|| ||Ingo Molnar <mingo-AT-elte.hu> |
|| ||KOSAKI Motohiro <kosaki.motohiro-AT-jp.fujitsu.com>,
Alexander Viro <viro-AT-ftp.linux.org.uk>,
Chris Wright <chrisw-AT-sous-sol.org>,
Andrew Morton <akpm-AT-linux-foundation.org> |
|| ||Re: [RFC][PATCH] Cross Memory Attach |
|| ||Thu, 16 Sep 2010 10:08:26 +0200|
|| ||Bryan Donlan <bdonlan-AT-gmail.com>, Avi Kivity <avi-AT-redhat.com>,
Christopher Yeoh <cyeoh-AT-au1.ibm.com>,
Linux Memory Management List <linux-mm-AT-kvack.org>,
Linus Torvalds <torvalds-AT-linux-foundation.org>|
|| ||Article, Thread
* KOSAKI Motohiro <email@example.com> wrote:
> > On Wed, Sep 15, 2010 at 19:58, Avi Kivity <firstname.lastname@example.org> wrote:
> > > Instead of those two syscalls, how about a vmfd(pid_t pid, ulong start,
> > > ulong len) system call which returns an file descriptor that represents a
> > > portion of the process address space. You can then use preadv() and
> > > pwritev() to copy memory, and io_submit(IO_CMD_PREADV) and
> > > io_submit(IO_CMD_PWRITEV) for asynchronous variants (especially useful with
> > > a dma engine, since that adds latency).
> > >
> > > With some care (and use of mmu_notifiers) you can even mmap() your vmfd and
> > > access remote process memory directly.
> > Rather than introducing a new vmfd() API for this, why not just add
> > implementations for these more efficient operations to the existing
> > /proc/$pid/mem interface?
> As far as I heared from my friend, old HP MPI implementation used
> /proc/$pid/mem for this purpose. (I don't know current status).
> However almost implementation doesn't do that because /proc/$pid/mem
> required the process is ptraced. As far as I understand , very old
> /proc/$pid/mem doesn't require it. but It changed for security
> concern. Then, Anybody haven't want to change this interface because
> they worry break security.
> But, I don't know what exactly protected "the process is ptraced"
> check. If anyone explain the reason and we can remove it. I'm not
> againt at all.
I did some Git digging - that ptrace check for /proc/$pid/mem read/write
goes all the way back to the beginning of written human history, aka
I researched the fragmented history of the stone ages as well, i checked
out numerous cave paintings, and while much was lost, i was able to
recover this old fragment of a clue in the cave called 'patch-2.3.27',
carbon-dated back as far as the previous millenium (!):
mem_read() in fs/proc/base.c:
+ * 1999, Al Viro. Rewritten. Now it covers the whole per-process part.
+ * Instead of using magical inumbers to determine the kind of object
+ * we allocate and fill in-core inodes upon lookup. They don't even
+ * go into icache. We cache the reference to task_struct upon lookup too.
+ * Eventually it should become a filesystem in its own. We don't use the
+ * rest of procfs anymore.
In such a long timespan language has changed much, so not all of this
scribbling can be interpreted - but one thing appears to be sure: this
is where the MAY_PTRACE() restriction was introduced to /proc/$pid/mem -
as part of a massive rewrite.
Alas, the reason for the restriction was not documented, and is feared
to be lost forever.
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