[PATCH v2 1/1] lib/vsprintf: Implement ssprintf() to catch truncated strings

[Lee Jones]

There is an ongoing effort to replace the use of {v}snprintf() variants
with safer alternatives - for a more in depth view, see Jon's write-up
on LWN [0] and/or Alex's on the Kernel Self Protection Project [1].

Whist executing the task, it quickly became apparent that the initial
thought of simply s/snprintf/scnprintf/ wasn't going to be adequate for
a number of cases.  Specifically ones where the caller needs to know
whether the given string ends up being truncated.  This is where
ssprintf() [based on similar semantics of strscpy()] comes in, since it
takes the best parts of both of the aforementioned variants.  It has the
testability of truncation of snprintf() and returns the number of Bytes
*actually* written, similar to scnprintf(), making it a very programmer
friendly alternative.

Here's some examples to show the differences:

  Success: No truncation - all 9 Bytes successfully written to the buffer

    ret = snprintf (buf, 10, "%s", "123456789");  // ret = 9
    ret = scnprintf(buf, 10, "%s", "123456789");  // ret = 9
    ret = ssprintf (buf, 10, "%s", "123456789");  // ret = 9

  Failure: Truncation - only 9 of 10 Bytes written; '-' is truncated

    ret = snprintf (buf, 10, "%s", "123456789-"); // ret = 10

      Reports: "10 Bytes would have been written if buf was large enough"
      Issue: Programmers need to know/remember to check ret against "10"

    ret = scnprintf(buf, 10, "%s", "123456789-"); // ret = 9

      Reports: "9 Bytes actually written"
      Issue: Returns 9 on success AND failure (see above)

    ret = ssprintf (buf, 10, "%s", "123456789-"); // ret = -E2BIG

      Reports: "Data provided is too large to fit in the buffer"
      Issue: No tangible impact: No way to tell how much data was lost

[0] https://lwn.net/Articles/69419/
[1] https://github.com/KSPP/linux/issues/105
Signed-off-by: Lee Jones <lee@xxxxxxxxxx>
---
Changelog:

v1 => v2:
 - Address Rasmus Villemoes's review comments:
   - Remove explicit check for zero sized buffer (-E2BIG is appropriate)
   - Remove unreachable branch in vssprintf()

 include/linux/sprintf.h |  2 ++
 lib/vsprintf.c          | 51 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Crutcher Dunnavant <crutcher+kernel@xxxxxxxxxxxxxx>
Cc: Juergen Quade <quade@xxxxxxx>
Cc: David Laight <David.Laight@xxxxxxxxxx>

diff --git a/include/linux/sprintf.h b/include/linux/sprintf.h
index 33dcbec719254..2a3db6285492a 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 ssprintf(char *buf, size_t size, const char *fmt, ...);
+__printf(3, 0) int vssprintf(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 552738f14275a..e2b51fc625564 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2936,6 +2936,34 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args)
 }
 EXPORT_SYMBOL(vscnprintf);
 
+/**
+ * vssprintf - Format a string and place it in a buffer
+ * @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 number of characters which have been written into
+ * the @buf not including the trailing '\0' or -E2BIG if the string was
+ * truncated.
+ *
+ * If you're not already dealing with a va_list consider using ssprintf().
+ *
+ * See the vsnprintf() documentation for format string extensions over C99.
+ */
+int vssprintf(char *buf, size_t size, const char *fmt, va_list args)
+{
+	int i;
+
+	i = vsnprintf(buf, size, fmt, args);
+
+	if (likely(i < size))
+		return i;
+
+	return -E2BIG;
+}
+EXPORT_SYMBOL(vssprintf);
+
 /**
  * snprintf - Format a string and place it in a buffer
  * @buf: The buffer to place the result into
@@ -2987,6 +3015,29 @@ int scnprintf(char *buf, size_t size, const char *fmt, ...)
 }
 EXPORT_SYMBOL(scnprintf);
 
+/**
+ * ssprintf - Format a string and place it in a buffer
+ * @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 number of characters written into @buf not including
+ * the trailing '\0' or -E2BIG if the string was truncated.
+ */
+int ssprintf(char *buf, size_t size, const char *fmt, ...)
+{
+	va_list args;
+	int i;
+
+	va_start(args, fmt);
+	i = vssprintf(buf, size, fmt, args);
+	va_end(args);
+
+	return i;
+}
+EXPORT_SYMBOL(ssprintf);
+
 /**
  * vsprintf - Format a string and place it in a buffer
  * @buf: The buffer to place the result into
-- 
2.43.0.429.g432eaa2c6b-goog