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

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Date:   Tue, 22 Feb 2022 13:39:45 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Qu Wenruo <wqu@...e.com>
Cc:     Qu Wenruo <quwenruo.btrfs@....com>, 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 Tue, Feb 22, 2022 at 1:34 PM Qu Wenruo <wqu@...e.com> wrote:
>
>
>
> 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.

Hmm, kernel test robot reported similar false alert in few other
modules as well.

>
>
> 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 &&
> >
>