linux-kernel - Re: [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer check in two

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Date:   Mon, 11 Jul 2022 15:08:55 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Jia He <justin.he@....com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>
Subject: Re: [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer
 check in two

On Fri 2022-06-17 09:15:53, Rasmus Villemoes wrote:
> On 17/06/2021 16.17, Petr Mladek wrote:
> > On Tue 2021-06-15 23:49:51, Jia He wrote:
> >> From: Rasmus Villemoes <linux@...musvillemoes.dk>
> >>
> >> Before each invocation of vsnprintf(), do_test() memsets the entire
> >> allocated buffer to a sentinel value. That buffer includes leading and
> >> trailing padding which is never included in the buffer area handed to
> >> vsnprintf (spaces merely for clarity):
> >>
> >>   pad  test_buffer      pad
> >>   **** **************** ****
> >>
> >> Then vsnprintf() is invoked with a bufsize argument <=
> >> BUF_SIZE. Suppose bufsize=10, then we'd have e.g.
> >>
> >>  |pad |   test_buffer    |pad |
> >>   **** pizza0 **** ****** ****
> >>  A    B      C    D           E
> >>
> >> where vsnprintf() was given the area from B to D.
> >>
> >> It is obviously a bug for vsnprintf to touch anything between A and B
> >> or between D and E. The former is checked for as one would expect. But
> >> for the latter, we are actually a little stricter in that we check the
> >> area between C and E.
> >>
> >> Split that check in two, providing a clearer error message in case it
> >> was a genuine buffer overrun and not merely a write within the
> >> provided buffer, but after the end of the generated string.
> >>
> >> So far, no part of the vsnprintf() implementation has had any use for
> >> using the whole buffer as scratch space, but it's not unreasonable to
> >> allow that, as long as the result is properly nul-terminated and the
> >> return value is the right one. However, it is somewhat unusual, and
> >> most %<something> won't need this, so keep the [C,D] check, but make
> >> it easy for a later patch to make that part opt-out for certain tests.
> > 
> > Excellent commit message.
> > 
> >> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> >> Tested-by: Jia He <justin.he@....com>
> >> Signed-off-by: Jia He <justin.he@....com>
> > 
> > Reviewed-by: Petr Mladek <pmladek@...e.com>
> 
> Hi Petr
> 
> It seems Justin's series got stalled, but I still think this patch makes
> sense on its own (especially since another series in flight mucks about
> in this area), so can you please pick it up directly?

I have just committed this patch into printk/linux.git, branch for-5.20.

I am sorry that it took so long. Too many things have happened during
last weeks.

Best Regards,
Petr