Re: [PATCH 2.4.33.2] enforce RLIMIT_NOFILE in poll()

[Willy Tarreau <w@1wt.eu>]

On Fri, Sep 01, 2006 at 05:24:01PM -0400, Chris Snook wrote:
> Willy Tarreau wrote:
> >On Fri, Sep 01, 2006 at 12:03:24PM -0400, Chris Snook wrote:
> >>Willy and Vadim --
> >>
> >>	We have received reports of apps which poll a large set of 
> >>not-necessarily-valid file descriptors which worked fine under 2.4.18, 
> >>when the check was only against NR_OPEN, which is 1024*1024, that fail 
> >>under newer kernels.  So there is a real motivation to change the 
> >>current code.  As for the patch breaking existing apps, there are really 
> >>3 scenarios:
> >>
> >>1)	RLIMIT_NOFILE is at the default value of 1024
> >>
> >>	In this (default) case, the patch changes nothing.  Calls with nfds 
> >>	> 1024 fail with EINVAL both before and after the patch, and calls 
> >>	with nfds <= 1024 pass the check both before and after the patch, since 
> >>1024 is the initial value of max_fdset.
> >>
> >>2)	RLIMIT_NOFILE has been raised above the default
> >>
> >>	In this case, poll() becomes more permissive, allowing polling up to 
> >>RLIMIT_NOFILE file descriptors even if less than 1024 have been opened. 
> >>The patch won't introduce new errors here.  If an application somehow 
> >>depends on poll() failing when it polls with duplicate or invalid file 
> >>descriptors, it's already broken, since this is already allowed below 
> >>1024, and will also work above 1024 if enough file descriptors have been 
> >>open at some point to cause max_fdset to have been increased above nfds.
> >>
> >>3)	RLIMIT_NOFILE has been lowered below the default
> >>
> >>	In this case, the system administrator or the user has gone out of 
> >>their way to protect the system from inefficient (or malicious) 
> >>applications wasting kernel memory.  The current code allows polling up 
> >>to 1024 file descriptors even if RLIMIT_NOFILE is much lower, which is 
> >>not what the user or administrator intended.  Well-written applications 
> >>which only poll valid, unique file descriptors will never notice the 
> >>difference, because they'll hit the limit on open() first.  If an 
> >>application gets broken because of the patch in this case, then it was 
> >>already poorly/maliciously designed, and allowing it to work in the past 
> >>was a violation of POSIX and a DoS risk on low-resource systems.
> >
> >
> >OK, thanks very much for the details. Now, call me an idiot, but why
> >don't you consider broken the apps which are currently failing on
> >newer kernels ? I'm starting to suspect that we have to sets of apps :
> 
> I do consider them poorly designed, but they're out there, and they used 
> to work, and it doesn't violate POSIX to allow them to work again, so 
> all things being equal, I'd like them to work on new kernels.
> 
> >  - those which rely on poll() failing for invalid fds (do they really
> >    exist ?)
> 
> I hope not.  If so, they're already broken in most situations anyway.
> 
> >  - those which rely on poll() not failing for invalid fds.
> 
> This is what we've gotten reports of.  I suspect we haven't heard much 
> about this because in most cases the offending apps get fixed to not 
> poll invalid fds, but for some deployed proprietary apps that may not be 
> an option.
> 
> >The poll(2) man page suggests what you're saying. Man pages from other
> >OSes found on the net suggest various behaviours. I guess it's better
> >to stick to what has been documented (ie: your fix) but with *infinite
> >care*. Apps which need more than 1024 fds are not end-user mp3 players.
> >Breaking them in a stable branch can have a huge impact. I'd like this
> >patch to be tested in 2.6 long before 2.4, and also it would be good
> >if we could find some feedback from affected people which could confirm
> >that your patch really fixes their problems. If you have some customers
> >reporting the problem in RHEL who confirm the fix, it would be nice if
> >they accepted to inform us about the application(s) which need this fix. 
> 
> I agree.  The 2.6 patch is in -mm now.  The patch has been tested 
> successfully with a synthetic reproducer under various vanilla and RHEL 
> 2.4 and 2.6 kernels, but we're still waiting on real-world customer 
> results.  Let's wait and see how the customer tests and the -mm patch go.

OK Chris,

let's wait and see then !

Regards,
Willy


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