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

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Message-ID: <YMtZcVy4gvmMtYv+@alley>
Date:   Thu, 17 Jun 2021 16:17:21 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Jia He <justin.he@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer
 check in two

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>

Best Regards,
Petr