Messages in this thread | | | From | Florian Weimer <> | Subject | Re: [PATCH v11 3/4] random: introduce generic vDSO getrandom() implementation | Date | Mon, 05 Dec 2022 19:56:47 +0100 |
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* 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. Likewise, the signal safety mechanism needs compiler barriers (signal fences).
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”?
Thanks, Florian
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