linux-kernel - Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20120407173318.GA5076@redhat.com>
Date:	Sat, 7 Apr 2012 19:33:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...nvz.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Roland Dreier <roland@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem

On 04/06, Hugh Dickins wrote:
>
> I've long detested that behaviour of GUP write,force, and my strong
> preference would be not to layer more strangeness upon strangeness,
> but limit the damage by making GUP write,force fail in that case,
> instead of inserting a PageAnon page into a VM_SHARED mapping.
>
> I think it's unlikely that it will cause a regression in real life
> (it already fails if you did not open the mmap'ed file for writing),

Yes, and this is what looks confusing to me. Assuming I understand
you (and the code) correctly ;)

If we have a (PROT_READ, MAP_SHARED) file mapping, then FOLL_FORCE
works depending on "file->f_mode & FMODE_WRITE".

Afaics, because do_mmap_pgoff(MAP_SHARED) clears VM_MAYWRITE if
!FMODE_WRITE, and gup(FORCE) checks "vma->vm_flags & VM_MAYWRITE"
before follow_page/etc.

OTOH, if the file was opened without FMODE_WRITE, then I do not
really understand how (PROT_READ, MAP_SHARED) differs from
(PROT_READ, MAP_PRIVATE). However, in the latter case FOLL_FORCE
works, VM_MAYWRITE was not cleared.

Speaking of the difference above, I'd wish I could understand
what VM_MAYSHARE actually means except "MAP_SHARED was used".

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/