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

From: Geert Uytterhoeven
Date: Tue Jan 25 2011 - 15:45:11 EST

On Tue, Jan 25, 2011 at 19:57, Phillip Lougher
<phillip@xxxxxxxxxxxxxxxxxxx> wrote:
> Geert Uytterhoeven wrote:
>> On Tue, Jan 25, 2011 at 02:33, Phillip Lougher
>> <phillip@xxxxxxxxxxxxxxxxxxx> 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.



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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.
             Â Â -- Linus Torvalds
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