linux-kernel - Re: [PATCH] fs/btrfs: Fix uninitialized variable

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Message-ID: <20210503084949.GA27017@ard0534>
Date:   Mon, 3 May 2021 09:49:49 +0100
From:   Khaled Romdhani <khaledromdhani216@...il.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     clm@...com, josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] fs/btrfs: Fix uninitialized variable

On Sun, May 02, 2021 at 12:17:51PM +0200, Christophe JAILLET wrote:
> Le 02/05/2021 à 00:50, Khaled ROMDHANI a écrit :
> > Fix the warning: variable 'zone' is used
> > uninitialized whenever '?:' condition is true.
> > 
> > Fix that by preventing the code to reach
> > the last assertion. If the variable 'mirror'
> > is invalid, the assertion fails and we return
> > immediately.
> > 
> > Reported-by: kernel test robot <lkp@...el.com>
> > Signed-off-by: Khaled ROMDHANI <khaledromdhani216@...il.com>
> > ---
> >   fs/btrfs/zoned.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> > index 8250ab3f0868..23da9d8dc184 100644
> > --- a/fs/btrfs/zoned.c
> > +++ b/fs/btrfs/zoned.c
> > @@ -145,7 +145,7 @@ static inline u32 sb_zone_number(int shift, int mirror)
> >   	case 2: zone = 1ULL << (BTRFS_SB_LOG_SECOND_SHIFT - shift); break;
> >   	default:
> >   		ASSERT((u32)mirror < 3);
> > -		break;
> > +		return 0;
> >   	}
> >   	ASSERT(zone <= U32_MAX);
> > 
> > base-commit: b5c294aac8a6164ddf38bfbdd1776091b4a1eeba
> > 
> Hi,
> 
> just a few comments.
> 
> If I understand correctly, what you try to do is to silence a compiler
> warning if no case branch is taken.
> 
> First, all your proposals are based on the previous one.
> I find it hard to follow because we don't easily see what are the
> differences since the beginning.
> 
> The "base-commit" at the bottom of your mail, is related to your own local
> tree, I guess. It can't be used by any-one.
> 
> My understanding it that a patch, should it be v2, v3..., must apply to the
> current tree. (In my case, it is the latest linux-next)
> This is not the case here and you have to apply each step to see the final
> result.
> 
> Should this version be fine, a maintainer wouldn't be able to apply it
> as-is.
> 
> You also try to take into account previous comments to check for incorrect
> negative values for minor and catch (the can't happen today) cases, should
> BTRFS_SUPER_MIRROR_MAX change and this function remain the same.
> 
> So, why hard-coding '3'?
> The reason of magic numbers are hard to remember. You should avoid them or
> add a comment about it.
> 
> My own personal variation would be something like the code below (untested).
> 
> Hope this helps.
> 
> CJ
> 
> 
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 70b23a0d03b1..75fe5f001d8b 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -138,11 +138,14 @@ static inline u32 sb_zone_number(int shift, int
> mirror)
>  {
>  	u64 zone;
> 
> -	ASSERT(mirror < BTRFS_SUPER_MIRROR_MAX);
> +	ASSERT(mirror >= 0 && mirror < BTRFS_SUPER_MIRROR_MAX);
>  	switch (mirror) {
>  	case 0: zone = 0; break;
>  	case 1: zone = 1ULL << (BTRFS_SB_LOG_FIRST_SHIFT - shift); break;
>  	case 2: zone = 1ULL << (BTRFS_SB_LOG_SECOND_SHIFT - shift); break;
> +	default:
> +		ASSERT(! "mirror < BTRFS_SUPER_MIRROR_MAX but not handled above.");
> +		return 0;
>  	}
> 
>  	ASSERT(zone <= U32_MAX);
>

Thank you for all of your comments. Yes, of course, they will help me.
I will try to handle that more properly.
Thanks again.