nfsd: escape high characters in binary data

I'm exposing some information about NFS clients in pseudofiles.  I
expect to eventually have simple tools to help read those pseudofiles.

But it's also helpful if the raw files are human-readable to the extent
possible.  It aids debugging and makes them usable on systems that don't
have the latest nfs-utils.

A minor challenge there is opaque client-generated protocol objects like
state owners and client identifiers.  Some clients generate those to
include handy information in plain ascii.  But they may also include
arbitrary byte sequences.

I think the simplest approach is to limit to isprint(c) && isascii(c)
and escape everything else.

That means you can just cat the file and get something that looks OK.
Also, I'm trying to keep these files legal YAML, which requires them to
UTF-8, and this is a simple way to guarantee that.
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/seq_file.c

@@ -384,6 +384,17 @@ void seq_escape(struct seq_file *m, const char *s, const char *esc)
 }
 EXPORT_SYMBOL(seq_escape);
 
+void seq_escape_mem_ascii(struct seq_file *m, const char *src, size_t isz)
+{
+	char *buf;
+	size_t size = seq_get_buf(m, &buf);
+	int ret;
+
+	ret = string_escape_mem_ascii(src, isz, buf, size);
+	seq_commit(m, ret < size ? ret : -1);
+}
+EXPORT_SYMBOL(seq_escape_mem_ascii);
+
 void seq_vprintf(struct seq_file *m, const char *f, va_list args)
 {
 	int len;

include/linux/seq_file.h

@@ -127,6 +127,7 @@ void seq_put_hex_ll(struct seq_file *m, const char *delimiter,
 		    unsigned long long v, unsigned int width);
 
 void seq_escape(struct seq_file *m, const char *s, const char *esc);
+void seq_escape_mem_ascii(struct seq_file *m, const char *src, size_t isz);
 
 void seq_hex_dump(struct seq_file *m, const char *prefix_str, int prefix_type,
 		  int rowsize, int groupsize, const void *buf, size_t len,

include/linux/string_helpers.h

@@ -54,6 +54,9 @@ static inline int string_unescape_any_inplace(char *buf)
 int string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
 		unsigned int flags, const char *only);
 
+int string_escape_mem_ascii(const char *src, size_t isz, char *dst,
+					size_t osz);
+
 static inline int string_escape_mem_any_np(const char *src, size_t isz,
 		char *dst, size_t osz, const char *only)
 {

lib/string_helpers.c

@@ -540,6 +540,25 @@ int string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
 }
 EXPORT_SYMBOL(string_escape_mem);
 
+int string_escape_mem_ascii(const char *src, size_t isz, char *dst,
+					size_t osz)
+{
+	char *p = dst;
+	char *end = p + osz;
+
+	while (isz--) {
+		unsigned char c = *src++;
+
+		if (!isprint(c) || !isascii(c) || c == '"' || c == '\\')
+			escape_hex(c, &p, end);
+		else
+			escape_passthrough(c, &p, end);
+	}
+
+	return p - dst;
+}
+EXPORT_SYMBOL(string_escape_mem_ascii);
+
 /*
  * Return an allocated string that has been escaped of special characters
  * and double quotes, making it safe to log in quotes.