Date: Thu, 15 Dec 2016 13:18:07 -0800 From: Greg KH <gregkh@...uxfoundation.org> To: kernel-hardening@...ts.openwall.com Cc: linux-kernel@...r.kernel.org Subject: Re: [PATCH 3/4] Make static usermode helper binaries constant On Thu, Dec 15, 2016 at 03:51:01PM -0500, Daniel Micay wrote: > > 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. Thanks for the explanation. I don't think we need to worry about merging these strings, but I'll keep it in mind. However, the "folklore" of the kernel was to never do: char *foo = "bar"; but instead do: char foo = "bar"; to save on the extra variable that the former creates. Is that no longer the case and we really should be using '*' to allow gcc to be smarter about optimizations? > 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. Many thanks for the explanations, greg k-h
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