[PATCH] Improve execl* functions

[Federico Terraneo]

On 27/01/25 18:26, Corinna Vinschen wrote:
> But it would still be nice to have a more dynamic solution which doesn't
> need ARG_NUM_MAX.

Hi Corinna,
I spent some time into it, and it's more complicated than I wanted, but 
I have a patch in this direction nonetheless, that replaces the previous 
patch proposal.

The complexity is caused by POSIX.1-2008 which as far as I could tell 
requires execl* functions to be callable from signal handlers, while 
malloc isn't. That leaves alloca as the only way I can think of to lift 
the argument number limitation.

The issue is, newlib targets also microcontroller targets where there's 
no virtual memory and stacks are small (our Miosix OS is one example), 
thus unbounded stack allocation (or just large stack allocations like 
the 1024bytes array in the current version of the execl* code) cause 
stack overflows and undefined behavior.

Thus I propose the attached patch that by default uses alloca but allows 
individual targets to define _EXECL_USE_MALLOC if in their use case the 
risk of stack overflows and consequent undefined behavior is a more 
pressing requirement than the corner case of allowing execl* calls from 
signal handlers.

Even with the default alloca implementation, the patched code uses 
*less* stack than the current code when execl* functions are called with 
less that 256 arguments, and on platforms where the stack can grow (i.e, 
when virtual memory is available) it entirely avoids the stack buffer 
overflow that was present in the current code, so I think it's an 
overall improvement.

Best regards,
Federico Terraneo


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