[v3,6/7] lib/string: Add strscpy_pad() function
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Message ID 20190306214226.14598-7-tobin@kernel.org
State New
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Series
  • lib/string: Add strscpy_pad() function
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Commit Message

Tobin C. Harding March 6, 2019, 9:42 p.m. UTC
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.

Signed-off-by: Tobin C. Harding <tobin@kernel.org>
---
 include/linux/string.h |  4 ++++
 lib/string.c           | 47 +++++++++++++++++++++++++++++++++++-------
 2 files changed, 44 insertions(+), 7 deletions(-)

Comments

Kees Cook April 2, 2019, 9:35 p.m. UTC | #1
On Wed, Mar 6, 2019 at 1:43 PM Tobin C. Harding <tobin@kernel.org> wrote:
>
> 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.
>
> Signed-off-by: Tobin C. Harding <tobin@kernel.org>

Lovely. :)

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  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 7927b875f80c..bfe95bf5d07e 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -31,6 +31,10 @@ 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 38e4ca08e757..3a3353512184 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -159,11 +159,9 @@ 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
> @@ -173,8 +171,10 @@ 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)
>  {
> @@ -237,6 +237,39 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
>  EXPORT_SYMBOL(strscpy);
>  #endif
>
> +/**
> + * 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
> --
> 2.20.1
>

Patch
diff mbox series

diff --git a/include/linux/string.h b/include/linux/string.h
index 7927b875f80c..bfe95bf5d07e 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -31,6 +31,10 @@  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 38e4ca08e757..3a3353512184 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -159,11 +159,9 @@  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
@@ -173,8 +171,10 @@  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)
 {
@@ -237,6 +237,39 @@  ssize_t strscpy(char *dest, const char *src, size_t count)
 EXPORT_SYMBOL(strscpy);
 #endif
 
+/**
+ * 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