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// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <stddef.h>
#include <stdint.h>
#include <memory>
#include <utility>
#include <vector>
#include "base/macros.h"
#include "sandbox/linux/bpf_dsl/bpf_dsl_forward.h"
#include "sandbox/linux/bpf_dsl/cons.h"
#include "sandbox/linux/bpf_dsl/trap_registry.h"
#include "sandbox/sandbox_export.h"
// The sandbox::bpf_dsl namespace provides a domain-specific language
// to make writing BPF policies more expressive. In general, the
// object types all have value semantics (i.e., they can be copied
// around, returned from or passed to function calls, etc. without any
// surprising side effects), though not all support assignment.
// An idiomatic and demonstrative (albeit silly) example of this API
// would be:
// #include "sandbox/linux/bpf_dsl/bpf_dsl.h"
// using namespace sandbox::bpf_dsl;
// class SillyPolicy : public Policy {
// public:
// SillyPolicy() {}
// ~SillyPolicy() override {}
// ResultExpr EvaluateSyscall(int sysno) const override {
// if (sysno == __NR_fcntl) {
// Arg<int> fd(0), cmd(1);
// Arg<unsigned long> flags(2);
// const uint64_t kGoodFlags = O_ACCMODE | O_NONBLOCK;
// return If(AllOf(fd == 0,
// cmd == F_SETFL,
// (flags & ~kGoodFlags) == 0),
// Allow())
// .ElseIf(AnyOf(cmd == F_DUPFD, cmd == F_DUPFD_CLOEXEC),
// Error(EMFILE))
// .Else(Trap(SetFlagHandler, NULL));
// } else {
// return Allow();
// }
// }
// private:
// };
// More generally, the DSL currently supports the following grammar:
// result = Allow() | Error(errno) | Kill() | Trace(aux)
// | Trap(trap_func, aux) | UnsafeTrap(trap_func, aux)
// | If(bool, result)[.ElseIf(bool, result)].Else(result)
// | Switch(arg)[.Case(val, result)].Default(result)
// bool = BoolConst(boolean) | Not(bool) | AllOf(bool...) | AnyOf(bool...)
// | arg == val | arg != val
// arg = Arg<T>(num) | arg & mask
// The semantics of each function and operator are intended to be
// intuitive, but are described in more detail below.
// (Credit to Sean Parent's "Inheritance is the Base Class of Evil"
// talk at Going Native 2013 for promoting value semantics via shared
// pointers to immutable state.)
namespace sandbox {
namespace bpf_dsl {
// ResultExpr is an opaque reference to an immutable result expression tree.
using ResultExpr = std::shared_ptr<const internal::ResultExprImpl>;
// BoolExpr is an opaque reference to an immutable boolean expression tree.
using BoolExpr = std::shared_ptr<const internal::BoolExprImpl>;
// Allow specifies a result that the system call should be allowed to
// execute normally.
SANDBOX_EXPORT ResultExpr Allow();
// Error specifies a result that the system call should fail with
// error number |err|. As a special case, Error(0) will result in the
// system call appearing to have succeeded, but without having any
// side effects.
SANDBOX_EXPORT ResultExpr Error(int err);
// Kill specifies a result to kill the process (task) immediately.
SANDBOX_EXPORT ResultExpr Kill();
// Trace specifies a result to notify a tracing process via the
// PTRACE_EVENT_SECCOMP event and allow it to change or skip the system call.
// The value of |aux| will be available to the tracer via PTRACE_GETEVENTMSG.
SANDBOX_EXPORT ResultExpr Trace(uint16_t aux);
// Trap specifies a result that the system call should be handled by
// trapping back into userspace and invoking |trap_func|, passing
// |aux| as the second parameter.
Trap(TrapRegistry::TrapFnc trap_func, const void* aux);
// UnsafeTrap is like Trap, except the policy is marked as "unsafe"
// and allowed to use SandboxSyscall to invoke any system call.
// NOTE: This feature, by definition, disables all security features of
// the sandbox. It should never be used in production, but it can be
// very useful to diagnose code that is incompatible with the sandbox.
// If even a single system call returns "UnsafeTrap", the security of
// entire sandbox should be considered compromised.
UnsafeTrap(TrapRegistry::TrapFnc trap_func, const void* aux);
// BoolConst converts a bool value into a BoolExpr.
SANDBOX_EXPORT BoolExpr BoolConst(bool value);
// Not returns a BoolExpr representing the logical negation of |cond|.
SANDBOX_EXPORT BoolExpr Not(BoolExpr cond);
// AllOf returns a BoolExpr representing the logical conjunction ("and")
// of zero or more BoolExprs.
SANDBOX_EXPORT BoolExpr AllOf(BoolExpr lhs, BoolExpr rhs);
template <typename... Rest>
SANDBOX_EXPORT BoolExpr AllOf(BoolExpr first, Rest&&... rest);
// AnyOf returns a BoolExpr representing the logical disjunction ("or")
// of zero or more BoolExprs.
SANDBOX_EXPORT BoolExpr AnyOf(BoolExpr lhs, BoolExpr rhs);
template <typename... Rest>
SANDBOX_EXPORT BoolExpr AnyOf(BoolExpr first, Rest&&... rest);
template <typename T>
// Initializes the Arg to represent the |num|th system call
// argument (indexed from 0), which is of type |T|.
explicit Arg(int num);
Arg(const Arg& arg) : num_(arg.num_), mask_(arg.mask_) {}
// Returns an Arg representing the current argument, but after
// bitwise-and'ing it with |rhs|.
friend Arg operator&(const Arg& lhs, uint64_t rhs) {
return Arg(lhs.num_, lhs.mask_ & rhs);
// Returns a boolean expression comparing whether the system call argument
// (after applying any bitmasks, if appropriate) equals |rhs|.
friend BoolExpr operator==(const Arg& lhs, T rhs) { return lhs.EqualTo(rhs); }
// Returns a boolean expression comparing whether the system call argument
// (after applying any bitmasks, if appropriate) does not equal |rhs|.
friend BoolExpr operator!=(const Arg& lhs, T rhs) { return Not(lhs == rhs); }
Arg(int num, uint64_t mask) : num_(num), mask_(mask) {}
BoolExpr EqualTo(T val) const;
int num_;
uint64_t mask_;
// If begins a conditional result expression predicated on the
// specified boolean expression.
SANDBOX_EXPORT Elser If(BoolExpr cond, ResultExpr then_result);
class SANDBOX_EXPORT Elser {
Elser(const Elser& elser);
// ElseIf extends the conditional result expression with another
// "if then" clause, predicated on the specified boolean expression.
Elser ElseIf(BoolExpr cond, ResultExpr then_result) const;
// Else terminates a conditional result expression using |else_result| as
// the default fallback result expression.
ResultExpr Else(ResultExpr else_result) const;
using Clause = std::pair<BoolExpr, ResultExpr>;
explicit Elser(cons::List<Clause> clause_list);
cons::List<Clause> clause_list_;
friend Elser If(BoolExpr, ResultExpr);
template <typename T>
friend Caser<T> Switch(const Arg<T>&);
// Switch begins a switch expression dispatched according to the
// specified argument value.
template <typename T>
SANDBOX_EXPORT Caser<T> Switch(const Arg<T>& arg);
template <typename T>
class SANDBOX_EXPORT Caser {
Caser(const Caser<T>& caser) : arg_(caser.arg_), elser_(caser.elser_) {}
~Caser() {}
// Case adds a single-value "case" clause to the switch.
Caser<T> Case(T value, ResultExpr result) const;
// Cases adds a multiple-value "case" clause to the switch.
// See also the SANDBOX_BPF_DSL_CASES macro below for a more idiomatic way
// of using this function.
template <typename... Values>
Caser<T> CasesImpl(ResultExpr result, const Values&... values) const;
// Terminate the switch with a "default" clause.
ResultExpr Default(ResultExpr result) const;
Caser(const Arg<T>& arg, Elser elser) : arg_(arg), elser_(elser) {}
Arg<T> arg_;
Elser elser_;
template <typename U>
friend Caser<U> Switch(const Arg<U>&);
// Recommended usage is to put
// near the top of the .cc file (e.g., nearby any "using" statements), then
// use like:
// Switch(arg).CASES((3, 5, 7), result)...;
#define SANDBOX_BPF_DSL_CASES(values, result) \
CasesImpl(result, SANDBOX_BPF_DSL_CASES_HELPER values)
// Helper macro to strip parentheses.
// =====================================================================
// Official API ends here.
// =====================================================================
namespace internal {
// Make argument-dependent lookup work. This is necessary because although
// BoolExpr is defined in bpf_dsl, since it's merely a typedef for
// scoped_refptr<const internal::BoolExplImpl>, argument-dependent lookup only
// searches the "internal" nested namespace.
using bpf_dsl::Not;
using bpf_dsl::AllOf;
using bpf_dsl::AnyOf;
// Returns a boolean expression that represents whether system call
// argument |num| of size |size| is equal to |val|, when masked
// according to |mask|. Users should use the Arg template class below
// instead of using this API directly.
ArgEq(int num, size_t size, uint64_t mask, uint64_t val);
// Returns the default mask for a system call argument of the specified size.
SANDBOX_EXPORT uint64_t DefaultMask(size_t size);
} // namespace internal
template <typename T>
Arg<T>::Arg(int num)
: num_(num), mask_(internal::DefaultMask(sizeof(T))) {
// Definition requires ArgEq to have been declared. Moved out-of-line
// to minimize how much internal clutter users have to ignore while
// reading the header documentation.
// Additionally, we use this helper member function to avoid linker errors
// caused by defining operator== out-of-line. For a more detailed explanation,
// see
template <typename T>
BoolExpr Arg<T>::EqualTo(T val) const {
if (sizeof(T) == 4) {
// Prevent sign-extension of negative int32_t values.
return internal::ArgEq(num_, sizeof(T), mask_, static_cast<uint32_t>(val));
return internal::ArgEq(num_, sizeof(T), mask_, static_cast<uint64_t>(val));
template <typename T>
SANDBOX_EXPORT Caser<T> Switch(const Arg<T>& arg) {
return Caser<T>(arg, Elser(nullptr));
template <typename T>
Caser<T> Caser<T>::Case(T value, ResultExpr result) const {
return SANDBOX_BPF_DSL_CASES((value), std::move(result));
template <typename T>
template <typename... Values>
Caser<T> Caser<T>::CasesImpl(ResultExpr result, const Values&... values) const {
// Theoretically we could evaluate arg_ just once and emit a more efficient
// dispatch table, but for now we simply translate into an equivalent
// If/ElseIf/Else chain.
return Caser<T>(arg_,
elser_.ElseIf(AnyOf((arg_ == values)...), std::move(result)));
template <typename T>
ResultExpr Caser<T>::Default(ResultExpr result) const {
return elser_.Else(std::move(result));
template <typename... Rest>
BoolExpr AllOf(BoolExpr first, Rest&&... rest) {
return AllOf(std::move(first), AllOf(std::forward<Rest>(rest)...));
template <typename... Rest>
BoolExpr AnyOf(BoolExpr first, Rest&&... rest) {
return AnyOf(std::move(first), AnyOf(std::forward<Rest>(rest)...));
} // namespace bpf_dsl
} // namespace sandbox