linux-kernel - Re: [PATCH] Squashfs: Fix use of uninitialised variable in zlib & xz decompressors

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Date:	Tue, 25 Jan 2011 21:45:02 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Phillip Lougher <phillip@...gher.demon.co.uk>
Cc:	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	Jesper Juhl <jj@...osbits.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Squashfs: Fix use of uninitialised variable in zlib & xz decompressors

On Tue, Jan 25, 2011 at 19:57, Phillip Lougher
<phillip@...gher.demon.co.uk> wrote:
> Geert Uytterhoeven wrote:
>>
>> On Tue, Jan 25, 2011 at 02:33, Phillip Lougher
>> <phillip@...gher.demon.co.uk> wrote:
>>>
>>> Incidentally, on most architectures (bar Mips and Parisc), no
>>> uninitialised variable warning is generated by gcc, this is because
>>> the while condition test on continue is optimised out and not performed
>>> (when executing continue zlib_err has not been changed since entering the
>>> loop, and logically if the while condition was true previously, then it's
>>> still true).
>>
>> As this is a "do { ... } while (...);" construct and not a "while
>> (...) { ... }" construct,
>> the condition is not checked before doing the first iteration. Furthermore
>> the
>> "continue" may happen during the first iteration (this depends on
>> parameters
>> passed to the function), so the compiler cannot make any assumptions about
>> the
>> value of zlib_err, except that may be uninitialized.
>>
>
> No, I've checked the assembly language produced - the while condition test
> has
> been optimised out in the case of continue.
>
> This is the assembly language output for x86_64, the relevant
> bits manually annotated by me,

While the continue skips the test at the end of the loop, that doesn't
change the
fact that zlib_err was never initialized in the first place.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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