Core Rule Set reads request bodies up to SWWAF_WAF_BODY_LIMIT (closes #116)

SWWAF_WAF_BODY_LIMIT (default off, at most 1G) has the Core Rule Set read
form data and multipart up to the limit, the rest streaming on, and JSON
and XML (with +json, text/json and +xml) no larger than it. The part read
is held for the app. A size or time limit met while reading ends the
request. Content-Encoding is refused again on these kinds. A body Coraza
cannot parse, or a multipart body failing its strict checks, adds 5, but
not a multipart body reaching the limit. Coraza is built with
no_fs_access, so writes no file. Rule 900300 moves to phase 2.

Judgement call: Content-Encoding is refused on a JSON or XML body too
large to read, as SPEC.md allows.

Model: opus-5-5

Dockerfile

@@ -36,11 +36,12 @@ RUN go mod tidy -diff || \
     { echo "go.mod or go.sum is not tidy: run make tidy" >&2; exit 1; }
 
 # Go's build cache is kept on a tmpfs, out of the image: nothing uses it
-# after this step, and writing it into the image takes seconds.
+# after this step, and writing it into the image takes seconds. The tests
+# are built with the no_fs_access tag, as the binary is in the build stage.
 RUN --mount=type=tmpfs,target=/root/.cache/go-build \
-    go test -timeout 90s -race -cover ./... || \
+    go test -tags no_fs_access -timeout 90s -race -cover ./... || \
     { echo "--- Rerunning with -v for details ---"; \
-      go test -timeout 90s -race -v ./...; exit 1; }
+      go test -tags no_fs_access -timeout 90s -race -v ./...; exit 1; }
 
 # Tidy stage: `go mod tidy` in the test phase's Go, so that the files it
 # writes pass the test phase's check. Nothing else depends on it, so only
@@ -84,7 +85,10 @@ COPY . .
 # The VERSION build arg when one is given, otherwise
 # `git describe --tags --always` on the .git in the build context. With
 # .git present, a version that is still empty, dev or unknown fails the
-# build: git is missing or could not read the checkout.
+# build: git is missing or could not read the checkout. The no_fs_access
+# tag keeps Coraza from writing the files of a multipart body to the
+# system's temporary directory, since smallwebwaf writes only to its state
+# directory.
 ARG VERSION
 RUN VERSION="${VERSION:-$(git describe --tags --always)}"; \
     if [ -e .git ]; then \
@@ -93,7 +97,7 @@ RUN VERSION="${VERSION:-$(git describe --tags --always)}"; \
             exit 1 ;; \
         esac; \
     fi; \
-    CGO_ENABLED=0 go build -trimpath \
+    CGO_ENABLED=0 go build -tags no_fs_access -trimpath \
         -ldflags="-s -w -X main.Version=${VERSION}" \
         -o /usr/local/bin/smallwebwaf ./cmd/smallwebwaf