Re: [Xen-devel] [PATCH] Fix blkback/blktap sysfs read bug.

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> "Joe Jin" <joe.jin@xxxxxxxxxx> 19.01.10 12:32 >>>
>On 2010-01-19 10:25, Jan Beulich wrote:
>> >>> "Joe Jin" <joe.jin@xxxxxxxxxx> 19.01.10 10:52 >>>
>> >At backend driver blkback and blktap, when checking statistics information,
>> >at the time vbd device remove, kernel will crash.
>> >
>> >Below patch will fix it, please review and apply.
>> 
>> This isn't a complete fix if I follow your analysis: There's still a race
>> between blk{back,tap}_remove() freeing be->blkif/be and the sysfs
>> code. dev->dev.driver_data (and possibly be->blkif) must be cleared
>> before freeing it (them).
>> 
>
>Thanks for you point.
>Anyway, I think need to add some check at VBD_SHOW even cleared 
>dev->dev.driver_data before free it, sysfs->fops() have been 
>initialized when call open(), and later when read the file, call
>trace will fall VBD_SHOW defined function(s), so it should crashed.

I think I understand what you're saying. But then there's only one
solution - avoid freeing those two data structures from the .remove
handler.

>The checks may look like below?

Checking dev to be non-NULL is certainly pointless in any case.

But wait - how did you see the crash you're trying to fix occur in the
first place? blkback_remove() calls xenvbd_sysfs_delif(), so the sysfs
entries are gone by the time driver_data gets freed.

Jan


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