linux-kernel - Re: [RFD] how to get something like chkspace / kinfo

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Date:	Wed, 14 Mar 2012 23:42:24 +0100
From:	Peter Waechtler <pwaechtler@....com>
To:	"EXTERNAL Waechtler Peter (Fa. TCP, CM-AI/PJ-CF31)" 
	<external.Peter.Waechtler@...bosch.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFD] how to get something like chkspace / kinfo_getvmmap?

On 14.03.2012 15:31, EXTERNAL Waechtler Peter (Fa. TCP, CM-AI/PJ-CF31) 
wrote:
> Hi,
>
> for implementing robust exception handlers that print out a backtrace in
> the signal handler, it would be nice to have a syscall to check if a
> memory access would fail.
>
> I know that one can parse /proc/self/maps (like libsigsegv does).
> libunwind employs an addrspace->access_mem() method that does an
> unprotected access because there is no (easy) way on Linux.
>
>
> How about enhancing mincore() with a few bits that tells the caller how
> the protection bits are?
> The main use case is in an embedded system to avoid that the handler
> (already running on an alternate signal stack) faults if the stack pointer
> achieved to point into the guard page - causing backtrace() to fail.
>
> mincore() on FreeBSD provides some more bits, but still the access bits
> are missing.
>
>       MINCORE_INCORE            Page is in core (resident).
>       MINCORE_REFERENCED        Page has been referenced by us.
>       MINCORE_MODIFIED          Page has been modified by us.
>       MINCORE_REFERENCED_OTHER  Page has been referenced.
>       MINCORE_MODIFIED_OTHER    Page has been modified.
>       MINCORE_SUPER             Page is part of a "super" page. (only i386&
>                                 amd64)
>
> add
>
>          MINCORE_PROT_W
>          MINCORE_PROT_R
>          MINCORE_PROT_X

Well, I guess some code will help?

This will also help in recognizing a guard page, for a clear and 
unambiguous:
"stack overflow" in multi-threaded programs.

     Peter

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