Date: Wed, 20 Feb 2019 08:52:45 +1100 From: "Tobin C. Harding" <me@...in.cc> To: Jann Horn <jannh@...gle.com> Cc: "Tobin C. Harding" <tobin@...nel.org>, 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 On Tue, Feb 19, 2019 at 01:51:45AM +0100, Jann Horn wrote: > +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? Thanks for the review Jaan. Seems I was in a world of confused, I misread count to be the size of the destination buffer not the size of the source string. Also, re-reading strscpy() and why its better I think I may not be the correct person to implement strscpy_from_user() since I'm not across all the subtleties. If v2 seems wanted for the first patches in this set I'll drop the *_from_user() stuff. thanks, Tobin.
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