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Message-ID: <4febf11b-31ea-82a1-bf08-b6bebe08bc75@huawei.com> Date: Wed, 11 Jan 2017 17:07:29 +0800 From: "zhangyi (F)" <yi.zhang@...wei.com> To: "Theodore Ts'o" <tytso@....edu> CC: "Darrick J. Wong <darrick.wong@...cle.com>; Jan Kara" <jack@...e.cz>, <Valdis.Kletnieks@...edu>, <linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>, <adilger.kernel@...ger.ca> Subject: Re: [RFC PATCH] ext4: increase the protection of drop nlink and ext4 inode destroy on 2017/1/5 7:35, Theodore Ts'o wrote: > On Wed, Jan 04, 2017 at 01:54:24PM -0800, Darrick J. Wong wrote: >> >> if (inode->i_nlink == 0) { >> ext4_warning_inode(inode, "nlink is already 0"); >> return; >> } > > We can't do that because the place where Zhangyi is proposing to > change is in fs/inode.c:drop_nlink(), so we can't add a call to > ext4_error() or ext4_warning(). > > So how exactly how did we get into this state? When we read the inode > into memory, if i_nlink is zero, we declare the file system as > corrupted immediately. > > So I assume this is happening the on-disk i_links_count (which is read > into inode->i_nlink) was too low. So I think the way we should be > handling this is in unlink and rename, before we let i_nlink drop to > zero, we need to check to see if there are other dcache entries > pointing at the inode. If so, we need to call ext4_error(), and in > the errors=continue case, return EFSCORRUPTED (aka EUCLEAN). > > - Ted > Hi Theodore: The i_nlink underflow and memory corruption problem on ext4fs remains inconclusive. You suggest we can check dcache entries when i_nlink drop to zero in unlink and rename. But I think it may still have some problems, assume the following situation: (1) The file we want to unlink have many hard links, but only one dcache entry in memory. (2) open this file, but it's inode->i_nlink read from disk was 1 (too low). (3) some one call rename and drop it's i_nlink to zero. (4) it's inode is still in use and do not destroy (not closed), at the same time, some others open it's hard link and create a dcache entry. (5) call rename again and it's i_nlink will still underflow and cause memory corruption. For simplicity, I think we can add underflow protection in ext4_rename or drop_nlink as V2 and V3 patch wrote. What do you think? yi zhang -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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