vsprintf: deal with format specifiers with a lookup table

We did the flags as an array earlier, they had simpler rules.  The final
format specifiers are a bit more complex since they have more fields to
deal with, and we want to handle the length modifiers at the same time.
But like the flags, we're better off just making it a data-driven table
rather than some case statement.

Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

lib/vsprintf.c

@@ -2566,7 +2566,7 @@ static noinline_for_stack
 struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 {
 	const char *start = fmt.str;
-	char flag, qualifier;
+	char flag;
 
 	/* we finished early by reading the field width */
 	if (fmt.state == FORMAT_STATE_WIDTH) {
@@ -2637,96 +2637,71 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	}
 
 qualifier:
-	/* get the conversion qualifier */
-	qualifier = 0;
-	if (*fmt.str == 'h' || _tolower(*fmt.str) == 'l' ||
-	    *fmt.str == 'z' || *fmt.str == 't') {
-		qualifier = *fmt.str++;
-		if (unlikely(qualifier == *fmt.str)) {
-			if (qualifier == 'l') {
-				qualifier = 'L';
-				fmt.str++;
-			} else if (qualifier == 'h') {
-				qualifier = 'H';
-				fmt.str++;
-			}
-		}
-	}
-
-	/* default base */
+	/* Set up default numeric format */
 	spec->base = 10;
-	switch (*fmt.str) {
-	case 'c':
-		fmt.state = FORMAT_STATE_CHAR;
-		fmt.str++;
-		return fmt;
-
-	case 's':
-		fmt.state = FORMAT_STATE_STR;
-		fmt.str++;
-		return fmt;
-
-	case 'p':
-		fmt.state = FORMAT_STATE_PTR;
-		fmt.str++;
-		return fmt;
-
-	case '%':
-		fmt.state = FORMAT_STATE_PERCENT_CHAR;
-		fmt.str++;
-		return fmt;
-
-	/* integer number formats - set up the flags and "break" */
-	case 'o':
-		spec->base = 8;
-		break;
-
-	case 'x':
-		spec->flags |= SMALL;
-		fallthrough;
-
-	case 'X':
-		spec->base = 16;
-		break;
+	fmt.state = FORMAT_STATE_SIZE(int);
+	static const struct format_state {
+		unsigned char state;
+		unsigned char flags_or_double_state;
+		unsigned char modifier;
+		unsigned char base;
+	} lookup_state[256] = {
+		// Qualifiers
+		['l'] = { FORMAT_STATE_SIZE(long), FORMAT_STATE_SIZE(long long), 1 },
+		['L'] = { FORMAT_STATE_SIZE(long long), 0, 1 },
+		['h'] = { FORMAT_STATE_SIZE(short), FORMAT_STATE_SIZE(char), 1 },
+		['H'] = { FORMAT_STATE_SIZE(char), 0, 1 }, // Questionable, historic
+		['z'] = { FORMAT_STATE_SIZE(size_t), 0, 1 },
+		['t'] = { FORMAT_STATE_SIZE(ptrdiff_t), 0, 1 },
+
+		// Non-numeric formats
+		['c'] = { FORMAT_STATE_CHAR },
+		['s'] = { FORMAT_STATE_STR },
+		['p'] = { FORMAT_STATE_PTR },
+		['%'] = { FORMAT_STATE_PERCENT_CHAR },
+
+		// Numerics
+		['o'] = { 0, 0, 0, 8 },
+		['x'] = { 0, SMALL, 0, 16 },
+		['X'] = { 0, 0, 0, 16 },
+		['d'] = { 0, SIGN, 0, 10 },
+		['i'] = { 0, SIGN, 0, 10 },
+		['u'] = { 0, 0, 0, 10, },
 
-	case 'd':
-	case 'i':
-		spec->flags |= SIGN;
-		break;
-	case 'u':
-		break;
-
-	case 'n':
 		/*
 		 * Since %n poses a greater security risk than
 		 * utility, treat it as any other invalid or
 		 * unsupported format specifier.
 		 */
-		fallthrough;
+	};
 
-	default:
-		WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt.str);
-		fmt.state = FORMAT_STATE_INVALID;
+	const struct format_state *p = lookup_state + (u8)*fmt.str;
+	if (p->modifier) {
+		fmt.state = p->state;
+		if (p->flags_or_double_state && fmt.str[0] == fmt.str[1]) {
+			fmt.state = p->flags_or_double_state;
+			fmt.str++;
+		}
+		fmt.str++;
+		p = lookup_state + *fmt.str;
+		if (unlikely(p->modifier))
+			goto invalid;
+	}
+	if (p->base) {
+		spec->base = p->base;
+		spec->flags |= p->flags_or_double_state;
+		fmt.str++;
 		return fmt;
 	}
-
-	if (qualifier == 'L')
-		fmt.state = FORMAT_STATE_SIZE(long long);
-	else if (qualifier == 'l') {
-		fmt.state = FORMAT_STATE_SIZE(long);
-	} else if (qualifier == 'z') {
-		fmt.state = FORMAT_STATE_SIZE(size_t);
-	} else if (qualifier == 't') {
-		fmt.state = FORMAT_STATE_SIZE(ptrdiff_t);
-	} else if (qualifier == 'H') {
-		fmt.state = FORMAT_STATE_SIZE(char);
-	} else if (qualifier == 'h') {
-		fmt.state = FORMAT_STATE_SIZE(short);
-	} else {
-		fmt.state = FORMAT_STATE_SIZE(int);
+	if (p->state) {
+		fmt.state = p->state;
+		fmt.str++;
+		return fmt;
 	}
 
-	fmt.str++;
+invalid:
+	WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt.str);
+	fmt.state = FORMAT_STATE_INVALID;
 	return fmt;
 }