Messages in this thread | | | From | Florian Weimer <> | Subject | Re: [PATCH v11 3/4] random: introduce generic vDSO getrandom() implementation | Date | Mon, 05 Dec 2022 20:35:09 +0100 |
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* 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
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