Re: [patch] fs: revert 8ab22b9a

Hisashi Hifumi
2008-09-11T05:31:45Z

At 19:19 08/09/10, Nick Piggin wrote:
>On Wed, Sep 10, 2008 at 05:47:00PM +0900, Hisashi Hifumi wrote:
>>
>> At 13:52 08/09/10, Nick Piggin wrote:
>> >
>> >Patch 8ab22b9a, "vfs: pagecache usage optimization for pagesize!=blocksize",
>> >introduces a data race that might cause uninitialized data to be exposed to
>> >userland. The race is conceptually the same as the one fixed for page
>> >uptodateness, fixed by 0ed361de.
>> >
>> >The problem is that a buffer_head flags will be set uptodate after the
>> >stores to bring its pagecache data uptodate[*]. This patch introduces a
>> >possibility to read that pagecache data if the buffer_head flag has been
>> >found uptodate. The problem is there are no barriers or locks ordering
>> >the store/store vs the load/load.
>> >
>> >To illustrate:
>> > CPU0: write(2) (1024 bytes) CPU1: read(2) (1024 bytes)
>> > 1. allocate new pagecache page A. locate page, not fully uptodate
>> > 2. copy_from_user to part of page B. partially uptodate? load bh flags
>> > 3. mark that buffer uptodate C. if yes, then copy_to_user
>> >
>> >So if the store 3 is allowed to execute before the store 2, and/or the
>> >load in C is allowed to execute before the load in B, then we can wind
>> >up loading !uptodate data.
>> >
>>
>> >
>> >One way to solve this is to add barriers to the buffer head operations
>> >similarly to the fix for the page issue. The problem is that, unlike the
>> >page race, we don't actually *need* to do that if we decide not to support
>> >this functionality. The barriers are quite heavyweight on some
>> >architectures, and we haven't seen really compelling numbers in favour of
>> >this patch yet (a best-case microbenchmark showed some improvement of
>> >course, but with memory barriers we could also produce a worst-case bench
>> >that shows some slowdown on many architectures).
>>
>> I think that adding wmb/rmb to all buffer_uptodate/set_buffer_uptodate is heavy
>> on some architectures using BUFFER_FNS macros, but it can be possible
>> to mitigate performance slowdown by minimizing memory barrier utilization.
>> The patch "vfs: pagecache usage optimization for pagesize!=blocksize" is now
>> just for ext2/3/4, so is it not sufficient to solve the above
>uninitialized data
>> exposure problem that adding one rmb to block_is_partially_uptodate()
>> and wmb to __block_commit_write() ?
>
>I guess it could be... if you have audited all those filesystems to ensure
>they don't set the buffer uptodate via any other paths.
>
>But still, forcing a wmb for everyone in the block path is... not so nice.
>As I said, I think the _best_ way to solve the problem is to ensure the
>buffer is only brought uptodate under the page lock, which will then give
>you serialisation against block_is_partially_uptodate (which is called with
>the page locked). If you are *sure* this is the case for ext2/3/4, then there
>should actually be no memory ordering problem in practice. You will have to
>document the API to say that users of it must obey that rule.
>

I again investigated write() path on ext2/3/4.
On these filesystems set_buffer_uptodate is done through __block_prepare_write
or __block_commit_write(), and is inside lock_page. buffer_uptodate test in
block_is_partially_uptodate and set_buffer_uptodate in __block_prepare_write or
__block_commit_write() are done inside lock_page, so I think these bitops
is serialized and there is no memory ordering problem regarding
buffer_uptodate/set_buffer_uptodate as far as ext2/3/4 is concerned.