hooks: Add pre-create hooks for runtime-config manipulation

There's been a lot of discussion over in [1] about how to support the
NVIDIA folks and others who want to be able to create devices
(possibly after having loaded kernel modules) and bind userspace
libraries into the container.  Currently that's happening in the
middle of runc's create-time mount handling before the container
pivots to its new root directory with runc's incorrectly-timed
prestart hook trigger [2].  With this commit, we extend hooks with a
'precreate' stage to allow trusted parties to manipulate the config
JSON before calling the runtime's 'create'.

I'm recycling the existing Hook schema from pkg/hooks for this,
because we'll want Timeout for reliability and When to avoid the
expense of fork/exec when a given hook does not need to make config
changes [3].

[1]: https://github.com/opencontainers/runc/pull/1811
[2]: https://github.com/opencontainers/runc/issues/1710
[3]: https://github.com/containers/libpod/issues/1828#issuecomment-439888059

Signed-off-by: W. Trevor King <wking@tremily.us>

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/libpod.conf.5.md b/docs/libpod.conf.5.md
index c02d247fb..98eb5bece 100644
--- a/docs/libpod.conf.5.md
+++ b/docs/libpod.conf.5.md
@@ -37,7 +37,9 @@ libpod to manage containers.
 
   For the bind-mount conditions, only mounts explicitly requested by the caller via `--volume` are considered.  Bind mounts that libpod inserts by default (e.g. `/dev/shm`) are not considered.
 
-  If `hooks_dir` is unset for root callers, Podman and libpod will currently default to `/usr/share/containers/oci/hooks.d` and `/etc/containers/oci/hooks.d` in order of increasing precedence.  Using these defaults is deprecated, and callers should migrate to explicitly setting `hooks_dir`.
+  Podman and libpod currently support an additional `precreate` state which is called before the runtime's `create` operation.  Unlike the other stages, which receive the container state on their standard input, `precreate` hooks receive the proposed runtime configuration on their standard input.  They may alter that configuration as they see fit, and write the altered form to their standard output.
+
+  **WARNING**: the `precreate` hook lets you do powerful things, such as adding additional mounts to the runtime configuration.  That power also makes it easy to break things.  Before reporting libpod errors, try running your container with `precreate` hooks disabled to see if the problem is due to one of your hooks.
 
 **static_dir**=""
   Directory for persistent libpod files (database, etc)