[Pal] Rework manifest loading

This is a major refactor of the way manifests are loaded and handled,
which will be followed by a complete rework of the loader code (which
will include e.g. centralized config).

Changes/fixes:
- Huge part of manifest handling was refactored and untangled.
- Starting without a manifest is now disallowed. This was actually
  accidentally broken for some time and no one complained. It also makes
  little sense in practice and in Graphene's overall design, e.g. it
  conflicts with protected argv.
- Now we only allow starting by giving the executable, not manifest (the
  magic resolution logic was removed).
- Now manifests are sent over pipes between parent and children, instead
  of children finding and loading them on their own. This is a
  preparation for the upcoming centralized manifests change.
- Previously manifests were parsed 2 times on Linux and 3 times on
  Linux-SGX (by untrusted PAL, trusted PAL and LibOS). This is now
  fixed.
- The common `pal_main()` now requires that the backend-specific PAL
  loader loads the manifest before calling it. SGX code already has to
  do it (for proper initialization), so let's unify this interface for
  all PALs.
- Fix for a PAL crash when manifest size was divisible by page size
  (sic!). NULL termination was missing, but most of the time the padding
  to page size saved Graphene from crashing.

Examples/python-simple/README.md

@@ -42,14 +42,14 @@ Here's an example of running Python scripts under Graphene:
 
 Without SGX:
 ```
-./pal_loader python.manifest scripts/helloworld.py
-./pal_loader python.manifest scripts/fibonacci.py
+./pal_loader ./python scripts/helloworld.py
+./pal_loader ./python scripts/fibonacci.py
 ```
 
 With SGX:
 ```
-SGX=1 ./pal_loader python.manifest scripts/helloworld.py
-SGX=1 ./pal_loader python.manifest scripts/fibonacci.py
+SGX=1 ./pal_loader ./python scripts/helloworld.py
+SGX=1 ./pal_loader ./python scripts/fibonacci.py
 ```
 
 You can also manually run included tests:

Examples/python-simple/run-tests.sh

@@ -4,23 +4,23 @@ set -e
 
 # === hellworld ===
 echo -e "\n\nRunning helloworld.py:"
-./pal_loader python.manifest scripts/helloworld.py > OUTPUT
+./pal_loader ./python scripts/helloworld.py > OUTPUT
 grep -q "Hello World" OUTPUT && echo "[ Success 1/3 ]"
 rm OUTPUT
 
 # === fibonacci ===
 echo -e "\n\nRunning fibonacci.py:"
-./pal_loader python.manifest scripts/fibonacci.py > OUTPUT
+./pal_loader ./python scripts/fibonacci.py > OUTPUT
 grep -q "fib2              55" OUTPUT && echo "[ Success 2/3 ]"
 rm OUTPUT
 
 # === web server and client (on port 8005) ===
 echo -e "\n\nRunning HTTP server dummy-web-server.py in the background:"
-./pal_loader python.manifest scripts/dummy-web-server.py 8005 & echo $! > server.PID
+./pal_loader ./python scripts/dummy-web-server.py 8005 & echo $! > server.PID
 sleep 30  # Graphene-SGX takes a lot of time to initialize
 
 echo -e "\n\nRunning HTTP client test-http.py:"
-./pal_loader python.manifest scripts/test-http.py localhost 8005 > OUTPUT1
+./pal_loader ./python scripts/test-http.py localhost 8005 > OUTPUT1
 wget -q http://localhost:8005/ -O OUTPUT2
 echo >> OUTPUT2  # include newline since wget doesn't add it
 # check if all lines from OUTPUT2 are included in OUTPUT1