linux-kernel - Re: [PATCH 5/6] lib: Fix function documentation for strncpy_from

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Message-ID: <CAG48ez0utQJjoJW69OwGiuH1EQtcD939-5u71ruD_1nr0eJK0g@mail.gmail.com>
Date:   Tue, 19 Feb 2019 01:51:45 +0100
From:   Jann Horn <jannh@...gle.com>
To:     "Tobin C. Harding" <tobin@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, Shuah Khan <shuah@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 5/6] lib: Fix function documentation for strncpy_from_user

+cc Andy because he's also preparing a patch for this function

On Tue, Feb 19, 2019 at 12:25 AM Tobin C. Harding <tobin@...nel.org> wrote:
> Current function documentation for strncpy_from_user() is incorrect.  If
> @count (size of destination buffer) is less than the length of the user
> string the function does _not_ return @count but rather returns -EFAULT.
>
> Document correctly the function return value, also add note that string
> may be left non-null terminated.
>
> Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> ---
>  lib/strncpy_from_user.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index 58eacd41526c..11fe9a4a00fd 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -82,22 +82,19 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
>  }
>
>  /**
> - * strncpy_from_user: - Copy a NUL terminated string from userspace.
> + * strncpy_from_user() - Copy a NUL terminated string from userspace.
>   * @dst:   Destination address, in kernel space.  This buffer must be at
>   *         least @count bytes long.
>   * @src:   Source address, in user space.
> - * @count: Maximum number of bytes to copy, including the trailing NUL.
> + * @count: Maximum number of bytes to copy, including the trailing %NUL.
>   *
>   * Copies a NUL-terminated string from userspace to kernel space.
>   *
> - * On success, returns the length of the string (not including the trailing
> - * NUL).
> - *
> - * If access to userspace fails, returns -EFAULT (some data may have been
> - * copied).
> - *
> - * If @count is smaller than the length of the string, copies @count bytes
> - * and returns @count.
> + * Return: If access to userspace fails, returns -EFAULT.  Otherwise,
> + *         return the number of characters copied excluding the trailing
> + *         %NUL, if the length of the user string exceeds @count return
> + *         -EFAULT (in which case @dst will be left without a %NUL
> + *         terminator).
>   */

AFAICS the byte_at_a_time loop exits when max==0 is reached, and then
if `res >= count` (in other words, if we've copied as many bytes as
requested, haven't encountered a null byte so far, and haven't reached
the end of the address space), we return `res`, which is the same as
`count`. Are you sure?