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

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Message-ID: <CAGXu5jKeS3bkTSXEw2nsjrWoqhbkDZD4C=PdYTHMtM0GNk6tSg@mail.gmail.com>
Date:   Wed, 20 Feb 2019 17:05:10 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     "Tobin C. Harding" <tobin@...nel.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>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Daniel Micay <danielmicay@...il.com>
Subject: Re: [PATCH 5/6] lib: Fix function documentation for strncpy_from_user

On Mon, Feb 18, 2019 at 4:52 PM Jann Horn <jannh@...gle.com> wrote:
> 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?

Oh, whew, there is only 1 arch-specific implementation of this. I
thought you meant there was multiple implementations.

So, generally speaking, I'd love to split all strncpy* uses into
strscpy_zero() (when expecting to do str->str copies), and some new
function, named like mempadstr() or str2mem() that copies a str to a
__nonstring char array, with trailing padding, if there is space. Then
there is no more mixing the two cases and botching things.

-- 
Kees Cook