linux-kernel - Re: [PATCH] Add prctl support for controlling PF

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Message-ID: <20191021225234.GC2642@dread.disaster.area>
Date:   Tue, 22 Oct 2019 09:52:34 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Mike Christie <mchristi@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, martin@...ackup.org,
        Damien.LeMoal@....com
Subject: Re: [PATCH] Add prctl support for controlling PF_MEMALLOC V2

On Mon, Oct 21, 2019 at 04:41:37PM -0500, Mike Christie wrote:
> There are several storage drivers like dm-multipath, iscsi, tcmu-runner,
> amd nbd that have userspace components that can run in the IO path. For
> example, iscsi and nbd's userspace deamons may need to recreate a socket
> and/or send IO on it, and dm-multipath's daemon multipathd may need to
> send IO to figure out the state of paths and re-set them up.
> 
> In the kernel these drivers have access to GFP_NOIO/GFP_NOFS and the
> memalloc_*_save/restore functions to control the allocation behavior,
> but for userspace we would end up hitting a allocation that ended up
> writing data back to the same device we are trying to allocate for.

I think this needs to describe the symptoms this results in. i.e.
that this can result in deadlocking the IO path.

> This patch allows the userspace deamon to set the PF_MEMALLOC* flags
> with prctl during their initialization so later allocations cannot
> calling back into them.
> 
> Signed-off-by: Mike Christie <mchristi@...hat.com>
> ---

....
> +	case PR_SET_MEMALLOC:
> +		if (!capable(CAP_SYS_ADMIN))
> +			return -EPERM;

Wouldn't CAP_SYS_RAWIO (because it's required by kernel IO path
drivers) or CAP_SYS_RESOURCE (controlling memory allocation
behaviour) be more appropriate here?

Which-ever is selected, the use should be added to the list above
the definition of the capability in include/linux/capability.h...

Otherwise looks fine to me.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com