[PATCH v2 1/3] posix: Remove dynamic memory allocation from execl{e,p}

[Adhemerval Zanella]

On 02-02-2016 09:24, Florian Weimer wrote:
> On 02/01/2016 06:18 PM, Adhemerval Zanella wrote:
> 
>> Right, it is not ensuring the safeness. Is '-fstack-check' the suffice 
>> option to ensure it or do we need a more strict one?
> 
> In my tests, the initial stack banging probe is sometimes more than a
> page away from the current stack pointer, so it does not look safe to me.

I am not aware of a better option, if any, to avoid stack overflow in case of
a exec function call with large arguments.  Also, check at least on x86_64
I do see trying to overflow the stack with new implementation does force a
segfault on the stack protector code (it tries to orq a memory beyond stack).

> 
> For posix_spawn, it's probably simpler for now to compute the shell
> invocation in the parent.  That is, perform two vforks in case the first
> execve fails.
> 
> Florian
> 

I do not think it would require to clone(VFORK) twice in parent: we can
calculate the total argument size prior any call and calculate the
total stack to mmap based on this plus a slack for possible local
variables (128 or 256 bytes or even higher). I will add some latency
in any posix_spawn{p} case.

Another option is to just create a guard page in the end of the allocated
stack page (as pthread_create does) to force a segfaults in case of
a stack allocation higher. However, as for execl using stack-check will
also force a segfault in case of stack overflow in this case.