As of r281843 it is possible to activate a bootloader prompt for the GELI passphrase in 10.1-STABLE by setting geom_eli_passphrase_prompt="YES" in /boot/loader.conf However this does not work as expected since the passphrase does not seem to be received by the kernel. (i.e. the kernel prompts again for the GELI passphrase) AFAIK, a function to receive the passphrase in the kernel was committed in -HEAD in r273489. However a look at a recent -STABLE (r283270) sys/geom/eli/g_eli.c suggests that this particular piece of code was never MFC'd.
A commit references this bug: Author: gjb Date: Tue Jul 7 18:52:23 UTC 2015 New revision: 285249 URL: https://svnweb.freebsd.org/changeset/base/285249 Log: MFC r273489 (cperciva): Populate the GELI passphrase cache with the kern.geom.eli.passphrase variable (if any) provided in the boot environment. Unset it from the kernel environment after doing this, so that the passphrase is no longer present in kernel memory once we enter userland. This will make it possible to provide a GELI passphrase via the boot loader. PR: 200448 Approved by: re (kib) Sponsored by: The FreeBSD Foundation Changes: _U stable/10/ stable/10/sys/geom/eli/g_eli.c
Fixed, thank you for the report.
A commit references this bug: Author: gjb Date: Wed Jul 8 09:54:18 UTC 2015 New revision: 285263 URL: https://svnweb.freebsd.org/changeset/base/285263 Log: MFC r273489 (cperciva): Populate the GELI passphrase cache with the kern.geom.eli.passphrase variable (if any) provided in the boot environment. Unset it from the kernel environment after doing this, so that the passphrase is no longer present in kernel memory once we enter userland. This will make it possible to provide a GELI passphrase via the boot loader. Note: head and stable/10 differ as a result of r273174, which renames the getenv(), setenv(), and unsetenv() functions with kern_getenv(), kern_setenv(), and kern_unsetenv(), which was reverted in the relevant parts of this change in 10-STABLE. PR: 200448 Approved by: re (kib) Sponsored by: The FreeBSD Foundation Changes: _U stable/10/ stable/10/sys/geom/eli/g_eli.c