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Date:   Fri, 30 Oct 2020 17:26:25 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Shuah Khan <shuah@kernel.org>, Kees Cook <keescook@chromium.org>,
        Willy Tarreau <w@1wt.eu>, linux-kselftest@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Arpitha Raghunandan <98.arpi@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Brendan Higgins <brendanhiggins@google.com>
Subject: Re: [PATCH 4/4] lib/test_printf.c: use deterministic sequence of
 random numbers
Message-ID: <20201030162625.GE20201@alley>
References: <20201025214842.5924-1-linux@rasmusvillemoes.dk>
 <20201025214842.5924-5-linux@rasmusvillemoes.dk>
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On Sun 2020-10-25 22:48:42, Rasmus Villemoes wrote:
> The printf test suite does each test with a few different buffer sizes
> to ensure vsnprintf() behaves correctly with respect to truncation and
> size reporting. It calls vsnprintf() with a buffer size that is
> guaranteed to be big enough, a buffer size of 0 to ensure that nothing
> gets written to the buffer, but it also calls vsnprintf() with a
> buffer size chosen to guarantee the output gets truncated somewhere in
> the middle.
> 
> That buffer size is chosen randomly to increase the chance of finding
> some corner case bug (for example, there used to be some %p<foo>
> extension that would fail to produce any output if there wasn't room
> enough for it all, despite the requirement of producing as much as
> there's room for). I'm not aware of that having found anything yet,
> but should it happen, it's annoying not to be able to repeat the
> test with the same sequence of truncated lengths.
> 
> For demonstration purposes, if we break one of the test cases
> deliberately, we still get different buffer sizes if we don't pass the
> seed parameter:
> 
> root@(none):/# modprobe test_printf
> [   15.317783] test_printf: vsnprintf(buf, 18, "%piS|%pIS", ...) wrote '127.000.000.001|1', expected '127-000.000.001|1'
> [   15.323182] test_printf: failed 3 out of 388 tests
> [   15.324034] test_printf: random seed used was 0x278bb9311979cc91
> modprobe: ERROR: could not insert 'test_printf': Invalid argument
> 
> root@(none):/# modprobe test_printf
> [   13.940909] test_printf: vsnprintf(buf, 22, "%piS|%pIS", ...) wrote '127.000.000.001|127.0', expected '127-000.000.001|127.0'
> [   13.944744] test_printf: failed 3 out of 388 tests
> [   13.945607] test_printf: random seed used was 0x9f72eee1c9dc02e5
> modprobe: ERROR: could not insert 'test_printf': Invalid argument
> 
> but to repeat a specific sequence of tests, we can do
> 
> root@(none):/# modprobe test_printf seed=0x9f72eee1c9dc02e5
> [  448.328685] test_printf: vsnprintf(buf, 22, "%piS|%pIS", ...) wrote '127.000.000.001|127.0', expected '127-000.000.001|127.0'
> [  448.331650] test_printf: failed 3 out of 388 tests
> [  448.332295] test_printf: random seed used was 0x9f72eee1c9dc02e5
> modprobe: ERROR: could not insert 'test_printf': Invalid argument
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Great feature!

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr