On Thu, 2016-11-03 at 15:36 -0700, Patrick J. LoPresti wrote:
> Right now, if I have code like this:
> int a; /* invalid value */
> int b = a + 1; /* operation on invalid value */
> ...memcheck does not produce a warning for the addition. It just
> taints b as invalid and only generates a warning if I try to use b in
> a behavior-changing way. This is good, and it is exactly what I want.
>
>
> However, if instead I have this code:
> int a; /* invalid value */
> int b;
> if (a > 0) /* conditional on invalid value */
> b = a;
> else
> b = 0;
> ...memcheck produces a warning on the conditional branch. But if you
> look at what this code actually computes, it is just "b = max(a,0)",
> which is not so different from "b = a + 1". (That is, b is just some
> simple function of a.) I want to teach memcheck to treat this second
> example like the first; that is, just taint b as invalid if a is
> invalid.
I am not sure to understand how you will differentiate the above
if (a > 0)
b = a;
else
b = 0;
from
if (a > 0) {
b = a;
launch_all_missiles();
} else
b = 0;
What we want is an error when the 'outside visible behaviour' of the
program depends on uninit values.
When a jump is processed, how do we know that the jump is jumping
to a location that will have no outside visible effect ?
For sure, we need/must have a valgrind error for the above case,
otherwise we risk a nuclear 3rd world war :).
Or maybe the idea is to do that only for very specific
way to compile
if (a > 0)
when the then/else branches are only assignment to b ?
This case seems very specialised to me, probably depending
a lot from the compiler and compiler optimisation options.
Is such code creating many false positive ?
Philippe
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