Re: When scrubbing secrets in memory doesn't work

[Valdis.Kletnieks () vt edu]

On Wed, 06 Nov 2002 12:55:26 GMT, Gianni Tedesco <gianni () ecsc co uk>  said:

> FYI: tested on gcc version 2.96 20000731 (Red Hat Linux 7.3 2.96-112)
> which doesn't seem to do this. What compiler version/flags, if any does
> this depend on?

gcc 3 and later (3.2 is current) are well able to do this optimization,
and I'm surprised that the 2.96 version isn't able to as well.

> Should this optimisation actually even remove function calls? What if
> you were depending on that function doing something? eg: writing
> something to a file... I can understand with memset/memcpy or other
> builtin functions because it could be hardcoded in the compiler to say
> 'allow optimiser to remove calls to this function' or whatever.

If it's not flagged 'volatile', and dead code, it can be pruned.

memset()/memcpy() is no different than any other function that the compiler
is able to inline - the only difference for those "magic" functions is
that the compiler already has a body to inline, so you don't have to provide
source for it.  And once it's inlined, it's fair game for the optimizer.

If you were depending on the function doing something, then the function has a
side effect and the something won't be optimized away, unless the side effect
is also dead code.  If all the code in the function is demonstrably dead, the
entire function itself may go bye-bye (especially if it's declared as a
'static' function only visible in this source file).  I suspect that for
non-static functions, it will emit a function prolog/return just to ensure
that you don't have an unresolved symbol if other modules call the function.

An interesting corner case exists in gcc - often, it's able to inline all
references to a function.  There's a -fkeep-inline-functions to force the
compiler to emit a non-inlined copy as well (which can be a problem with
C++ initializers invoked from another module).
-- 
                                Valdis Kletnieks
                                Computer Systems Senior Engineer
                                Virginia Tech

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