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Date:   Wed, 3 Mar 2021 11:44:22 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Colin Ian King <colin.king@canonical.com>
Cc:     Changman Lee <cm224.lee@samsung.com>,
        Chao Yu <chao2.yu@samsung.com>,
        linux-f2fs-devel@lists.sourceforge.net,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: f2fs_convert_inline_inode causing rebalance based on random
 uninitialized value in dn.node_changed
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On 03/02, Colin Ian King wrote:
> Hi,
> 
> Static analysis on linux-next detected a potential uninitialized
> variable dn.node_changed that does not get set when a call to
> f2fs_get_node_page() fails.  This uninitialized value gets used in the
> call to f2fs_balance_fs() that may or not may not balances dirty node
> and dentry pages depending on the uninitialized state of the variable.
> 
> I believe the issue was introduced by commit:
> 
> commit 2a3407607028f7c780f1c20faa4e922bf631d340
> Author: Jaegeuk Kim <jaegeuk@kernel.org>
> Date:   Tue Dec 22 13:23:35 2015 -0800
> 
>     f2fs: call f2fs_balance_fs only when node was changed
> 
> 
> The analysis is a follows:
> 
> 184 int f2fs_convert_inline_inode(struct inode *inode)
> 185 {
> 186        struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> 
>    1. var_decl: Declaring variable dn without initializer.
> 
> 187        struct dnode_of_data dn;
> 
>    NOTE dn is not initialized here.
> 
> 188        struct page *ipage, *page;
> 189        int err = 0;
> 190
> 
>    2. Condition !f2fs_has_inline_data(inode), taking false branch.
>    3. Condition f2fs_hw_is_readonly(sbi), taking false branch.
>    4. Condition f2fs_readonly(sbi->sb), taking false branch.
> 
> 191        if (!f2fs_has_inline_data(inode) ||
> 192                        f2fs_hw_is_readonly(sbi) ||
> f2fs_readonly(sbi->sb))
> 193                return 0;
> 194
> 195        err = dquot_initialize(inode);
> 
>    5. Condition err, taking false branch.
> 
> 196        if (err)
> 197                return err;
> 198
> 199        page = f2fs_grab_cache_page(inode->i_mapping, 0, false);
> 
>    6. Condition !page, taking false branch.
> 
> 200        if (!page)
> 201                return -ENOMEM;
> 202
> 203        f2fs_lock_op(sbi);
> 204
> 205        ipage = f2fs_get_node_page(sbi, inode->i_ino);
> 
>    7. Condition IS_ERR(ipage), taking true branch.
> 
> 206        if (IS_ERR(ipage)) {
> 207                err = PTR_ERR(ipage);
> 
>    8. Jumping to label out.
> 
> 208                goto out;
> 209        }
> 210
> 
>    NOTE: set_new_dnode memset's dn so sets the flag to false, but we
> don't get to this memset if IS_ERR(ipage) above is true.
> 
> 211        set_new_dnode(&dn, inode, ipage, ipage, 0);
> 212
> 213        if (f2fs_has_inline_data(inode))
> 214                err = f2fs_convert_inline_page(&dn, page);
> 215
> 216        f2fs_put_dnode(&dn);
> 217 out:
> 218        f2fs_unlock_op(sbi);
> 219
> 220        f2fs_put_page(page, 1);
> 221
> 
> Uninitialized scalar variable:
> 
>    9. uninit_use_in_call: Using uninitialized value dn.node_changed when
> calling f2fs_balance_fs.
> 
> 222        f2fs_balance_fs(sbi, dn.node_changed);
> 223
> 224        return err;
> 225 }
> 
> I think a suitable fix will be to set dn.node_changed to false on in
> line 207-208 but I'm concerned if I'm missing something subtle to the
> rebalancing if I do this.
> 
> Comments?

Thank you for the report. Yes, it seems that's a right call and we need to
check the error to decide calling f2fs_balance_fs() in line 222, since
set_new_dnode() is used to set all the fields in dnode_of_data. So, if you
don't mind, could you please post a patch?

Thanks,

> 
> Colin
>