Re: [PATCH 1/2] pipe: fix race with fcntl

From: Al Viro
Date: Sun Apr 12 2015 - 13:35:23 EST

On Sun, Apr 12, 2015 at 09:08:21PM +0400, Dmitry Monakhov wrote:
> Fix other long standing issues caused by fcntl(,F_SETFL,):
> - User can disable O_DIRECT for pipe[1] (paketized IO), but can not enable it again.
> - Currently we do not set O_APPEND on pipe[1] (IMHO it is wrong, but let it be)
> so it is reasonable to completely prohibit change O_APPEND flag on both
> end's of pipe. Add ->check_flags method in order to diallow O_APPEND toggling.
> Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx>
> ---

TBH, all those ->direct_IO != NULL checks seem to be a wrong approach.
If nothing else, it forces several filesystem into inventing a fake
->direct_IO just to fool those tests. How about we
* introduce FMODE_MAY_DIRECT and allow ->open() explicitly set it
* make open_check_o_direct() and fcntl.c check that instead of poking
in ->f_mapping->a_ops, etc.
* provide a variant of generic_file_open() that would set that
bit and use it on the filesystems that handle dio
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