Re: f2fs_convert_inline_inode causing rebalance based on random uninitialized value in dn.node_changed

[Jaegeuk Kim]

On 03/03, Colin Ian King wrote:
> On 03/03/2021 19:44, Jaegeuk Kim wrote:
> > 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@xxxxxxxxxx>
> >> 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?
> 
> Just to clarify, just setting dn.node_changes to false is enough?
> 
> I'm not entirely sure what you meant when you wrote "and we need to
> check the error to decide calling f2fs_balance_fs() in line 222".

I meant:

222	if (!err)
223		f2fs_balance_fs(sbi, dn.node_changed);

Thanks,

> 
> Colin
> 
> > 
> > Thanks,
> > 
> >>
> >> Colin
> >>