From: torvalds@transmeta.com (Linus Torvalds)
To: linux-kernel@vger.kernel.org
Subject: Re: RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.
Date: Fri, 8 Nov 2002 23:31:09 +0000 (UTC) [thread overview]
Message-ID: <aqhhft$19b$1@penguin.transmeta.com> (raw)
In-Reply-To: 24305.1036795742@passion.cambridge.redhat.com
In article <24305.1036795742@passion.cambridge.redhat.com>,
David Woodhouse <dwmw2@infradead.org> wrote:
>Why does a _readonly_ mapping fail if the file system has no writepage
>method?
>
>do_mmap_pgoff() sets VM_MAYWRITE on the vma and then generic_file_mmap()
>refuses to allow it.
>
>Suggested patch below.... or should I just hack fsx-linux to use
>MAP_PRIVATE for its readonly mappings and ignore it?
This is broken. Since it has VM_MAYWRITE, a subsequent mprotect() may
mark it writable, and you you went boom.
If you really want a shared mapping, you'd better open with O_RDONLY, at
which point the existing code should be perfectly happy and does the
right thing.
In other words: the code is correct as-is.
Linus
next prev parent reply other threads:[~2002-11-08 23:24 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-11-08 22:49 RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage David Woodhouse
2002-11-08 23:31 ` Linus Torvalds [this message]
2002-11-08 23:50 ` David Woodhouse
2002-11-09 2:46 ` Linus Torvalds
2002-11-09 16:36 ` David Woodhouse
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