On Fri, Jun 14, 2019 at 9:48 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> The getxattr interface returns a length after filling out the value
> buffer, and the convention with xattrs is to not NULL terminate string
> data.
>
> CephFS implements some virtual xattrs by using snprintf to fill the
> buffer, but that always NULL terminates the string. If userland sends
> down a buffer that is just the right length to hold the text without
> termination then we end up truncating the value.
>
> Factor the formatting piece of vsnprintf into a separate helper
> function, and have vsnprintf call that and then do the NULL termination
> afterward. Then add a snprintf_noterm function that calls the new helper
> to populate the string but skips the termination.
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> include/linux/kernel.h | 2 +
> lib/vsprintf.c | 145 ++++++++++++++++++++++++++++-------------
> 2 files changed, 103 insertions(+), 44 deletions(-)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 2d14e21c16c0..2f305a347482 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -462,6 +462,8 @@ extern int num_to_str(char *buf, int size,
> extern __printf(2, 3) int sprintf(char *buf, const char * fmt, ...);
> extern __printf(2, 0) int vsprintf(char *buf, const char *, va_list);
> extern __printf(3, 4)
> +int snprintf_noterm(char *buf, size_t size, const char *fmt, ...);
> +extern __printf(3, 4)
> int snprintf(char *buf, size_t size, const char *fmt, ...);
> extern __printf(3, 0)
> int vsnprintf(char *buf, size_t size, const char *fmt, va_list args);
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 791b6fa36905..ad5f4990eda3 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2296,53 +2296,24 @@ set_precision(struct printf_spec *spec, int prec)
> }
>
> /**
> - * vsnprintf - Format a string and place it in a buffer
> + * vsnprintf_noterm - Format a string and place it in a buffer without NULL
> + * terminating it
> * @buf: The buffer to place the result into
> - * @size: The size of the buffer, including the trailing null space
> + * @end: The end of the buffer
> * @fmt: The format string to use
> * @args: Arguments for the format string
> *
> - * This function generally follows C99 vsnprintf, but has some
> - * extensions and a few limitations:
> - *
> - * - ``%n`` is unsupported
> - * - ``%p*`` is handled by pointer()
> - *
> - * See pointer() or Documentation/core-api/printk-formats.rst for more
> - * extensive description.
> - *
> - * **Please update the documentation in both places when making changes**
> - *
> - * The return value is the number of characters which would
> - * be generated for the given input, excluding the trailing
> - * '\0', as per ISO C99. If you want to have the exact
> - * number of characters written into @buf as return value
> - * (not including the trailing '\0'), use vscnprintf(). If the
> - * return is greater than or equal to @size, the resulting
> - * string is truncated.
> - *
> - * If you're not already dealing with a va_list consider using snprintf().
> + * See the documentation over vsnprintf. This function does NOT add any NULL
> + * termination to the buffer. The caller must do that if necessary.
> */
> -int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
> +static int vsnprintf_noterm(char *buf, char *end, const char *fmt,
> + va_list args)
> {
> unsigned long long num;
> - char *str, *end;
> + char *str;
> struct printf_spec spec = {0};
>
> - /* Reject out-of-range values early. Large positive sizes are
> - used for unknown buffer sizes. */
> - if (WARN_ON_ONCE(size > INT_MAX))
> - return 0;
> -
> str = buf;
> - end = buf + size;
> -
> - /* Make sure end is always >= buf */
> - if (end < buf) {
> - end = ((void *)-1);
> - size = end - buf;
> - }
> -
> while (*fmt) {
> const char *old_fmt = fmt;
> int read = format_decode(fmt, &spec);
> @@ -2462,18 +2433,69 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
> str = number(str, end, num, spec);
> }
> }
> -
> out:
> + /* the trailing null byte doesn't count towards the total */
> + return str-buf;
> +}
> +EXPORT_SYMBOL(vsnprintf_noterm);
export static function?
> +
> +/**
> + * vsnprintf - 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
> + *
> + * This function generally follows C99 vsnprintf, but has some
> + * extensions and a few limitations:
> + *
> + * - ``%n`` is unsupported
> + * - ``%p*`` is handled by pointer()
> + *
> + * See pointer() or Documentation/core-api/printk-formats.rst for more
> + * extensive description.
> + *
> + * **Please update the documentation in both places when making changes**
> + *
> + * The return value is the number of characters which would
> + * be generated for the given input, excluding the trailing
> + * '\0', as per ISO C99. If you want to have the exact
> + * number of characters written into @buf as return value
> + * (not including the trailing '\0'), use vscnprintf(). If the
> + * return is greater than or equal to @size, the resulting
> + * string is truncated.
> + *
> + * If you're not already dealing with a va_list consider using snprintf().
> + */
> +int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
> +{
> + int ret;
> + char *end;
> +
> + /* Reject out-of-range values early. Large positive sizes are
> + used for unknown buffer sizes. */
> + if (WARN_ON_ONCE(size > INT_MAX))
> + return 0;
> +
> + end = buf + size;
> +
> + /* Make sure end is always >= buf */
> + if (end < buf) {
> + end = ((void *)-1);
> + size = end - buf;
> + }
> +
> + ret = vsnprintf_noterm(buf, end, fmt, args);
> +
> + /* NULL terminate the result */
> if (size > 0) {
> - if (str < end)
> - *str = '\0';
> + if (ret < size)
> + buf[ret] = '\0';
> else
> - end[-1] = '\0';
> + buf[size - 1] = '\0';
> }
>
> - /* the trailing null byte doesn't count towards the total */
> - return str-buf;
> -
> + return ret;
> }
> EXPORT_SYMBOL(vsnprintf);
>
> @@ -2506,6 +2528,41 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args)
> }
> EXPORT_SYMBOL(vscnprintf);
>
> +/**
> + * snprintf_noterm - 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
> + *
> + * Same as snprintf, but don't NULL terminate the result.
> + */
> +int snprintf_noterm(char *buf, size_t size, const char *fmt, ...)
> +{
> + va_list args;
> + int ret;
> + char *end;
> +
> + /* Reject out-of-range values early. Large positive sizes are
> + used for unknown buffer sizes. */
> + if (WARN_ON_ONCE(size > INT_MAX))
> + return 0;
> +
> + /* Make sure end is always >= buf */
> + end = buf + size;
> + if (end < buf) {
> + end = ((void *)-1);
> + size = end - buf;
> + }
> +
> + va_start(args, fmt);
> + ret = vsnprintf_noterm(buf, end, fmt, args);
> + va_end(args);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL(snprintf_noterm);
> +
> /**
> * snprintf - Format a string and place it in a buffer
> * @buf: The buffer to place the result into
> --
> 2.21.0
>
