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

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Message-ID: <1425163779.4645.151.camel@kernel.crashing.org>
Date:	Sun, 01 Mar 2015 09:49:39 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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, 2015-02-28 at 13:49 -0800, Linus Torvalds wrote:

 .../...

>  - 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.

I see, it should work yes, I'll still add that FAULT_FLAG_EXEC for
those who can tell reliably but it shouldn't hurt for x86 to not set it.

Cheers,
Ben.


>                      Linus
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