reg->name, reg->fields[f].name);
+	// append '<fieldname>=' to obtain '<name>.<fieldname>='
+	memcpy(opt + len, reg->fields[f].name, flen);
+	len += flen;
+	opt[len++] = '=';
 
 	if (memcmp(cmdline, opt, len))
 		return -1;
@@ -190,12 +191,18 @@ static const void * __init get_filter(const struct ftr_set_desc *reg, int idx)
 
 static void __init match_options(const char *cmdline)
 {
+	char opt[FTR_DESC_NAME_LEN + FTR_DESC_FIELD_LEN + 2];
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(regs); i++) {
 		const struct ftr_set_desc *reg = offset_to_ptr(&regs[i]);
+		int len = strlen(reg->name);
 		int f;
 
+		// set opt[] to '<name>.'
+		memcpy(opt, reg->name, len);
+		opt[len++] = '.';
+
 		for (f = 0; reg->fields[f].name[0] != '\0'; f++) {
 			u64 shift = reg->fields[f].shift;
 			u64 width = reg->fields[f].width ?: 4;
@@ -203,7 +210,7 @@ static void __init match_options(const char *cmdline)
 			bool (*filter)(u64) = get_filter(reg, f);
 			u64 v;
 
-			if (find_field(cmdline, reg, f, &v))
+			if (find_field(cmdline, opt, len, reg, f, &v))
 				continue;
 
 			/*
-- 
2.35.1