Re: C aggregate passing (Rust kernel policy)

[David Laight <david.laight.linux@gmail.com>]

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.

	David

> 
> -- Steve
>