Genode: constrain `osTryAllocPages` to RAM quota (#6883)

Genode software components all start with an explicit RAM resource quota
which may or may not be upgraded during runtime by the parent process.
With this patch `osTryAllocPages` will fail if allocation exceeds quotas
set by the parent and the `osAllocPages` procedure will trigger a
blocking request to the parent to increase quotas. The previous behavior
could potentially block both procedures indefinitely for a quota upgrade
rather than fail and trigger garbage collection.

This patch also adds tracking of Genode dataspace mappings into the
component address space so they can be detached and freed.

2 files changed, 115 insertions, 11 deletions

diff --git a/lib/system/genodealloc.nim b/lib/system/genodealloc.nim
new file mode 100644
index 000000000..3646a842d
--- /dev/null
+++ b/lib/system/genodealloc.nim
@@ -0,0 +1,114 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2017 Emery Hemingway
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+# Low level dataspace allocator for Genode.
+
+when not defined(genode):
+  {.error: "Genode only module".}
+
+type DataspaceCapability {.
+  importcpp: "Genode::Dataspace_capability", pure.} = object
+
+type
+  Map = object
+    attachment: pointer
+    size: int
+    ds: DataspaceCapability
+
+  SlabMeta = object
+    next: ptr MapSlab
+    ds: DataspaceCapability
+
+  MapSlab = object
+    meta: SlabMeta
+    maps: array[1,Map]
+
+const SlabBackendSize = 4096
+
+proc ramAvail(): int {.
+  importcpp: "genodeEnv->pd().avail_ram().value".}
+  ## Return number of bytes available for allocation.
+
+proc capsAvail(): int {.
+  importcpp: "genodeEnv->pd().avail_caps().value".}
+  ## Return the number of available capabilities.
+  ## Each dataspace allocation consumes a capability.
+
+proc allocDataspace(size: int): DataspaceCapability {.
+  importcpp: "genodeEnv->pd().alloc(@)".}
+  ## Allocate a dataspace and its capability.
+
+proc attachDataspace(ds: DataspaceCapability): pointer {.
+  importcpp: "genodeEnv->rm().attach(@)".}
+  ## Attach a dataspace into the component address-space.
+
+proc detachAddress(p: pointer) {.
+  importcpp: "genodeEnv->rm().detach(@)".}
+  ## Detach a dataspace from the component address-space.
+
+proc freeDataspace(ds: DataspaceCapability) {.
+  importcpp: "genodeEnv->pd().free(@)".}
+  ## Free a dataspace.
+
+proc newMapSlab(): ptr MapSlab =
+  let
+    ds = allocDataspace SlabBackendSize
+    p = attachDataspace ds
+  result = cast[ptr MapSlab](p)
+  result.meta.ds = ds
+
+iterator items(s: ptr MapSlab): ptr Map =
+  let mapCount = (SlabBackendSize - sizeof(SlabMeta)) div sizeof(Map)
+  for i in 0 .. <mapCount:
+    yield s.maps[i].addr
+
+var slabs: ptr MapSlab
+
+proc osAllocPages(size: int): pointer =
+  if slabs.isNil:
+    slabs = newMapSlab()
+  var
+    slab = slabs
+    map: ptr Map
+  let mapCount = (SlabBackendSize - sizeof(SlabMeta)) div sizeof(Map)
+  block findFreeMap:
+    while true:
+      # lookup first free spot in slabs
+      for m in slab.items:
+        if m.attachment.isNil:
+          map = m
+          break findFreeMap
+      if slab.meta.next.isNil:
+        slab.meta.next = newMapSlab()
+          # tack a new slab on the tail
+      slab = slab.meta.next
+        # move to next slab in linked list
+  map.ds = allocDataspace size
+  map.size = size
+  map.attachment = attachDataspace map.ds
+  result = map.attachment
+
+proc osTryAllocPages(size: int): pointer =
+  if ramAvail() >= size and capsAvail() > 1:
+    result = osAllocPages size
+
+proc osDeallocPages(p: pointer; size: int) =
+  var slab = slabs
+  while not slab.isNil:
+    # lookup first free spot in slabs
+    for m in slab.items:
+      if m.attachment == p:
+        if m.size != size:
+          echo "cannot partially detach dataspace"
+          quit -1
+        detachAddress m.attachment
+        freeDataspace m.ds
+        m[] = Map()
+        return
+    slab = slab.meta.next
diff --git a/lib/system/osalloc.nim b/lib/system/osalloc.nim
index 444f11306..1ad4cf695 100644
--- a/lib/system/osalloc.nim
+++ b/lib/system/osalloc.nim
@@ -78,17 +78,7 @@ when defined(emscripten):
     munmap(mmapDescr.realPointer, mmapDescr.realSize)
 
 elif defined(genode):
-
-  proc osAllocPages(size: int): pointer {.
-   importcpp: "genodeEnv->rm().attach(genodeEnv->ram().alloc(@))".}
-
-  proc osTryAllocPages(size: int): pointer =
-    {.emit: """try {""".}
-    result = osAllocPages size
-    {.emit: """} catch (...) { }""".}
-
-  proc osDeallocPages(p: pointer, size: int) {.
-    importcpp: "genodeEnv->rm().detach(#)".}
+  include genodealloc # osAllocPages, osTryAllocPages, osDeallocPages
 
 elif defined(posix):
   const