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

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Message-ID: <eba16312-9d50-9549-76c0-b0512a394669@wanadoo.fr>
Date:   Sun, 2 May 2021 12:17:51 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Khaled ROMDHANI <khaledromdhani216@...il.com>, clm@...com,
        josef@...icpanda.com, dsterba@...e.com
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] fs/btrfs: Fix uninitialized variable

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);