[RFC v6 1/8] vsprintf: Add [v]sprintf_trunc()

[Alejandro Colomar <alx@kernel.org>]

sprintf_trunc() is a function similar to strscpy().  It truncates the
string, and returns an error code on truncation or error.  On success,
it returns the length of the new string.

Cc: Kees Cook <kees@kernel.org>
Cc: Christopher Bazley <chris.bazley.wg14@gmail.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Marco Elver <elver@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
 include/linux/sprintf.h |  2 ++
 lib/vsprintf.c          | 53 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/include/linux/sprintf.h b/include/linux/sprintf.h
index 51cab2def9ec..5ea6ec9c2e59 100644
--- a/include/linux/sprintf.h
+++ b/include/linux/sprintf.h
@@ -13,6 +13,8 @@ __printf(3, 4) int snprintf(char *buf, size_t size, const char *fmt, ...);
 __printf(3, 0) int vsnprintf(char *buf, size_t size, const char *fmt, va_list args);
 __printf(3, 4) int scnprintf(char *buf, size_t size, const char *fmt, ...);
 __printf(3, 0) int vscnprintf(char *buf, size_t size, const char *fmt, va_list args);
+__printf(3, 4) int sprintf_trunc(char *buf, size_t size, const char *fmt, ...);
+__printf(3, 0) int vsprintf_trunc(char *buf, size_t size, const char *fmt, va_list args);
 __printf(2, 3) __malloc char *kasprintf(gfp_t gfp, const char *fmt, ...);
 __printf(2, 0) __malloc char *kvasprintf(gfp_t gfp, const char *fmt, va_list args);
 __printf(2, 0) const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list args);
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 01699852f30c..15e780942c56 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2923,6 +2923,34 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args)
 }
 EXPORT_SYMBOL(vscnprintf);
 
+/**
+ * vsprintf_trunc - va_list string truncate print formatted
+ * @buf: The buffer to place the result into
+ * @size: The size of the buffer, including the trailing null space
+ * @fmt: The format string to use
+ * @args: Arguments for the format string
+ *
+ * The return value is the length of the string.
+ * If the string is truncated, the function returns -E2BIG.
+ * If @size is invalid, the function returns -EOVERFLOW.
+ *
+ * See the vsnprintf() documentation for format string extensions over C99.
+ */
+int vsprintf_trunc(char *buf, size_t size, const char *fmt, va_list args)
+{
+	int len;
+
+	if (WARN_ON_ONCE(size == 0 || size > INT_MAX))
+		return -EOVERFLOW;
+
+	len = vsnprintf(buf, size, fmt, args);
+	if (unlikely(len >= size))
+		return -E2BIG;
+
+	return len;
+}
+EXPORT_SYMBOL(vsprintf_trunc);
+
 /**
  * snprintf - Format a string and place it in a buffer
  * @buf: The buffer to place the result into
@@ -2974,6 +3002,31 @@ int scnprintf(char *buf, size_t size, const char *fmt, ...)
 }
 EXPORT_SYMBOL(scnprintf);
 
+/**
+ * sprintf_trunc - string truncate print formatted
+ * @buf: The buffer to place the result into
+ * @size: The size of the buffer, including the trailing null space
+ * @fmt: The format string to use
+ * @...: Arguments for the format string
+ *
+ * The return value is the length of the string.
+ * If the string is truncated, the function returns -E2BIG.
+ * If @size is invalid, the function returns -EOVERFLOW.
+ */
+
+int sprintf_trunc(char *buf, size_t size, const char *fmt, ...)
+{
+	int len;
+	va_list args;
+
+	va_start(args, fmt);
+	len = vsprintf_trunc(buf, size, fmt, args);
+	va_end(args);
+
+	return len;
+}
+EXPORT_SYMBOL(sprintf_trunc);
+
 /**
  * vsprintf - Format a string and place it in a buffer
  * @buf: The buffer to place the result into
-- 
2.50.0