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
