| Message ID | 20190306214226.14598-7-tobin@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | lib/string: Add strscpy_pad() function | expand |
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 >
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
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(-)