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Re: [Qemu-devel] [PATCH 23/25] fdc: Move floppy geometry guessing back f

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 23/25] fdc: Move floppy geometry guessing back from block.c
Date: Mon, 09 Jul 2012 12:01:10 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 07/09/2012 11:46 AM, Eric Blake wrote:
On 07/09/2012 10:07 AM, Markus Armbruster wrote:

This is an unconditional use of fd_type[0].  If floppy == NULL, this is
dereferencing an uninitialized value.

I'm not sure why the explicit initialization was removed...

Brain fart on my part, sorry.  The old loop assigns only if the drive
exists.  The new loop assigns unconditionally.  Except the whole loop is
still conditional.

Testing can't flag this, because floppy is never null.

Looks broken indeed. I just wonder why my gcc (or the buildbots) didn't

Me too.  Looks like I should upgrade to a more recent gcc.

It's probably not the version of the gcc you used, but whether or not
your CFLAGS include -O2.  Gcc has the (IMO very annoying) limitation
that uninitialized-use analysis can only be performed if you are also
doing optimization.  You have to use a tool like clang or Coverity if
you want more reliable uninitialized-use analysis even while building
-O0 debug images.

Specifically, without -O, GCC doesn't do data flow analysis so any warning that requires DFA won't get triggered.

So in general, if you are normally building with -O0, make sure to also build with -O in order to get full warnings.


Anthony Liguori

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