Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.lang.c
Subject: Re: Memory protection between compilation units?
Date: Thu, 12 Jun 2025 11:40:20 +0300
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On 2025-06-11 13:32:39 +0000, Mateusz Viste said:

> This might not be a strictly C question, but it definitely concerns all
> C programmers.
> 
> Earlier today, I fixed an out-of-bounds write bug. An obvious issue:
> 
>   static int *socks[0xffff];
> 
>   void update_my_socks(int *sock, int val) {
>     socks[val & 0xffff] = sock;
>   }
> 
> While the presented issue is common knowledge for anyone familiar with
> C, *locating* the bug was challenging. The program did not crash at the
> moment of the out-of-bounds write but much later - somewhere entirely
> different, in a different object file that maintained a static pointer
> for tracking a position in a linked list. To my surprise, the pointer
> was randomly reset to NULL about once a week, causing a segfault.
> Tracing this back to an unrelated out-of-bounds write elsewhere in the
> code was tedious, to say the least.
> 
> This raises a question: how can such corruptions be detected sooner?
> Protected mode prevents interference between programs but doesn’t
> safeguard a program from corrupting itself. Is there a way to enforce
> memory protection between module files of the same program? After all,
> static objects shouldn't be accessible outside their compilation unit.
> 
> How would you approach this?

The traditional method to ensure that a program or a part of a program
does not do what it must not do is testing. In this case the tester
must modify the code so that the array socks is a part of a larger
data structure and and call update_my_socks with different values for
val, including the critical values -1, 0, 0xfffe, and 0xffff.

-- 
Mikko