From: Kyle Ackerman <kack@kyleackerman.net>
Subject: [Kyle Ackerman] Re: imsg_get_data Question
To: tech@openbsd.org
Date: Sun, 16 Aug 2026 13:14:18 -0600


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From: Kyle Ackerman <kack@kyleackerman.net>
To: Florian Obser <florian@openbsd.org>
Subject: Re: imsg_get_data Question
Date: Sun, 16 Aug 2026 12:52:15 -0600

Florian Obser <florian@openbsd.org> writes:

> imsg is usually used over a privilege boundary. The sender is supposed
> to send exactly len bytes of data. If it fails to do that the receiver
> has to assume the sender is compromised and terminate itself.
> E.g. (dhcpleased/engine.c):
>
> case IMSG_CTL_LOG_VERBOSE:
> 	if (imsg_get_data(&imsg, &verbose,
> 	    sizeof(verbose)) == -1)
> 		fatalx("%s: invalid %s", __func__, i2s(type));
For more context, there is a current use case in Got where we send over
a struct and a string in the same imsg.  I imagined a case where you can
do multiple imsg_get_data(3) calls, over predefined lengths, to utilize
the rpos and wpos fields to incrementally extract seprate data.
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