Linux/Android: Read a byte from /dev/random instead of polling it.
#449
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| // Poll /dev/random to make sure it is ok to read from /dev/urandom. | ||
| // Read a byte from /dev/random to make sure it is ok to read from | ||
| // /dev/urandom. reading a byte instead of polling is more compatible with | ||
| // sandboxes that disallow `poll()` but which allow reading /dev/random, |
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I think this sounds reasonable, but do we have any documentation or examples of sandboxes which have this issue? It's not obvious to me that read-ing from an open file descriptor would be better or worse than poll-ing or select-ing it.
The reason for the current implementation (discussed in #58 and briansmith/ring#558 (comment)) is to avoid "debiting" a single byte from the /dev/random "entropy estimate". Both libsodium and openssl try to avoid this read:
Personally, I think the downside of reading one byte from the /dev/random pool is not that bad, and I prefer the simplicity of this approch. I just want to understand why select/poll is not a good fit in practice.
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I just want to understand why select/poll is not a good fit in practice.
I am submitting this change is because I have to write documentation about how to configure sandboxes and when one can avoid doing pre-sandbox initializations. In investigating this, I found that it is actually really difficult to document the seccomp policy for poll. The Chromium sandbox has conditional logic for __NR_POLL vs __NR_PPOLL for example. I added some more commentary about this in the comments of this PR.
I am also trying to ensure that if somebody is already using OpenSSL (or a fork of it) then switching to a getrandom-based library will not affect their sandbox policies.
So, in some sense it is hypothetical, but that's mostly because I write libraries for orgs who make frameworks for people who product applications.
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openssl uses select() by default but can be configured to use read()
OpenSSL uses select() unless there are too many file descriptors open, in which case it will use read().
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Those additional comments look great! Thanks for the explanation, makes sense to me!
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Please hold off on merging this. I initially underestimated the potential problem of draining the estimated entropy from |
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Closing in favor of #452, which just adds the documentation from this. |
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