linux-kernel - Re: Generic page fault (Was: libsigsegv ....)

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Date:	Sat, 28 Feb 2015 13:49:17 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: Generic page fault (Was: libsigsegv ....)

On Sat, Feb 28, 2015 at 1:14 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
>
> BTW. I fail to see how x86 checks PF_INSTR vs. VM_NOEXEC ... or it doesn't ?

It doesn't. x86 traditionally doesn't have an execute bit, so
traditionally "read == exec".

So PF_INSTR really wasn't historically very useful, in that it would
only show if the *first* access to a page was an instruction fetch -
if you did a regular read to brign the page in, then subsequent
instruction fetches would just work.

Then NX came along, and what happens now is

 - we handle write faults separately (see the first part of access_error()

 - so now we know it was a read or an instruction fetch

 - if PF_PROT is set, that means that the present bit was set in the
page tables, so it must have been an exec access to a NX page

 - otherwise, we just say "PROTNONE means no access, otherwise
populate the page tables"

.. and if it turns out that it was a PF_INSTR to a NX page, we'll end
up taking the page fault *again* after it's been populated, and now
since the page table was populated, the access_error() will catch it
with the PF_PROT case.

Or something like that. I might have screwed up some detail, but it
should all work.

                     Linus
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