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SubjectRe: [PATCH] [RFC] Initialization of unused function parameters
On Tue, Jun 14, 2022 at 11:45 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> Hi!
>
> On Tue, Jun 14, 2022 at 10:19:53PM +0200, Alexander Potapenko wrote:
> > ================
> > char *kmalloc(int size);
> >
> > char *kmalloc_or_not(int flag, int size, char *p) {
> > if (flag)
> > return kmalloc(size);
> > else
> > return p;
> > }
> >
> > char global[16];
> >
> > char *p(int flag) {
> > char *c;
> > int size;
> > if (flag)
> > return kmalloc_or_not(1, 4, c);
> > else
> > return kmalloc_or_not(0, size, global);
> > }
> > ================
>
> Since C11, lvalue conversion of an automatic variable that does not have
> its address taken is explicitly undefined behaviour (6.3.2.1/2). So in
> function "p", both where "c" and where "size" are passed causes UB (so
> that executing "p" always causes UB btw).

Thanks for this reference to the standard. I've received another one
off-list, which lets the variables be address-taken:

6.2.4/6: "If an initialization is specified for the object, it is
performed each time the declaration or compound literal is reached in
the execution of the block; otherwise, the value becomes indeterminate
each time the declaration is reached."
3.19.2/1: "indeterminate value: either an unspecified value or a trap
representation"
6.2.6.1/5: "Certain object representations need not represent a value
of the object type. If the stored value of an object has such a
representation and is read by an lvalue expression that does not have
character type, the behavior is undefined. If such a representation is
produced by a side effect that modifies all or any part of the object
by an lvalue expression that does not have character type, the
behavior is undefined. [Footnote: Thus, an automatic variable can be
initialized to a trap representation without causing undefined
behavior, but the value of the variable cannot be used until a proper
value is stored in it.] Such a representation is called a trap
representation."


> > In this example `size` is passed into kmalloc_or_not() initialized,
> > however it is never used, so the code probably has defined behavior.
>
> No such luck: the passing itself already causes UB.

Looks like I've been missing this fact all the time.

> GCC does not warn, it has already optimised the code to what you expect
> by the time this warning is done. If you use -fno-inline it does warn
> for both "c" and "size" (via -Wmaybe-uninitialized).
>
> But it is still UB! All bets are off, no compiler can do any correct
> translation of your program, since there *is none*.

Then it makes sense for us to report non-trivial cases where
uninitialized values are actually passed to functions.
As Evgenii mentions, trivial inlinable cases are optimized away before
KMSAN instrumentation kicks in, so we won't be reporting them.

>
> Segher



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