Re: [PATCH 0/7 V2] Trust readonly sections if target has memory protection

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> From: Yao Qi <yao@codesourcery.com>
> Date: Sun, 8 Sep 2013 20:03:20 +0800
> 
> Here is the V2, to address comments to V1,
> 
>   - Warn about writes to readonly sections if trust-readonly-sections
> is enabled. (patch 1/7)
>   - Windows targets have memory protection. (patch 7/7)
> 
> Do some refactor in Windows related targets, add a new function
> windows_init_abi (done by patch 3/7).  It paves the way for patch
> 7/7.
> 
> Below the description of this series in V1:
> https://sourceware.org/ml/gdb-patches/2013-09/msg00193.html
> 
> When option "trust-readonly-sections" was introduced in this patch
> http://www.sourceware.org/ml/gdb-patches/2002-01/msg00711.html, there
> was the discussion to turn it on in default.  However, we didn't do
> that because we have a concern that bad or buggy program may modify the
> code on the non-memory protected system.  On the other hand, it is
> highly recommended to turn this on in remote debugging.
> 
> I am wondering if we can teach GDB to trust read-only sections if it
> knows the target system has memory protection.  For some targets, such
> as linux, we know they do have memory protection.  This is what this
> patch series tries to do.

What does "memory protection" mean?  That a target has an MMU that
allows pages to be marked read-only?  That really is more a hardware
feature than a OS aatribute.

Even on systems that have an MMU that can mark pages read-only, system
calls like mprotect(2) can be used to make read-only pages
(temporarily) writable.  This is done by the OpenBSD dynamic linker
during relocation processing.  I expect other systems implementing
strict W^X to do the same.  Enabling trust-readonly-sections on such
systems would be a bad idea.