linux-kernel - Re: [PATCH] btrfs: Initialize ret to 0 in scrub_simple

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Date:   Tue, 22 Feb 2022 16:04:10 +0800
From:   Qu Wenruo <wqu@...e.com>
To:     Souptick Joarder <jrdr.linux@...il.com>,
        Qu Wenruo <quwenruo.btrfs@....com>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        dsterba@...e.com, nathan@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] btrfs: Initialize ret to 0 in scrub_simple_mirror()



On 2022/2/22 15:50, Souptick Joarder wrote:
> On Mon, Feb 21, 2022 at 5:46 AM Qu Wenruo <quwenruo.btrfs@....com> wrote:
>>
>>
>>
>> On 2022/2/20 22:46, Souptick Joarder wrote:
>>> From: "Souptick Joarder (HPE)" <jrdr.linux@...il.com>
>>>
>>> Kernel test robot reported below warning ->
>>> fs/btrfs/scrub.c:3439:2: warning: Undefined or garbage value
>>> returned to caller [clang-analyzer-core.uninitialized.UndefReturn]
>>>
>>> Initialize ret to 0.
>>>
>>> Reported-by: kernel test robot <lkp@...el.com>
>>> Signed-off-by: Souptick Joarder (HPE) <jrdr.linux@...il.com>
>>
>> Although the patch is not yet merged, but I have to say, it's a false alert.
> 
> Yes, I agree it is a false positive but this patch will at least keep
> kernel test robot happy :)

I'd say we should enhance the compiler to fix the false alert.

Thus adding LLVM list here is correct.


To me, the root problem is that, we lack the hint to allow clang to know 
that, @logical_length passed in would not cause u64 overflow.

Unfortunately the sanity check to prevent overflow is hidden far away 
inside tree-checker.c.

Maybe some ASSERT() for overflow check would help LLVM to know that?

Thanks,
Qu

>>
>> Firstly, the while loop will always get at least one run.
>>
>> Secondly, in that loop, we either set ret to some error value and break,
>> or after at least one find_first_extent_item() and scrub_extent() call,
>> we increase cur_logical and reached the limit of the while loop and exit.
>>
>> So there is no possible routine to leave @ret uninitialized and returned
>> to caller.
>>
>> Thanks,
>> Qu
>>
>>> ---
>>>    fs/btrfs/scrub.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
>>> index 4baa8e43d585..5ca7e5ffbc96 100644
>>> --- a/fs/btrfs/scrub.c
>>> +++ b/fs/btrfs/scrub.c
>>> @@ -3325,7 +3325,7 @@ static int scrub_simple_mirror(struct scrub_ctx *sctx,
>>>        const u32 max_length = SZ_64K;
>>>        struct btrfs_path path = {};
>>>        u64 cur_logical = logical_start;
>>> -     int ret;
>>> +     int ret = 0;
>>>
>>>        /* The range must be inside the bg */
>>>        ASSERT(logical_start >= bg->start &&
>