RE: [PATCH v2 0/2] Lock and Pointer guards

David Laight
2023-06-08T16:26:10Z

From: Linus Torvalds
> Sent: 06 June 2023 16:46
>
> On Tue, Jun 6, 2023 at 8:31 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > nit: Linus's example was "(void *)8" (instead of 1) because we've had
> > issues in the past with alignment warnings on archs that are sensitive
> > to it. (e.g. see the __is_constexpr() macro which is doing NULL/!NULL
> > comparisons.)

__is_constexpr() is playing entirely different games.
Basically the type of (x ? (void *)y : (int *)z)
depends on whether 'y' is a compile-time 0 (int *) or not (void *).

...
> So I'm not sure the 1-vs-8 actually matters. We do other things that
> assume that low bits in a pointer are retained and valid, even if in
> theory the C type system might have issues with it.

Yes, given that gcc will assume a pointer is aligned if you try
to memcpy() from it, I'm surprised it doesn't always assume that.
In which case (long)ptr_to_int & 3 can be validly assumed to be zero.

I've found some 'day job' code that passed the address of a
member of a 'packed' structure to a function which then used
host ordered unsigned char[] accesses.
The compiler is certainly allowed to convert that back to
a word write - which would then fault.
(I've not looked to see if any modern compilers do.)
Of course the simple ptr->member would have be fine.

David

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