Re: Q about pagecache data never written to disk

[Nick Piggin <nickpiggin@yahoo.com.au>]

Pavel Machek wrote:

>>>kernel realizes that there's not enough disk space, and discard
>>>changes, therefore /tmp/sign.me reverts to previous, evil, content.
>>>
>>
>>root would have to make that change while user has the file open,
>>and should welcome the subsequent unleashing of evil content as a
>>valuable lesson.
> 
> 
> Really? I thought that writeback is not synchronous at close()
> time.... Hmm.... It probably could be in case of mmap....
> 

writeback isn't, but the pages will get marked dirty at unmap.
But I think I am wrong actually - I don't actually see why the
user would have to have the file open.

> It is still pretty unexpected. Like "root sees you have that file
> open, so he stops you via ptrace".... but ok....

Or maybe

cp /tmp/sign.me ~/
chown ... ~/sign.me
chmod ... ~/sign.me
mv ~/sign.me /tmp/signed