From: Amir Goldstein <firstname.lastname@example.org> To: Jan Kara <email@example.com> Cc: Ext4 <firstname.lastname@example.org>, email@example.com Subject: Re: Data exposure on IO error Date: Sat, 1 Aug 2020 10:32:53 +0300 Message-ID: <CAOQ4uxgovoBjs5BnYdPyV6K9AP17fCaeVgZ=wQMfx4hAuAf5RQ@mail.gmail.com> (raw) In-Reply-To: <20200731225621.GA7126@quack2.suse.cz> On Sat, Aug 1, 2020 at 1:59 AM Jan Kara <firstname.lastname@example.org> wrote: > > Hello! > > In bug 207729, Anthony reported a bug that can actually lead to a stale > data exposure on IO error. The problem is relatively simple: Suppose we > do: > > fd = open("file", O_WRONLY | O_CREAT | O_TRUNC, 0644); > write(fd, buf, 4096); > fsync(fd); > > And IO error happens when fsync writes the block of "file". The IO error > gets properly reported to userspace but otherwise the filesystem keeps > running. So the transaction creating "file" and allocating block to it can > commit. Then when page cache of "file" gets evicted, the user can read > stale block contents (provided the IO error was just temporary or involving > only writes). > > Now I understand in face of IO errors the behavior is really undefined but > potential exposure of stale data seems worse than strictly necessary. Also > if we run in data=ordered mode, especially if also data_err=abort is set, > user would rightfully expect that the filesystem gets aborted when such IO > error happens but that's not the case. Generally data_err=abort seems a bit > misnamed (and the manpage is wrong about this mount option) since what it > really does is that if jbd2 thread encounters error when writing back > ordered data, the filesystem is aborted. However the ordered data can be > written back by other processes as well and in that case the error is just > lost / reported to userspace but the filesystem doesn't get aborted. > > As I was thinking about it, it seems to me that in data=ordered mode, we > should just always abort the filesystem when writeback of newly allocated > block fails to avoid the stale data exposure mentioned above. And then, we > could just deprecate data_err= mount option because it wouldn't be any > useful anymore... What do people think? > It sounds worse than strictly necessary. In what way is that use case different from writing into a punched hole in the middle of the file and getting an IO error on writeback? It looks like ext4 already goes into a great deal of trouble to handle extent conversion to init at io end. So couldn't the described case be handled as a private case of filling a hole at the end of the file? Am I missing something beyond the fact that traditionally, extending a file enjoyed the protection of i_disksize, so did not need to worry about unwritten extents? Thanks, Amir.
next prev parent reply index Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-07-31 22:56 Jan Kara 2020-08-01 7:32 ` Amir Goldstein [this message] 2020-08-03 7:57 ` Jan Kara
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