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

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Sorry about the blank post last night. Looks like my non-patch-mangling webmailer has a UTF-8/ASCII conversion bug that eats messages. If anyone can recommend completely reliable mail clients for posting here, they'd be a good addition to the lkml.org FAQ

Comments inline...

Vadim Lobanov wrote:
On Thursday 31 August 2006 20:48, Willy Tarreau wrote:

Hi Chris,

On Thu, Aug 31, 2006 at 09:06:55PM -0400, Chris Snook wrote:

From: Chris Snook <[email protected]>

POSIX states that poll() shall fail with EINVAL if nfds > OPEN_MAX.  In
this context, POSIX is referring to sysconf(OPEN_MAX), which is the value
of current->rlim[RLIMIT_NOFILE].rlim_cur, not the compile-time constant
which happens to be named OPEN_MAX.  The current code will permit polling
up to 1024 file descriptors even if RLIMIT_NOFILE is less than 1024,
which POSIX forbids. The current code also breaks polling greater than
1024 file descriptors if the process has less than 1024 valid
descriptors, even if RLIMIT_NOFILE > 1024.  While it is silly to poll
duplicate or invalid file descriptors, POSIX permits this, and it worked
circa 2.4.18, and currently works up to 1024. This patch directly checks
the RLIMIT_NOFILE value, and permits exactly what POSIX suggests, no
more, no less.

While I understand that it was a bug before, I fear that it could break
existing apps. Are you aware of some apps which do not work as expected
because of this bug ? If not, I'd prefer to wait for some feedback from
2.6 with this fix before applying it (or maybe you're already using it
in RHEL with success ?).


I submitted a similar but different patch for this very same issue against the 2.6 kernel. It's currently residing in the -mm tree. Andrew Morton is somewhat reticent (and understandably so) to push it quickly into the vanilla tree; but, for what it's worth, I've yet to hear -- either directly or indirectly -- of any application breakages caused by this fix.

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.

	-- Chris

Thanks,
Willy


Signed-off-by: Chris Snook <[email protected]>
---

diff -urNp linux-2.4.33.2-orig/fs/select.c
linux-2.4.33.2-patch/fs/select.c ---
linux-2.4.33.2-orig/fs/select.c	2006-08-22 16:13:54.000000000 -0400 +++
linux-2.4.33.2-patch/fs/select.c	2006-08-31 13:43:39.000000000 -0400 @@
-417,7 +417,7 @@ asmlinkage long sys_poll(struct pollfd *
	int nchunks, nleft;

	/* Do a sanity check on nfds ... */
-	if (nfds > current->files->max_fdset && nfds > OPEN_MAX)
+	if (nfds > current->rlim[RLIMIT_NOFILE].rlim_cur)
		return -EINVAL;

	if (timeout) {


-- Vadim Lobanov

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