Entrypoint checks DATA_DIR in full before acting on it as root (closes #80)

`bin/entrypoint.sh` now checks `DATA_DIR` before it creates anything or
changes an owner or mode: it must be `/data` or a path below it with no
`.`, `..` or empty part, and no part of it that exists, `/data`
included, may be a symbolic link. Anything else stops the start with one
message naming `DATA_DIR`. Only then does it create `DATA_DIR`, give
`/data` and everything in it to `netwatch` (`chown -R -h`, so a link in
it is not followed) and set mode 750 on `/data` and `DATA_DIR`. The
README section "Running under upaas" says which values are accepted.

Model: opus-5-5

bin/entrypoint.sh

@@ -63,25 +63,43 @@ done > /etc/nginx/trusted-proxies.conf
 
 # netwatch-server keeps its report files in DATA_DIR, on the /data
 # volume, which may be a host directory owned by root or by another
-# uid. Both are given to the netwatch user here, with the mode the
-# server gives a directory it creates, so the host directory needs no
-# preparing.
+# uid. /data and everything in it are given to the netwatch user here,
+# and /data and DATA_DIR get the mode the server gives a directory it
+# creates, so the host directory needs no preparing.
 #
-# chown and chmod, run as root, change whatever a symbolic link on the
-# path points to, anywhere in the container, and the netwatch user can
-# put one in /data. So the start stops unless readlink -f, which
-# follows every link on a path, gives /data and DATA_DIR back as they
-# are. It also writes a path in full, so a DATA_DIR with '.', '..' or
-# an extra '/' in it is refused too.
+# This runs as root, so nothing is created or changed until DATA_DIR is
+# known to be /data or a path below it, with no '.', '..' or empty
+# part, and no part of it that exists, /data included, is a symbolic
+# link: the netwatch user can put one in /data, and root would follow
+# it anywhere in the container. Nothing else runs in the container yet,
+# so no link can appear after the check.
 export DATA_DIR="${DATA_DIR:-/data/reports}"
-mkdir -p "$DATA_DIR" || exit 1
-if [ "$(readlink -f /data)" != /data ] ||
-    [ "$(readlink -f "$DATA_DIR")" != "$DATA_DIR" ]; then
-    echo "entrypoint: DATA_DIR must be a full path with no '.', '..'," \
-        "extra '/' or symbolic link on it or on /data, not '$DATA_DIR'" >&2
+data_dir_ok() {
+    # With a / added at the end, a last part of '.' or '..', and a / at
+    # the end, match these patterns too.
+    case "$DATA_DIR/" in
+        */./* | */../* | *//*) return 1 ;;
+        /data/*) ;;
+        *) return 1 ;;
+    esac
+    # Each part from DATA_DIR up to /data. [ -L ] is false for a part
+    # that does not exist.
+    dir="$DATA_DIR"
+    while [ "$dir" != /data ]; do
+        [ -L "$dir" ] && return 1
+        dir="${dir%/*}"
+    done
+    [ ! -L /data ]
+}
+if ! data_dir_ok; then
+    echo "entrypoint: DATA_DIR must be /data or a path below it, with no" \
+        "'.', '..', extra '/' or symbolic link on it, not '$DATA_DIR'" >&2
     exit 1
 fi
-chown -R netwatch:netwatch /data "$DATA_DIR" || exit 1
+mkdir -p "$DATA_DIR" || exit 1
+# -h: a symbolic link in /data is itself given to netwatch, not what it
+# points to.
+chown -R -h netwatch:netwatch /data || exit 1
 chmod 750 /data "$DATA_DIR" || exit 1
 
 # A stop signal is only noted here; the loop below acts on it.