Re: [Tinycc-devel] The built-in memory and bounds checker cannot handle mmap'd memory and judges errno wrong

[Michael Matz]

Hi,

On Fri, 6 Apr 2018, George Gaarder wrote:

> I just tried -b to see whether I forgot to free something, and I found that 
> the memory and bounds checker will alert when I use the memory I mmap'd. Whi
> le I was writing a demo for report, I found that the checker also reported o
> ut of region when I accessed errno. I was to print errno to see if I really 
> opened the file to be mmap'd.
>
> I think the warning for the mmap'd memory is not a problem, just needed to b
> e pointed out in the document. But the errno one is quite strange.

Add these snippets to your program:

#ifdef __BOUNDS_CHECKING_ON
void __bound_new_region(void *p, size_t size);
int __bound_delete_region(void *p);
#endif

... somewhere early in main, before forking, and before accessing errno ...

#ifdef __BOUNDS_CHECKING_ON
  __bound_new_region(&errno, sizeof (errno));
#endif

Keep in mind that errno is a thread local variable (which is also the 
reason why it doesn't work out-of-box, like some other global vars like 
stdout work), so you would have to register each new errno location after 
you create a new thread.  With the above two functions you can also write 
a wrapper for mmap/munmap that suits you.

We could also register at least errno for the main thread at program start 
(in __bounds_init), but it'd still not be a complete solution for 
multi-threaded programs (let's ignore all other problems connected with 
TCCs bound checking and multiple threads), so I'm split-mind about this.


Ciao,
Michael.