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Date:   Tue, 26 Feb 2019 13:07:20 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Changbin Du <changbin.du@gmail.com>,
        Jann Horn <jannh@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Nadav Amit <namit@vmware.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH 2/4] uaccess: Add non-pagefault user-space read
 functions
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Hi Kees,

On Mon, 25 Feb 2019 09:06:55 -0800
Kees Cook <keescook@chromium.org> wrote:

> On Mon, Feb 25, 2019 at 6:06 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > +static __always_inline long strncpy_from_unsafe_common(char *dst,
> > +                               const char __user *unsafe_addr, long count)
> > +{
> > +       const char __user *src = unsafe_addr;
> > +       int ret;
> > +
> > +       pagefault_disable();
> > +       do {
> > +               ret = __get_user(*dst++, src++);
> > +       } while (dst[-1] && ret == 0 && src - unsafe_addr < count);
> > +       dst[-1] = '\0';
> > +       pagefault_enable();
> > +
> > +       return ret ? -EFAULT : src - unsafe_addr;
> > +}
> 
> I'm all for always NUL-truncating, but this isn't "strncpy" (which has
> the buggy maybe-I-didn't-NUL-terminate behavior). Can we call this
> strscpy_...() instead?

Yes, it is easy to me to fit it to strscpy spec and caller side too.
But if we reuse strncpy_from_user() as Linus suggested, it may be better
keep it or write a wrapper, since this function spec is still a bit
different from strscpy (this doesn't return -E2BIG but returns the
copied length of the string with NULL terminal byte).

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>