[gentoo-user] Re: Spectre-NG

[Ian Zimmerman]

On 2018-05-10 14:35, Wol's lists wrote:

> > Code may be "security-sensitive" but buggy.  Is the compiler writer
> > really responsible for guessing what the programmer meant to
> > accomplish with buggy code?
>
> What do you mean by "buggy"?

Relying on UB, or not telling the compiler the whole truth.  You have a
point in that I should have been more specific.

> So if the compiler can't detect undefined behaviour, how the hell do
> you expect the programmer to?

Number one reason is that UB is at least in part a run-time concept.
Clearly the compiler cannot try all possible inputs to a function and
run a simulation on them.  The programmer _can_ insert guard code at the
calling site to prevent the undefined cases from happening.

A "whole program" compiler (ie. one that analyzes code across source
modules) may be able to detect the _possibility_ of UB.  But if you put
such a compiler to work on the kernel (for example), you can probably
take a short vacation while you await the result ;-)

> Oh - and please explain - what is buggy about wanting the following
> program to compile and actually *do* what the code is asking, rather
> than compiling to a no-op ... and 0x00ff is the address of your
> network adaptor? Do you want THAT to be optimised away "because it
> doesn't do anything"?

> int main () {
> int a, b, c;
> a = 2;
> b = 4;
> c = 6;
> }

> int main () {
> void *a;
> a = 0x00ff;
> *a = 6;
> }

This is actually not UB, but a different problem.  Yes, if I write it
like this, I want it eliminated.  When I want to keep it, I will use
the "volatile" keyword which is in the language precisely for this
purpose.

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