linux-kernel - Re: [RFC] apparently broken RLIMIT

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Message-ID: <20131006213358.GZ13318@ZenIV.linux.org.uk>
Date:	Sun, 6 Oct 2013 22:33:58 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] apparently broken RLIMIT_CORE

On Sun, Oct 06, 2013 at 01:42:05PM -0700, Linus Torvalds wrote:
> On Sat, Oct 5, 2013 at 5:43 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> >         In other words, the size of notes section is ignored for
> > RLIMIT_CORE purposes.  Is that intentional?
> 
> I doubt it is intentional, but I also cannot really feel that we care
> deeply. Afaik we don't really honor the size limit exactly anyway, ie
> we tend to check only at page boundaries etc. So do we really care?

It's actually easier to put the counter of already written data into
coredump_params and have a new primitive both check that we won't
exceed the limit and update that counter, rather than doing it manually
(and somewhat inconsistently) in load_elf_binary() and the stuff it
calls.  As the matter of fact, after that change we get code looking
so:
        if (!elf_core_write_extra_phdrs(cprm, offset))
                goto end_coredump;

        size = cprm->written;
        /* write out the notes section */
        if (!write_note_info(&info, cprm))
                goto end_coredump;

        if (elf_coredump_extra_notes_write(cprm))
                goto end_coredump;

        /* Align to page */   
        if (!dump_seek(cprm->file, dataoff - cprm->written))
                goto end_coredump;

        cprm->written = size;
        for (vma = first_vma(current, gate_vma); vma != NULL;
                        vma = next_vma(vma, gate_vma)) {
and that's the only remaining reason to have size as local variable at all.
IOW, making the it consistent would mean getting rid of this "save and restore
cprm->written" thing around write_note_info()/elf_coredump_extra_notes_write()
in there...

Objections?

PS: I've ended up modifying aout32 coredump code (after fixing the bitrot in
there), now the problem is how to test it.  I'm grabbing something I hadn't
seen in a _long_ time - i386 Slackware 2.3; that should still contain live
a.out toolchain and as long as it's run under KVM and isolated from network...
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