Re: C aggregate passing (Rust kernel policy)

Paul E. McKenney
2025-02-27T21:41:16Z

On Thu, Feb 27, 2025 at 08:47:22PM +0000, David Laight wrote:
> On Wed, 26 Feb 2025 17:35:34 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > On Wed, 26 Feb 2025 14:22:26 -0800
> > Linus Torvalds <torvalds@linux-foundation.org> wrote:
> >
> > > > But if I used:
> > > >
> > > > if (global > 1000)
> > > > goto out;
> > > > x = global;
> > >
> > > which can have the TUCTOU issue because 'global' is read twice.
> >
> > Correct, but if the variable had some other protection, like a lock held
> > when this function was called, it is fine to do and the compiler may
> > optimize it or not and still have the same result.
> >
> > I guess you can sum this up to:
> >
> > The compiler should never assume it's safe to read a global more than the
> > code specifies, but if the code reads a global more than once, it's fine
> > to cache the multiple reads.
> >
> > Same for writes, but I find WRITE_ONCE() used less often than READ_ONCE().
> > And when I do use it, it is more to prevent write tearing as you mentioned.
>
> Except that (IIRC) it is actually valid for the compiler to write something
> entirely unrelated to a memory location before writing the expected value.
> (eg use it instead of stack for a register spill+reload.)
> Not gcc doesn't do that - but the standard lets it do it.

Or replace a write with a read, a check, and a write only if the read
returns some other value than the one to be written. Also not something
I have seen, but something that the standard permits.

Thanx, Paul