[OpenWrt-Devel] [RFC] fstools/overlayfs race condition

[Luiz Angelo Daros de Luca]

Maybe fsfreeze can avoid the race condition. (if this works with the
involved FS)
http://linux.die.net/man/8/fsfreeze
https://github.com/karelzak/util-linux/blob/master/sys-utils/fsfreeze.c

Or just temporary remount source fs as ro. It might make some writing
process unhappy but I'll keep file filesystem state.

My two cents.

Regards,

Em seg, 7 de mar de 2016 às 10:10, John Crispin <blogic at openwrt.org>
escreveu:

>
>
> On 07/03/2016 14:03, Roman Yeryomin wrote:
> > There is a race between `cp -a /tmp/root/* /rom/overlay` from
> > libfstools/overlay.c and a process creating new file(s) before
> > pivot(/rom, /mnt) occured.
> > That is a process can create a file and it will not be copied.
> >
> > Currently I do additional copy after jffs2 is ready, which is kind of
> > cumbersome (see attached patch), but there are still few potentially
> > erroneous scenarios:
> > 1. a process may recreate the file by the time second cp occurs
> > 2. a process may delete a file (not existing at that moment) and
> > second cp will copy it again
> > 3. a process may want to read created file before second cp occurs
> >
> > If attached patch is the way to go I will properly submit it.
> > Otherwise there should be a more fundamental fix but I don't see a way
> > to fix this properly.
> >
> >
>
> Hi Roman
>
> that race has been there since the day we do overlayfs. i am always
> surprised that it has not exploded in a big way yet. the only way i see
> are workarounds such as yours or sending out lots of SIGSTOP and the
> continues when we copied the files. either way it will be ugly and
> require protective gear.
>
> i'll ponder this and see if we can find a better way
>
>         John
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>
-- 

Luiz Angelo Daros de Luca
luizluca at gmail.com
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