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Subject[PATCH 4.4 08/35] lib/string: Add strscpy_pad() function
Date
From: Tobin C. Harding <tobin@kernel.org>

[ Upstream commit 458a3bf82df4fe1f951d0f52b1e0c1e9d5a88a3b ]

We have a function to copy strings safely and we have a function to copy
strings and zero the tail of the destination (if source string is
shorter than destination buffer) but we do not have a function to do
both at once. This means developers must write this themselves if they
desire this functionality. This is a chore, and also leaves us open to
off by one errors unnecessarily.

Add a function that calls strscpy() then memset()s the tail to zero if
the source string is shorter than the destination buffer.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/string.h | 4 ++++
lib/string.c | 47 +++++++++++++++++++++++++++++++++++-------
2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index 870268d42ae7d..7da409760cf18 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -28,6 +28,10 @@ size_t strlcpy(char *, const char *, size_t);
#ifndef __HAVE_ARCH_STRSCPY
ssize_t strscpy(char *, const char *, size_t);
#endif
+
+/* Wraps calls to strscpy()/memset(), no arch specific code required */
+ssize_t strscpy_pad(char *dest, const char *src, size_t count);
+
#ifndef __HAVE_ARCH_STRCAT
extern char * strcat(char *, const char *);
#endif
diff --git a/lib/string.c b/lib/string.c
index 7f4baad6fb193..4351ec43cd6b8 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -157,11 +157,9 @@ EXPORT_SYMBOL(strlcpy);
* @src: Where to copy the string from
* @count: Size of destination buffer
*
- * Copy the string, or as much of it as fits, into the dest buffer.
- * The routine returns the number of characters copied (not including
- * the trailing NUL) or -E2BIG if the destination buffer wasn't big enough.
- * The behavior is undefined if the string buffers overlap.
- * The destination buffer is always NUL terminated, unless it's zero-sized.
+ * Copy the string, or as much of it as fits, into the dest buffer. The
+ * behavior is undefined if the string buffers overlap. The destination
+ * buffer is always NUL terminated, unless it's zero-sized.
*
* Preferred to strlcpy() since the API doesn't require reading memory
* from the src string beyond the specified "count" bytes, and since
@@ -171,8 +169,10 @@ EXPORT_SYMBOL(strlcpy);
*
* Preferred to strncpy() since it always returns a valid string, and
* doesn't unnecessarily force the tail of the destination buffer to be
- * zeroed. If the zeroing is desired, it's likely cleaner to use strscpy()
- * with an overflow test, then just memset() the tail of the dest buffer.
+ * zeroed. If zeroing is desired please use strscpy_pad().
+ *
+ * Return: The number of characters copied (not including the trailing
+ * %NUL) or -E2BIG if the destination buffer wasn't big enough.
*/
ssize_t strscpy(char *dest, const char *src, size_t count)
{
@@ -259,6 +259,39 @@ char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
}
EXPORT_SYMBOL(stpcpy);

+/**
+ * strscpy_pad() - Copy a C-string into a sized buffer
+ * @dest: Where to copy the string to
+ * @src: Where to copy the string from
+ * @count: Size of destination buffer
+ *
+ * Copy the string, or as much of it as fits, into the dest buffer. The
+ * behavior is undefined if the string buffers overlap. The destination
+ * buffer is always %NUL terminated, unless it's zero-sized.
+ *
+ * If the source string is shorter than the destination buffer, zeros
+ * the tail of the destination buffer.
+ *
+ * For full explanation of why you may want to consider using the
+ * 'strscpy' functions please see the function docstring for strscpy().
+ *
+ * Return: The number of characters copied (not including the trailing
+ * %NUL) or -E2BIG if the destination buffer wasn't big enough.
+ */
+ssize_t strscpy_pad(char *dest, const char *src, size_t count)
+{
+ ssize_t written;
+
+ written = strscpy(dest, src, count);
+ if (written < 0 || written == count - 1)
+ return written;
+
+ memset(dest + written + 1, 0, count - written - 1);
+
+ return written;
+}
+EXPORT_SYMBOL(strscpy_pad);
+
#ifndef __HAVE_ARCH_STRCAT
/**
* strcat - Append one %NUL-terminated string to another
--
2.27.0


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