sandbox/linux/bpf_dsl/codegen.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "sandbox/linux/bpf_dsl/codegen.h"
   6
   7 #include <limits>
   8 #include <utility>
   9
  10 #include "base/logging.h"
  11 #include "sandbox/linux/system_headers/linux_filter.h"
  12
  13 // This CodeGen implementation strives for simplicity while still
  14 // generating acceptable BPF programs under typical usage patterns
  15 // (e.g., by PolicyCompiler).
  16 //
  17 // The key to its simplicity is that BPF programs only support forward
  18 // jumps/branches, which allows constraining the DAG construction API
  19 // to make instruction nodes immutable. Immutable nodes admits a
  20 // simple greedy approach of emitting new instructions as needed and
  21 // then reusing existing ones that have already been emitted. This
  22 // cleanly avoids any need to compute basic blocks or apply
  23 // topological sorting because the API effectively sorts instructions
  24 // for us (e.g., before MakeInstruction() can be called to emit a
  25 // branch instruction, it must have already been called for each
  26 // branch path).
  27 //
  28 // This greedy algorithm is not without (theoretical) weakness though:
  29 //
  30 //   1. In the general case, we don't eliminate dead code.  If needed,
  31 //      we could trace back through the program in Compile() and elide
  32 //      any unneeded instructions, but in practice we only emit live
  33 //      instructions anyway.
  34 //
  35 //   2. By not dividing instructions into basic blocks and sorting, we
  36 //      lose an opportunity to move non-branch/non-return instructions
  37 //      adjacent to their successor instructions, which means we might
  38 //      need to emit additional jumps. But in practice, they'll
  39 //      already be nearby as long as callers don't go out of their way
  40 //      to interleave MakeInstruction() calls for unrelated code
  41 //      sequences.
  42
  43 namespace sandbox {
  44
  45 // kBranchRange is the maximum value that can be stored in
  46 // sock_filter's 8-bit jt and jf fields.
  47 const size_t kBranchRange = std::numeric_limits<uint8_t>::max();
  48
  49 const CodeGen::Node CodeGen::kNullNode;
  50
  51 CodeGen::CodeGen() : program_(), equivalent_(), memos_() {
  52 }
  53
  54 CodeGen::~CodeGen() {
  55 }
  56
  57 CodeGen::Program CodeGen::Compile(CodeGen::Node head) {
  58   return Program(program_.rbegin() + Offset(head), program_.rend());
  59 }
  60
  61 CodeGen::Node CodeGen::MakeInstruction(uint16_t code,
  62                                        uint32_t k,
  63                                        Node jt,
  64                                        Node jf) {
  65   // To avoid generating redundant code sequences, we memoize the
  66   // results from AppendInstruction().
  67   auto res = memos_.insert(std::make_pair(MemoKey(code, k, jt, jf), kNullNode));
  68   CodeGen::Node* node = &res.first->second;
  69   if (res.second) {  // Newly inserted memo entry.
  70     *node = AppendInstruction(code, k, jt, jf);
  71   }
  72   return *node;
  73 }
  74
  75 CodeGen::Node CodeGen::AppendInstruction(uint16_t code,
  76                                          uint32_t k,
  77                                          Node jt,
  78                                          Node jf) {
  79   if (BPF_CLASS(code) == BPF_JMP) {
  80     CHECK_NE(BPF_JA, BPF_OP(code)) << "CodeGen inserts JAs as needed";
  81
  82     // Optimally adding jumps is rather tricky, so we use a quick
  83     // approximation: by artificially reducing |jt|'s range, |jt| will
  84     // stay within its true range even if we add a jump for |jf|.
  85     jt = WithinRange(jt, kBranchRange - 1);
  86     jf = WithinRange(jf, kBranchRange);
  87     return Append(code, k, Offset(jt), Offset(jf));
  88   }
  89
  90   CHECK_EQ(kNullNode, jf) << "Non-branch instructions shouldn't provide jf";
  91   if (BPF_CLASS(code) == BPF_RET) {
  92     CHECK_EQ(kNullNode, jt) << "Return instructions shouldn't provide jt";
  93   } else {
  94     // For non-branch/non-return instructions, execution always
  95     // proceeds to the next instruction; so we need to arrange for
  96     // that to be |jt|.
  97     jt = WithinRange(jt, 0);
  98     CHECK_EQ(0U, Offset(jt)) << "ICE: Failed to setup next instruction";
  99   }
 100   return Append(code, k, 0, 0);
 101 }
 102
 103 CodeGen::Node CodeGen::WithinRange(Node target, size_t range) {
 104   // Just use |target| if it's already within range.
 105   if (Offset(target) <= range) {
 106     return target;
 107   }
 108
 109   // Alternatively, look for an equivalent instruction within range.
 110   if (Offset(equivalent_.at(target)) <= range) {
 111     return equivalent_.at(target);
 112   }
 113
 114   // Otherwise, fall back to emitting a jump instruction.
 115   Node jump = Append(BPF_JMP | BPF_JA, Offset(target), 0, 0);
 116   equivalent_.at(target) = jump;
 117   return jump;
 118 }
 119
 120 CodeGen::Node CodeGen::Append(uint16_t code, uint32_t k, size_t jt, size_t jf) {
 121   if (BPF_CLASS(code) == BPF_JMP && BPF_OP(code) != BPF_JA) {
 122     CHECK_LE(jt, kBranchRange);
 123     CHECK_LE(jf, kBranchRange);
 124   } else {
 125     CHECK_EQ(0U, jt);
 126     CHECK_EQ(0U, jf);
 127   }
 128
 129   CHECK_LT(program_.size(), static_cast<size_t>(BPF_MAXINSNS));
 130   CHECK_EQ(program_.size(), equivalent_.size());
 131
 132   Node res = program_.size();
 133   program_.push_back(sock_filter{
 134       code, static_cast<uint8_t>(jt), static_cast<uint8_t>(jf), k});
 135   equivalent_.push_back(res);
 136   return res;
 137 }
 138
 139 size_t CodeGen::Offset(Node target) const {
 140   CHECK_LT(target, program_.size()) << "Bogus offset target node";
 141   return (program_.size() - 1) - target;
 142 }
 143
 144 // TODO(mdempsky): Move into a general base::Tuple helper library.
 145 bool CodeGen::MemoKeyLess::operator()(const MemoKey& lhs,
 146                                       const MemoKey& rhs) const {
 147   if (base::get<0>(lhs) != base::get<0>(rhs))
 148     return base::get<0>(lhs) < base::get<0>(rhs);
 149   if (base::get<1>(lhs) != base::get<1>(rhs))
 150     return base::get<1>(lhs) < base::get<1>(rhs);
 151   if (base::get<2>(lhs) != base::get<2>(rhs))
 152     return base::get<2>(lhs) < base::get<2>(rhs);
 153   if (base::get<3>(lhs) != base::get<3>(rhs))
 154     return base::get<3>(lhs) < base::get<3>(rhs);
 155   return false;
 156 }
 157
 158 }  // namespace sandbox