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

[Florian Weimer]

* 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?

Compiler barrier.

>> Likewise, the signal safety mechanism needs compiler barriers (signal
>> fences).
>
> READ_ONCE() should prevent the compiler from reordering the read.

READ_ONCE looks just like a volatile read.  Other reads can be ordered
around it.

For example, this:

void f1 (int, int, int);

extern int a;
extern int b;

void
f2 (volatile int *p)
{
  int a1 = a;
  int p1 = *p;
  int b1 = b;
  return f1 (a1, p1, b1);
}

Turns into:

	.globl	f2
	.type	f2, @function
f2:
	movl	(%rdi), %esi
	movl	b(%rip), %edx
	movl	a(%rip), %edi
	jmp	f1

Looks like compiler reodering to me.

>> 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.

Maybe add a comment that it was considered?

Thanks,
Florian