Re: pipe/page fault oddness.

From: Linus Torvalds
Date: Tue Sep 30 2014 - 00:27:15 EST

On Mon, Sep 29, 2014 at 8:33 PM, Dave Jones <davej@xxxxxxxxxx> wrote:
> Looking at the dump, there's only one running trinity child,
> with all the others blocking on it.
> trinity-c49 R running task 12856 19464 7633 0x00000004
> ffff8800a09bf960 0000000000000002 ffff8800a09bf9f8 ffff880219650000
> 00000000001d4080 0000000000000000 ffff8800a09bffd8 00000000001d4080
> ffff88023f755bc0 ffff880219650000 ffff8800a09bffd8 ffff88010b017e00
> Call Trace:
> [<ffffffff911c3c67>] handle_mm_fault+0x3a7/0xcd0
> [<ffffffff91042c84>] __do_page_fault+0x1a4/0x600
> [<ffffffff910430fe>] do_page_fault+0x1e/0x70
> [<ffffffff918264b2>] page_fault+0x22/0x30
> [<ffffffff911bd7e3>] ? copy_page_to_iter+0x3b3/0x500
> [<ffffffff9120eddf>] pipe_read+0xdf/0x330
> Running the function tracer on that pid shows it spinning forever..
> Kernel bug (missing EFAULT check somewhere perhaps?), or is this a
> case where the fuzzer asked the kernel to do something stupid, and it obliged ?

Hmm. It looks like copy_page_to_iter_iovec() is broken and keeps not
making any progress while just faulting.

I don't see how that could happen, though. All the loops there are
conditional on the user copies *not* failing (ie "!left"), and they
seem to properly update "iov".

Mind sending a disassembly of your "copy_page_to_iter" function, in
particular around that whole "0x3b3/0x500" area which is where the
page fault seems to happen?

Adding Al to the cc, since this code is from his commit 6e58e79db8a1
("introduce copy_page_to_iter, kill loop over iovec in
generic_file_aio_read()") but I don't see anything obviously wrong

Al? Do you see something I don't? Dave's function trace does seem to
say that it doesn't even get back to pipe_read(), though, so the loop
really must be inside copy_page_to_iter().

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