linux-kernel - Re: [kernel-hardening] [PATCH 3/4] Make static usermode helper binaries constant

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Date:   Thu, 15 Dec 2016 15:51:01 -0500
From:   Daniel Micay <danielmicay@...il.com>
To:     kernel-hardening@...ts.openwall.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [kernel-hardening] [PATCH 3/4] Make static usermode helper
 binaries constant

> To follow up on this, and after staring at too many outputs of the
> compiler, I think what this really should be is:
> 	static char const critical_overtemp_path[] =
> "/sbin/critical_overtemp";
> right?
> 
> That way both the variable, and the data, end up in read-only memory
> from what I can tell.
> 
> But, if I do:
> 	static char const char critical_overtemp_path[] =
> "/sbin/critical_overtemp";
> then sparse complains to me about:
> 	warning: duplicate const
> 
> Is that just sparse being dense, or is the latter one really better
> here?  It seems that both of the above put the data and variable into
> the same segment (.rodata).
> 
> thanks,
> 
> greg k-h

Either 'char *const foo = "bar"' or 'const char *const foo = "bar" will
also be a string constant in rodata with a pointer in rodata referring
to them. Duplicate string constants get merged without any analysis as
there's no guarantee of a unique address for the data itself since it's
not a variable. 'const char foo[] = "bar"' goes into rodata too, but is
the toolchain can't assume it can't safely merge strings + sizeof works
but gcc/clang know how to optimize constant strlen anyway.

The 'const' qualifier for pointers doesn't really do anything, it's when
it's used on the variable (after the pointer) that it can do more than
acting as a programming guide.
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