linux-kernel - Re: [PATCH] sdhci: 'scratch' may be used uninitialized

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Date:	Sun, 5 Oct 2008 16:48:49 -0700
From:	"Steven Noonan" <steven@...inklabs.net>
To:	"Adrian Bunk" <bunk@...nel.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, drzeus@...eus.cx
Subject: Re: [PATCH] sdhci: 'scratch' may be used uninitialized

On Sun, Oct 5, 2008 at 4:16 PM, Adrian Bunk <bunk@...nel.org> wrote:
> On Sun, Oct 05, 2008 at 03:53:28PM -0700, Steven Noonan wrote:
>> On Sun, Oct 5, 2008 at 7:28 AM, Adrian Bunk <bunk@...nel.org> wrote:
>> > On Wed, Oct 01, 2008 at 01:50:25AM -0700, Steven Noonan wrote:
>> >> The variable 'scratch' is always initialized before it's used. The
>> >> conditional which is responsible for initialization of 'scratch' will
>> >> always evaluate 'true' when the first loop iteration occurs, and thus,
>> >> it's properly initialized. GCC doesn't see this, of course, so using
>> >> the uninitialized_var() macro seems to work for silencing this case.
>> >>
>> >> Signed-off-by: Steven Noonan <steven@...inklabs.net>
>> >> ---
>> >>  drivers/mmc/host/sdhci.c |    2 +-
>> >>  1 files changed, 1 insertions(+), 1 deletions(-)
>> >>
>> >> diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
>> >> index e3a8133..6257677 100644
>> >> --- a/drivers/mmc/host/sdhci.c
>> >> +++ b/drivers/mmc/host/sdhci.c
>> >> @@ -177,7 +177,7 @@ static void sdhci_read_block_pio(struct sdhci_host *host)
>> >>  {
>> >>       unsigned long flags;
>> >>       size_t blksize, len, chunk;
>> >> -     u32 scratch;
>> >> +     u32 uninitialized_var(scratch);
>> >>...
>> >
>> > With which gcc version?
>> >
>> > I'm not getting this warning with gcc 4.3, and IMHO it doesn't make
>> > sense to clutter the source code with such workarounds for older gcc
>> > versions (we officially support 6 years old compilers, and warning-free
>> > compilations with all of them are not reasonably possible).
>> >
>> > cu
>> > Adrian
>>
>> I've seen it on GCC 4.1 and 4.2. Since lots of distributions still
>> haven't marked GCC >4.1 stable, it makes sense to me to kill warnings
>> for GCC 4.1 and above. I don't know of any current distribution
>> releases using less than GCC 4.1 at the moment.
>
> It will clutter our code with these workarounds forever.
>
> And due to silencing these false warnings we will no longer get a
> warning when one of them becomes a real bug.
>
> Working on the remaining warnings that are visible with gcc 4.3 is a
> worthwhile goal, but I see no point for silencing some warnings that
> only occur with older gcc versions (especially as long as warnings
> that are present with all gcc versions stay unfixed).
>
I feel like there's a logical fallacy here. Sure, we can fix GCC 4.3
warnings, but what about when GCC 4.3 becomes an "old version"?
uninitialized_var and other such workarounds will still exist in the
code. It seems like the logical progression of your argument should be
to never fix false warnings.

- Steven
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