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// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "sandbox/linux/bpf_dsl/codegen.h"
#include <stddef.h>
#include <stdint.h>
#include <limits>
#include <ostream>
#include <utility>
#include "base/check_op.h"
#include "sandbox/linux/system_headers/linux_filter.h"
// This CodeGen implementation strives for simplicity while still
// generating acceptable BPF programs under typical usage patterns
// (e.g., by PolicyCompiler).
//
// The key to its simplicity is that BPF programs only support forward
// jumps/branches, which allows constraining the DAG construction API
// to make instruction nodes immutable. Immutable nodes admits a
// simple greedy approach of emitting new instructions as needed and
// then reusing existing ones that have already been emitted. This
// cleanly avoids any need to compute basic blocks or apply
// topological sorting because the API effectively sorts instructions
// for us (e.g., before MakeInstruction() can be called to emit a
// branch instruction, it must have already been called for each
// branch path).
//
// This greedy algorithm is not without (theoretical) weakness though:
//
// 1. In the general case, we don't eliminate dead code. If needed,
// we could trace back through the program in Compile() and elide
// any unneeded instructions, but in practice we only emit live
// instructions anyway.
//
// 2. By not dividing instructions into basic blocks and sorting, we
// lose an opportunity to move non-branch/non-return instructions
// adjacent to their successor instructions, which means we might
// need to emit additional jumps. But in practice, they'll
// already be nearby as long as callers don't go out of their way
// to interleave MakeInstruction() calls for unrelated code
// sequences.
namespace sandbox {
// kBranchRange is the maximum value that can be stored in
// sock_filter's 8-bit jt and jf fields.
const size_t kBranchRange = std::numeric_limits<uint8_t>::max();
const CodeGen::Node CodeGen::kNullNode;
CodeGen::CodeGen() : program_(), equivalent_(), memos_() {
}
CodeGen::~CodeGen() {
}
CodeGen::Program CodeGen::Compile(CodeGen::Node head) {
return Program(program_.rbegin() + Offset(head), program_.rend());
}
CodeGen::Node CodeGen::MakeInstruction(uint16_t code,
uint32_t k,
Node jt,
Node jf) {
// To avoid generating redundant code sequences, we memoize the
// results from AppendInstruction().
auto res = memos_.insert(std::make_pair(MemoKey(code, k, jt, jf), kNullNode));
CodeGen::Node* node = &res.first->second;
if (res.second) { // Newly inserted memo entry.
*node = AppendInstruction(code, k, jt, jf);
}
return *node;
}
CodeGen::Node CodeGen::AppendInstruction(uint16_t code,
uint32_t k,
Node jt,
Node jf) {
if (BPF_CLASS(code) == BPF_JMP) {
CHECK_NE(BPF_JA, BPF_OP(code)) << "CodeGen inserts JAs as needed";
// Optimally adding jumps is rather tricky, so we use a quick
// approximation: by artificially reducing |jt|'s range, |jt| will
// stay within its true range even if we add a jump for |jf|.
jt = WithinRange(jt, kBranchRange - 1);
jf = WithinRange(jf, kBranchRange);
return Append(code, k, Offset(jt), Offset(jf));
}
CHECK_EQ(kNullNode, jf) << "Non-branch instructions shouldn't provide jf";
if (BPF_CLASS(code) == BPF_RET) {
CHECK_EQ(kNullNode, jt) << "Return instructions shouldn't provide jt";
} else {
// For non-branch/non-return instructions, execution always
// proceeds to the next instruction; so we need to arrange for
// that to be |jt|.
jt = WithinRange(jt, 0);
CHECK_EQ(0U, Offset(jt)) << "ICE: Failed to setup next instruction";
}
return Append(code, k, 0, 0);
}
CodeGen::Node CodeGen::WithinRange(Node target, size_t range) {
// Just use |target| if it's already within range.
if (Offset(target) <= range) {
return target;
}
// Alternatively, look for an equivalent instruction within range.
if (Offset(equivalent_.at(target)) <= range) {
return equivalent_.at(target);
}
// Otherwise, fall back to emitting a jump instruction.
Node jump = Append(BPF_JMP | BPF_JA, Offset(target), 0, 0);
equivalent_.at(target) = jump;
return jump;
}
CodeGen::Node CodeGen::Append(uint16_t code, uint32_t k, size_t jt, size_t jf) {
if (BPF_CLASS(code) == BPF_JMP && BPF_OP(code) != BPF_JA) {
CHECK_LE(jt, kBranchRange);
CHECK_LE(jf, kBranchRange);
} else {
CHECK_EQ(0U, jt);
CHECK_EQ(0U, jf);
}
CHECK_LT(program_.size(), static_cast<size_t>(BPF_MAXINSNS));
CHECK_EQ(program_.size(), equivalent_.size());
Node res = program_.size();
program_.push_back(sock_filter{
code, static_cast<uint8_t>(jt), static_cast<uint8_t>(jf), k});
equivalent_.push_back(res);
return res;
}
size_t CodeGen::Offset(Node target) const {
CHECK_LT(target, program_.size()) << "Bogus offset target node";
return (program_.size() - 1) - target;
}
} // namespace sandbox
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