From a39fefd9509883a8e267b770990b5e7daf9895f3 Mon Sep 17 00:00:00 2001
From: Peter Rugg <pdr32@cam.ac.uk>
Date: Thu, 5 Sep 2019 12:10:11 +0100
Subject: [PATCH] Add safe maskAddr function

---
 CHERICC_Fat.bsv | 13 ++++++++++---
 CHERICap.bsv    | 10 +++++++---
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/CHERICC_Fat.bsv b/CHERICC_Fat.bsv
index 43d7059..564cffe 100644
--- a/CHERICC_Fat.bsv
+++ b/CHERICC_Fat.bsv
@@ -849,7 +849,7 @@ typedef struct {
   TempFields tempFields;
 } CapPipe deriving (Bits, FShow);
 
-instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW);
+instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
   function isValidCap (x);
     CapabilityInMemory capMem = unpack(x);
     return capMem.isCapability;
@@ -870,6 +870,7 @@ instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW);
   function setType = error("feature not implemented for this cap type");
   function getAddr = error("feature not implemented for this cap type");
   function setAddr = error("feature not implemented for this cap type");
+  function maskAddr = error("feature not implemented for this cap type");
   function getOffset = error("feature not implemented for this cap type");
   function setOffset = error("feature not implemented for this cap type");
   function getBase = error("feature not implemented for this cap type");
@@ -885,7 +886,7 @@ instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW);
   function toMem = error("feature not implemented for this cap type");
 endinstance
 
-instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW);
+instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
   function isValidCap = error("feature not implemented for this cap type");
   function setValidCap = error("feature not implemented for this cap type");
   function getFlags = error("feature not implemented for this cap type");
@@ -899,6 +900,7 @@ instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW);
   function setType = error("feature not implemented for this cap type");
   function getAddr = error("feature not implemented for this cap type");
   function setAddr = error("feature not implemented for this cap type");
+  function maskAddr = error("feature not implemented for this cap type");
   function getOffset = error("feature not implemented for this cap type");
   function setOffset = error("feature not implemented for this cap type");
   function getBase = error("feature not implemented for this cap type");
@@ -914,7 +916,7 @@ instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW);
   function toMem = error("feature not implemented for this cap type");
 endinstance
 
-instance CHERICap #(CapPipe, OTypeW, FlagsW, CapAddressW, CapW);
+instance CHERICap #(CapPipe, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
 
   function isValidCap (x) = x.capFat.isCapability;
 
@@ -1004,6 +1006,11 @@ instance CHERICap #(CapPipe, OTypeW, FlagsW, CapAddressW, CapW);
     return Exact {exact: result.v, value: cap};
   endfunction
 
+  function CapPipe maskAddr (CapPipe cap, Bit#(TSub#(MW, 3)) mask);
+    cap.capFat.addrBits[valueOf(TSub#(MW, 4)):0] = cap.capFat.addrBits[valueOf(TSub#(MW, 4)):0] & mask;
+    return cap;
+  endfunction
+
   function getOffset (x) = getOffsetFat(x.capFat, x.tempFields);
 
   function Exact#(CapPipe) setOffset (CapPipe cap, Bit#(CapAddressW) offset, Bool increment);
diff --git a/CHERICap.bsv b/CHERICap.bsv
index bdab959..0b17924 100644
--- a/CHERICap.bsv
+++ b/CHERICap.bsv
@@ -78,8 +78,8 @@ typedef enum {
   SEALED_WITH_TYPE
 } Kind deriving (Bits, Eq, FShow);
 
-typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, numeric type mem_sz)
-  dependencies (t determines (ot, flg, n, mem_sz));
+typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, numeric type mem_sz, numeric type maskable_bits)
+  dependencies (t determines (ot, flg, n, mem_sz, maskable_bits));
 
   // Return whether the Capability is valid
   function Bool isValidCap (t cap);
@@ -123,6 +123,10 @@ typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, n
   function Bit#(n) getAddr (t cap);
   // Set the address of the capability. Result invalid if not exact
   function Exact#(t) setAddr (t cap, Bit#(n) addr);
+  // Mask the least significant bits of capability address with a mask
+  // maskable_width should be small enough to make this
+  // safe with respect to representability
+  function t maskAddr (t cap, Bit#(maskable_bits) mask);
 
   // Get the offset of the capability
   function Bit#(n) getOffset (t cap) = getAddr(cap) - getBase(cap);
@@ -164,7 +168,7 @@ typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, n
 
 endtypeclass
 
-function Fmt showCHERICap(t cap) provisos (CHERICap#(t, ot, flg, n, mem_sz));
+function Fmt showCHERICap(t cap) provisos (CHERICap#(t, ot, flg, n, mem_sz, maskable_bits));
   return $format( "Valid: 0x%0x", isValidCap(cap)) +
          $format(" Perms: 0x%0x", getPerms(cap)) +
          $format(" Kind: ", fshow(getKind(cap))) +