lib/string_helpers: allow to append additional characters to be escaped

Introduce a new flag to append additional characters, passed in 'only'
parameter, to be escaped if they fall in the corresponding class.

Link: https://lkml.kernel.org/r/20210504180819.73127-7-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/string_helpers.h

@@ -54,6 +54,7 @@ static inline int string_unescape_any_inplace(char *buf)
 #define ESCAPE_HEX		BIT(5)
 #define ESCAPE_NA		BIT(6)
 #define ESCAPE_NAP		BIT(7)
+#define ESCAPE_APPEND		BIT(8)
 
 int string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
 		unsigned int flags, const char *only);

lib/string_helpers.c

@@ -493,6 +493,11 @@ static bool escape_hex(unsigned char c, char **dst, char *end)
  *		escape only non-ascii characters, checked by isascii()
  *	%ESCAPE_NAP:
  *		escape only non-printable or non-ascii characters
+ *	%ESCAPE_APPEND:
+ *		append characters from @only to be escaped by the given classes
+ *
+ * %ESCAPE_APPEND would help to pass additional characters to the escaped, when
+ * one of %ESCAPE_NP, %ESCAPE_NA, or %ESCAPE_NAP is provided.
  *
  * One notable caveat, the %ESCAPE_NAP, %ESCAPE_NP and %ESCAPE_NA have the
  * higher priority than the rest of the flags (%ESCAPE_NAP is the highest).
@@ -513,9 +518,11 @@ int string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
 	char *p = dst;
 	char *end = p + osz;
 	bool is_dict = only && *only;
+	bool is_append = flags & ESCAPE_APPEND;
 
 	while (isz--) {
 		unsigned char c = *src++;
+		bool in_dict = is_dict && strchr(only, c);
 
 		/*
 		 * Apply rules in the following sequence:
@@ -531,20 +538,24 @@ int string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
 		 *	  defined by given @flags
 		 * In these cases we just pass through a character to the
 		 * output buffer.
+		 *
+		 * When %ESCAPE_APPEND is passed, the characters from @only
+		 * have been excluded from the %ESCAPE_NAP, %ESCAPE_NP, and
+		 * %ESCAPE_NA cases.
 		 */
-		if (is_dict && !strchr(only, c) &&
+		if (!(is_append || in_dict) && is_dict &&
 					  escape_passthrough(c, &p, end))
 			continue;
 
-		if (isascii(c) && isprint(c) &&
+		if (!(is_append && in_dict) && isascii(c) && isprint(c) &&
 		    flags & ESCAPE_NAP && escape_passthrough(c, &p, end))
 			continue;
 
-		if (isprint(c) &&
+		if (!(is_append && in_dict) && isprint(c) &&
 		    flags & ESCAPE_NP && escape_passthrough(c, &p, end))
 			continue;
 
-		if (isascii(c) &&
+		if (!(is_append && in_dict) && isascii(c) &&
 		    flags & ESCAPE_NA && escape_passthrough(c, &p, end))
 			continue;