linux-kernel - Re: [PATCH 0/3] extend get/setrlimit to support setting rlimits external to a process (v7)

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Message-ID: <20091118231532.GA13039@redhat.com>
Date:	Thu, 19 Nov 2009 00:15:32 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Neil Horman <nhorman@...driver.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	marcin.slusarz@...il.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/3] extend get/setrlimit to support setting rlimits
	external to a process (v7)

On 11/18, Jiri Slaby wrote:
>
> On the last point: I added explicit ACCESS_ONCE all over there. If you
> don't like I will trash it.

Just curious, why?

I mean, do you have any example of the "bad" behaviour which is
fixed by ACCESS_ONCE() ?

> The only remaining weird user is in
> kernel/acct.c:
>         /*
>          * Accounting records are not subject to resource limits.
>          */
>         flim = current->signal->rlim[RLIMIT_FSIZE].rlim_cur;
>         current->signal->rlim[RLIMIT_FSIZE].rlim_cur = RLIM_INFINITY;
>         file->f_op->write(file, (char *)&ac,
>                                sizeof(acct_t), &file->f_pos);
>         current->signal->rlim[RLIMIT_FSIZE].rlim_cur = flim;
>
> It means that threads of the process with PACCT caps have unlimited file
> size for a short while. If there is setrlimit in between, it gets wiped
> out as well.

This is called when the whole thread-group exits, there are no
live threads except current.

We don't care if the new rlimit is lost afaics, but if RLIMIT_FSIZE
is changed in between ->write() can fail. Not sure what can we do,
perhaps just ignore this problem ;)

At least, given that do_acct_process() does override_creds(), an
ordinary user can't fool the accounting.

Oleg.

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