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Re: [Tinycc-devel] The built-in memory and bounds checker cannot handle

From: Michael Matz
Subject: Re: [Tinycc-devel] The built-in memory and bounds checker cannot handle mmap'd memory and judges errno wrong
Date: Fri, 6 Apr 2018 21:44:17 +0200 (CEST)
User-agent: Alpine 2.20 (LSU 67 2015-01-07)


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:

void __bound_new_region(void *p, size_t size);
int __bound_delete_region(void *p);

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

  __bound_new_region(&errno, sizeof (errno));

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.


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