Re: [djwong-xfs:realtime-rmap 245/249] fs/xfs/scrub/repair.c:2202:6: warning: variable 'is_free' is uninitialized when used here

[Nathan Chancellor <natechancellor@gmail.com>]

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On Tue, Dec 22, 2020 at 03:22:03AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git realtime-rmap
> head:   62eb0024d510607ecb04789e8b4f8c0975efb08b
> commit: f4188e5ab1984e1b140433a80f223ab4644801af [245/249] xfs: online repair of realtime file bmaps
> config: x86_64-randconfig-a011-20201217 (attached as .config)
> compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project cee1e7d14f4628d6174b33640d502bff3b54ae45)
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # install x86_64 cross compiling tool for clang build
>         # apt-get install binutils-x86-64-linux-gnu
>         # https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?id=f4188e5ab1984e1b140433a80f223ab4644801af
>         git remote add djwong-xfs https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git
>         git fetch --no-tags djwong-xfs realtime-rmap
>         git checkout f4188e5ab1984e1b140433a80f223ab4644801af
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=x86_64 
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
> 
> All warnings (new ones prefixed by >>):
> 
> >> fs/xfs/scrub/repair.c:2202:6: warning: variable 'is_free' is uninitialized when used here [-Wuninitialized]
>            if (is_free)
>                ^~~~~~~
>    fs/xfs/scrub/repair.c:2189:16: note: initialize the variable 'is_free' to silence this warning
>            bool                    is_free;
>                                           ^
>                                            = 0
>    1 warning generated.
> 
> 
> vim +/is_free +2202 fs/xfs/scrub/repair.c
> 
>   2173	
>   2174	/*
>   2175	 * Check if any part of this range of rt blocks is free, so that we don't
>   2176	 * rebuild things with bad records.  Returns -EFSCORRUPTED if bad.
>   2177	 */
>   2178	int
>   2179	xrep_rtext_is_free(
>   2180		struct xfs_scrub	*sc,
>   2181		xfs_rtblock_t		rtbno,
>   2182		xfs_filblks_t		len)
>   2183	{
>   2184		struct xfs_mount	*mp = sc->mp;
>   2185		xfs_rtblock_t		startext;
>   2186		xfs_rtblock_t		endext;
>   2187		xfs_rtblock_t		extcount;
>   2188		uint32_t		mod;
>   2189		bool			is_free;
>   2190		int			error;
>   2191	
>   2192		/* Convert rt blocks to rt extents. */
>   2193		startext = div_u64_rem(rtbno, mp->m_sb.sb_rextsize, &mod);
>   2194		endext = div_u64_rem(rtbno + len - 1, mp->m_sb.sb_rextsize, &mod);
>   2195	
>   2196		/* Make sure this isn't free space. */
>   2197		extcount = endext - startext + 1;
>   2198		error = xfs_rtalloc_extent_is_free(mp, sc->tp, startext, extcount,
>   2199				&is_free);
>   2200		if (error)
>   2201			return error;
> > 2202		if (is_free)
>   2203			return -EFSCORRUPTED;
>   2204	
>   2205		return 0;
>   2206	}
>   2207	
> 
> ---
> 0-DAY CI Kernel Test Service, Intel Corporation
> https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org
> 

This appears to be a false positive. From the config:

# CONFIG_XFS_RT is not set

which means xfs_rtalloc_extent_is_free evaluates to ENOSYS and we never
actually make it to that if statement evaluation; however, clang cannot
see this because it is doing simple static analysis at this stage of the
pipeline, not seeing that it will never have to use is_free in this
configuration. A simple reproducer:

$ cat test.c
#define func(a) 22

int main() {
    int error;
    _Bool is_free;

    error = func(&is_free);
    if (error)
        return error;
    if (is_free)
        return -117;

    return 0;
}

$ clang -Wuninitialized -fsyntax-only test.c
test.c:10:9: warning: variable 'is_free' is uninitialized when used here [-Wuninitialized]
    if (is_free)
        ^~~~~~~
test.c:5:18: note: initialize the variable 'is_free' to silence this warning
    _Bool is_free;
                 ^
                  = 0
1 warning generated.

Obviously initializing is_free to false will avoid this warning, which
is probably the simplest fix.

Cheers,
Nathan