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QNX Operating System/Discussion/OSTech/Stack memory flags/List of Posts
Forum Topic - Stack memory flags: (4 Items)
   
Update
Mario Charest
07/18/2008 11:49 AM
post10734
Stack memory flags   
When looking at the memory information with the IDE I noticed that for programs with more then 1 thread the memory for 
the stack of these extra threads have the execute permission set.

Isn't that odd?
Brian Stecher
07/18/2008 11:54 AM
post10736
Re: Stack memory flags   
It's intentional. When GCC takes the address of a nested function (an
extension), it actually ends up dynamically generating a little code thunk
on to the thread's stack so they have to be executable for that to work.
Actually, in the current code base, we've added a new suboption to the
-m switch to control this: -m~x will turn off PROT_EXEC for system allocated
stacks, while -mx (the default) will turn on PROT_EXEC.

	Brian

On Fri, Jul 18, 2008 at 11:49:20AM -0400, Mario Charest wrote:
> When looking at the memory information with the IDE I noticed that for programs with more then 1 thread the memory for
 the stack of these extra threads have the execute permission set.
> 
> Isn't that odd?
> 
> 
> 
> _______________________________________________
> OSTech
> http://community.qnx.com/sf/go/post10734
> 

-- 
Brian Stecher (bstecher@qnx.com)        QNX Software Systems
phone: +1 (613) 591-0931 (voice)        175 Terence Matthews Cr.
       +1 (613) 591-3579 (fax)          Kanata, Ontario, Canada K2M 1W8
Thomas Fletcher
07/18/2008 1:57 PM
post10747
Re: Stack memory flags   
Brian Stecher wrote:
> It's intentional. When GCC takes the address of a nested function (an
> extension), it actually ends up dynamically generating a little code thunk
> on to the thread's stack so they have to be executable for that to work.
> Actually, in the current code base, we've added a new suboption to the
> -m switch to control this: -m~x will turn off PROT_EXEC for system allocated
> stacks, while -mx (the default) will turn on PROT_EXEC.
>   

So does this only become an issue if you are using nested functions.  If 
you don't use them, you
should never need the PROT_EXEC flag turned on?

Thomas
> On Fri, Jul 18, 2008 at 11:49:20AM -0400, Mario Charest wrote:
>   
>> When looking at the memory information with the IDE I noticed that for programs with more then 1 thread the memory 
for the stack of these extra threads have the execute permission set.
>>
>> Isn't that odd?
>>
>>
>>
>> _______________________________________________
>> OSTech
>> http://community.qnx.com/sf/go/post10734
>>
>>     
>
>
Brian Stecher
07/18/2008 3:43 PM
post10753
Re: Stack memory flags   
On Fri, Jul 18, 2008 at 01:57:40PM -0400, Thomas Fletcher wrote:
> Brian Stecher wrote:
> > It's intentional. When GCC takes the address of a nested function (an
> > extension), it actually ends up dynamically generating a little code thunk
> > on to the thread's stack so they have to be executable for that to work.
> > Actually, in the current code base, we've added a new suboption to the
> > -m switch to control this: -m~x will turn off PROT_EXEC for system allocated
> > stacks, while -mx (the default) will turn on PROT_EXEC.
> >   
> 
> So does this only become an issue if you are using nested functions.  If 
> you don't use them, you
> should never need the PROT_EXEC flag turned on?

More specifically, it's only if you're taking the address of a nested 
function do you need the PROT_EXEC. However, a lot of CPU's don't have
a separate permission bit for PROT_EXEC in the MMU hardware, so turning 
it off doesn't buy you any extra security.

-- 
Brian Stecher (bstecher@qnx.com)        QNX Software Systems
phone: +1 (613) 591-0931 (voice)        175 Terence Matthews Cr.
       +1 (613) 591-3579 (fax)          Kanata, Ontario, Canada K2M 1W8

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