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Re: [PATCH v7] getrandom system call wrapper [BZ #17252]

On 11/17/2016 02:56 PM, Zack Weinberg wrote:
On 11/17/2016 08:50 AM, Florian Weimer wrote:
On 11/17/2016 02:45 PM, Zack Weinberg wrote:
On 11/17/2016 08:02 AM, Florian Weimer wrote:
On 11/16/2016 05:41 PM, Zack Weinberg wrote:
On Wed, Nov 16, 2016 at 10:52 AM, Florian Weimer <>
On 11/16/2016 04:20 PM, Zack Weinberg wrote:
I am seriously considering escalating my disagreement here to a
formal objection.  I would like to know why you think it is
NECESSARY - not merely convenient or consistent with other stuff -
for this function to be a cancellation point.

It's necessary if you ever want to cancel a hanging getrandom in a
context where you cannot install a signal handler (so that you can
trigger EINTR when getrandom is stuck).

That only pushes the question back a level.  When would it ever be
necessary to do that?  Be as concrete as you possibly can.  Actual
code from a real program, if possible.

It's not clear to me what you are asking here.

Do you mean cancellation in general, or cancellation in conjunction with
getrandom specifically?

Sorry.  I meant cancellation specifically of a thread hanging in

I'm not sure how I can provide that, considering that there is currently
no way to cancel a thread which hangs in getrandom because we do not
provide a way for applications to implement system calls as cancellation
points (unless we provide a wrapper for the specific system call, of

What I'm asking for is evidence that that is actually a problem for at
least one real application.

I found this:


ipsec_resolver_thread reads from /dev/random, which can block for a very long time. The thread for it is spawned in ipsec_process_prefs, which cancels it when got_terminate returns true. The intent seems to be that is terminated on graceful process termination: read in ipsec_process_prefs returns EINTR after a signal handler has run which makes got_terminate (defined in main.c in the parent directory) return true.

If the read from /dev/random is replaced with a call to getrandom which is not a cancellation point, this would like not work as intended.

Is this the kind of stuff you are looking for?

> Also evidence that making getrandom a
cancellation point _won't_ break programs that naively assume it can
never fail.

Cancellation does not add additional error return cases.


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