[PATCH 3/9] lib/string: add sysfs_buf_streq()

[Zev Weiss <zev@bewilderbeest.net>]

This is intended for use with mutable device-tree string properties.
As with sysfs_streq(), it accepts a trailing linefeed for convenient
shell access (e.g. 'echo foo > /sys/firmware/devicetree/...'), but
also accepts a trailing NUL terminator to match how DT string
properties are presented when read (so that we don't reject userspace
writing back exactly what it had previously read from the file).

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
---
 include/linux/string.h |  1 +
 lib/string.c           | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 5e96d656be7a..d066ff82d1ec 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -183,6 +183,7 @@ extern char **argv_split(gfp_t gfp, const char *str, int *argcp);
 extern void argv_free(char **argv);
 
 extern bool sysfs_streq(const char *s1, const char *s2);
+extern bool sysfs_buf_streq(const void *buf, size_t blen, const char *str);
 int match_string(const char * const *array, size_t n, const char *string);
 int __sysfs_match_string(const char * const *array, size_t n, const char *s);
 
diff --git a/lib/string.c b/lib/string.c
index b2de45a581f4..aab5cadb6b98 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -715,6 +715,40 @@ bool sysfs_streq(const char *s1, const char *s2)
 }
 EXPORT_SYMBOL(sysfs_streq);
 
+/**
+ * sysfs_buf_streq - check if a buffer matches a string, modulo trailing \n or \0
+ * @buf: pointer to the buffer to check
+ * @blen: length of the buffer to check
+ * @str: string to compare against
+ *
+ * This routine returns true iff the buffer equals the string, treating a
+ * trailing \n or \0 on the buffer as an accepted terminator, the length of
+ * which (aside from the optional terminator) must match the length of the
+ * string.  It's geared for use with sysfs bin_attribute inputs, which may
+ * terminate with newlines or NULs (the latter to match how device-tree string
+ * properties in particular are presented on read).
+ */
+bool sysfs_buf_streq(const void *buf, size_t blen, const char *str)
+{
+	const char *p = buf;
+	size_t slen = strlen(str);
+	if (blen < slen)
+		return false;
+
+	if (memcmp(p, str, slen))
+		return false;
+
+	switch (blen - slen) {
+	case 0:
+		return true;
+	case 1:
+		return p[slen] == '\n' || p[slen] == '\0';
+	default:
+		return false;
+	}
+}
+EXPORT_SYMBOL(sysfs_buf_streq);
+
 /**
  * match_string - matches given string in an array
  * @array:	array of strings
-- 
2.33.0