Re: [PATCH v11 3/4] random: introduce generic vDSO getrandom() implementation

[Jason A. Donenfeld]

Hi Florian,

On Mon, Dec 05, 2022 at 07:56:47PM +0100, Florian Weimer wrote:
> * Jason A. Donenfeld:
> 
> > +retry_generation:
> > +	/*
> > +	 * @rng_info->generation must always be read here, as it serializes @state->key with the
> > +	 * kernel's RNG reseeding schedule.
> > +	 */
> > +	current_generation = READ_ONCE(rng_info->generation);
> 
> > +		if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info-
> 
> I'm pretty sure you need some sort of barrier here.  We have a similar
> TM-lite construct in glibc ld.so for locating link maps by address, and
> there the compiler performed reordering.
> 
>   _dl_find_object miscompilation on powerpc64le
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=28745>
> 
> I'm not familiar with READ_ONCE, but Documentation/atomic_t.txt suggests
> it's a “regular LOAD”, and include/asm-generic/rwonce.h concurs.

Do you mean compiler barriers or SMP barriers?

> Likewise, the signal safety mechanism needs compiler barriers (signal
> fences).

READ_ONCE() should prevent the compiler from reordering the read.
 
> I'm also not sure how READ_ONCE realizes atomic 64-bit reads on 32-bit
> platforms.  i386 can do them in user space via the FPU worst-case (if
> the control word hasn't been corrupted).  CMPXCHG8B is not applicable
> here because it's a read-only mapping.  Maybe add a comment at least
> about that “strong prevailing wind”?

There's read tearing in that case, which isn't super, but perhaps not
all together harmful.

Jason