arm64: idreg-override: Avoid sprintf() for simple string concatenation

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@kernel.org>
Link: https://lore.kernel.org/r/20231129111555.3594833-61-ardb@google.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: ardbiesheuvel

arch/arm64/kernel/idreg-override.c

@@ -206,14 +206,15 @@ static int __init parse_nokaslr(char *unused)
 }
 early_param("nokaslr", parse_nokaslr);
 
-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;
@@ -223,23 +224,29 @@ static int __init find_field(const char *cmdline,
 
 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 = prel64_pointer(regs[i].reg);
 		struct arm64_ftr_override *override;
+		int len = strlen(reg->name);
 		int f;
 
 		override = prel64_pointer(reg->override);
 
+		// 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;
 			u64 mask = GENMASK_ULL(shift + width - 1, shift);
 			bool (*filter)(u64 val);
 			u64 v;
 
-			if (find_field(cmdline, reg, f, &v))
+			if (find_field(cmdline, opt, len, reg, f, &v))
 				continue;
 
 			/*