|From:||Theodore Tso <tytso-AT-mit.edu>|
|To:||Kay Sievers <kay.sievers-AT-vrfy.org>|
|Subject:||Re: Runaway loop with the current git.|
|Date:||Sun, 7 Dec 2008 20:18:51 -0500|
|Cc:||Alan Cox <alan-AT-lxorguk.ukuu.org.uk>, Evgeniy Polyakov <zbr-AT-ioremap.net>, Herbert Xu <herbert-AT-gondor.apana.org.au>, linux-kernel-AT-vger.kernel.org, Linux Crypto Mailing List <linux-crypto-AT-vger.kernel.org>|
Let's take a step back, shall we? Fundamentally, what's going on here is that a particular distribution's initrd (Debian's, to be precise) is running into an error in response to a modprobe request for char-major-5-1, and it is attempting to write to the console, which is resulting in another modprobe request.... ad infinitum. There is a dispute about whether it is looping forever, or whether it should be getting caught by kernel/kmod.c's modprobe recursion detector. Alan has checked the recursion detector and reports that it works just fine; Evgeniy and Kay are claiming that it in fact loops forever, and the recursion detector is not working. I'm going to guess that Alan tested on Fedora, where it did work just fine, and reason why people using a Debian-derived initrd is seeing a recursive loop is because the recursive loop detector works by detecting up to five concurrent calls to modprobe. That is, while the userspace helper process is running, another userspace helper is invoked, and so on, so that there are five userspace helpers piled up on one another, this will trigger the automatic recursion detector. I'm guessing why it isn't working given Debian's initrd setup is that whatever is ultimately opening /dev/console isn't being called until after the helper script has exited. Given that the general purpose recursion detector is apparently not working at least in this case, Kay has proposed that we special case a kludge wihch prevents the userspace helper be called in the case of 5:1. His argument in favor of doing this is that /dev/console is never a module, so requesting char-major-5-1 will never be helpful, and this error can only happen in early userspace, when the tty subsystem hasn't been initialized yet. Alan claims this could also happen if the appropriate low-level console driver hasn't been loaded, and so perhaps the right thing in response to the request for char-major-5-1 is to load 8250_pci. Here, I think Alan is wrong, and Kay is right. From looking at the source, if there is no low-level console driver loaded, there is no call to request_module(); the only time this can happen is when tty driver hasn't been initialized in early startup. On the other hand, Alan is right that in general it is the usermode helper and initrd's responsibility not create a recursive dependency. This is in general true, not just for /dev/console. So based on that, it can be argued that the recursion kludge checking for 5:1 should just as much be put in userspace. In addition, the fact that recursion detection isn't working also seems to indicate that initrd in question is also doing something very wrong. So I would think the best thing to do is to figure out what Debian's initrd is doing that is evading the recursion detection. Fixing that is going to make things much more robust. Regards, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to email@example.com More majordomo info at http://vger.kernel.org/majordomo-info.html
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