netdev - Re: DMA allocations from CMA and fatal_signal

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <5457CEB4.9020700@gmail.com>
Date:	Mon, 03 Nov 2014 10:51:32 -0800
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Michal Nazarewicz <mina86@...a86.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	linux-arm-kernel@...ts.infradead.org,
	Brian Norris <computersforpeace@...il.com>,
	Gregory Fong <gregory.0xf0@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	lauraa@...eaurora.org, gioh.kim@....com,
	aneesh.kumar@...ux.vnet.ibm.com, m.szyprowski@...sung.com,
	akpm@...ux-foundation.org,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: DMA allocations from CMA and fatal_signal_pending check

On 11/03/2014 08:45 AM, Michal Nazarewicz wrote:
> On Fri, Oct 31 2014, Florian Fainelli wrote:
>> I agree that the CMA allocation should not be allowed to succeed, but
>> the dma_alloc_coherent() allocation should succeed. If we look at the
>> sysport driver, there are kmalloc() calls to initialize private
>> structures, those will succeed (except under high memory pressure), so
>> by the same token, a driver expects DMA allocations to succeed (unless
>> we are under high memory pressure)
>>
>> What are we trying to solve exactly with the fatal_signal_pending()
>> check here? Are we just optimizing for the case where a process has
>> allocated from a CMA region to allow this region to be returned to the
>> pool of free pages when it gets killed? Could there be another mechanism
>> used to reclaim those pages if we know the process is getting killed
>> anyway?
> 
> We're guarding against situations where process may hang around
> arbitrarily long time after receiving SIGKILL.  If user does “kill -9
> $pid” the usual expectation is that the $pid process will die within
> seconds and anything longer is perceived by user as a bug.
> 
> What problem are *you* trying to solve?  If user sent SIGKILL to
> a process that imitated device initialisation, what is the point of
> continuing initialising the device?  Just recover and return -EINTR.

I have two problems with the current approach:

- behavior of a dma_alloc_coherent() call is not consistent between a
CONFIG_CMA=y vs. CONFIG_CMA=n build, which is probably fine as long as
we document that properly

- there is currently no way for a caller of dma_alloc_coherent to tell
whether the allocation failed because it was interrupted by a signal, a
genuine OOM or something else, this is largely made worse by problem 1

> 
>> Well, not really. This driver is not an isolated case, there are tons of
>> other networking drivers that do exactly the same thing, and we do
>> expect these dma_alloc_* calls to succeed.
> 
> Again, why do you expect them to succeed?  The code must handle failures
> correctly anyway so why do you wish to ignore fatal signal?

I guess expecting them to succeed is probably not good, but at we should
at least be able to report an accurate error code to the caller and down
to user-space.

Thanks
--
Florian

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html