Added a new SafeSPrintf() function that implements snprintf() in an async-safe-fashion

base/strings/safe_sprintf.h

+// Copyright (c) 2013 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.
+
+#ifndef BASE_STRINGS_SAFE_SPRINTF_H_
+#define BASE_STRINGS_SAFE_SPRINTF_H_
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#if defined(__unix__)
+// For ssize_t
+#include <unistd.h>
+#endif
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+
+namespace base {
+namespace strings {
+
+#if defined(_MSC_VER)
+// Define ssize_t inside of our namespace.
+#if defined(_WIN64)
+typedef __int64 ssize_t;
+#else
+typedef long ssize_t;
+#endif
+#endif
+
+// SafeSPrintf() is a type-safe and completely self-contained version of
+// snprintf().
+//
+// SafeSNPrintf() is an alternative function signature that can be used when
+// not dealing with fixed-sized buffers. When possible, SafeSPrintf() should
+// always be used instead of SafeSNPrintf()
+//
+// These functions allow for formatting complicated messages from contexts that
+// require strict async-signal-safety. In fact, it is safe to call them from
+// any low-level execution context, as they are guaranteed to make no library
+// or system calls. It deliberately never touches "errno", either.
+//
+// The only exception to this rule is that in debug builds the code calls
+// RAW_CHECK() to help diagnose problems when the format string does not
+// match the rest of the arguments. In release builds, no CHECK()s are used,
+// and SafeSPrintf() instead returns an output string that expands only
+// those arguments that match their format characters. Mismatched arguments
+// are ignored.
+//
+// The code currently only supports a subset of format characters:
+//   %c, %o, %d, %x, %X, %p, and %s.
+//
+// SafeSPrintf() aims to be as liberal as reasonably possible. Integer-like
+// values of arbitrary width can be passed to all of the format characters
+// that expect integers. Thus, it is explicitly legal to pass an "int" to
+// "%c", and output will automatically look at the LSB only. It is also
+// explicitly legal to pass either signed or unsigned values, and the format
+// characters will automatically interpret the arguments accordingly.
+//
+// It is still not legal to mix-and-match integer-like values with pointer
+// values. For instance, you cannot pass a pointer to %x, nor can you pass an
+// integer to %p.
+//
+// The one exception is "0" zero being accepted by "%p". This works-around
+// the problem of C++ defining NULL as an integer-like value.
+//
+// All format characters take an optional width parameter. This must be a
+// positive integer. For %d, %o, %x, %X and %p, if the width starts with
+// a leading '0', padding is done with '0' instead of ' ' characters.
+//
+// There are a few features of snprintf()-style format strings, that
+// SafeSPrintf() does not support at this time.
+//
+// If an actual user showed up, there is no particularly strong reason they
+// couldn't be added. But that assumes that the trade-offs between complexity
+// and utility are favorable.
+//
+// For example, adding support for negative padding widths, and for %n are all
+// likely to be viewed positively. They are all clearly useful, low-risk, easy
+// to test, don't jeopardize the async-signal-safety of the code, and overall
+// have little impact on other parts of SafeSPrintf() function.
+//
+// On the other hands, adding support for alternate forms, positional
+// arguments, grouping, wide characters, localization or floating point numbers
+// are all unlikely to ever be added.
+//
+// SafeSPrintf() and SafeSNPrintf() mimic the behavior of snprintf() and they
+// return the number of bytes needed to store the untruncated output. This
+// does *not* include the terminating NUL byte.
+//
+// They return -1, iff a fatal error happened. This typically can only happen,
+// if the buffer size is a) negative, or b) zero (i.e. not even the NUL byte
+// can be written). The return value can never be larger than SSIZE_MAX-1.
+// This ensures that the caller can always add one to the signed return code
+// in order to determine the amount of storage that needs to be allocated.
+//
+// While the code supports type checking and while it is generally very careful
+// to avoid printing incorrect values, it tends to be conservative in printing
+// as much as possible, even when given incorrect parameters. Typically, in
+// case of an error, the format string will not be expanded. (i.e. something
+// like SafeSPrintf(buf, "%p %d", 1, 2) results in "%p 2"). See above for
+// the use of RAW_CHECK() in debug builds, though.
+//
+// The pre-C++11 version cannot handle more than ten arguments.
+//
+// Basic example:
+//   char buf[20];
+//   base::strings::SafeSPrintf(buf, "The answer: %2d", 42);
+//
+// Example with dynamically sized buffer (async-signal-safe). This code won't
+// work on Visual studio, as it requires dynamically allocating arrays on the
+// stack. Consider picking a smaller value for |kMaxSize| if stack size is
+// limited and known. On the other hand, if the parameters to SafeSNPrintf()
+// are trusted and not controllable by the user, you can consider eliminating
+// the check for |kMaxSize| altogether. The current value of SSIZE_MAX is
+// essentially a no-op that just illustrates how to implement an upper bound:
+//   const size_t kInitialSize = 128;
+//   const size_t kMaxSize = std::numeric_limits<ssize_t>::max();
+//   size_t size = kInitialSize;
+//   for (;;) {
+//     char buf[size];
+//     size = SafeSNPrintf(buf, size, "Error message \"%s\"\n", err) + 1;
+//     if (sizeof(buf) < kMaxSize && size > kMaxSize) {
+//       size = kMaxSize;
+//       continue;
+//     } else if (size > sizeof(buf))
+//       continue;
+//     write(2, buf, size-1);
+//     break;
+//   }
+
+namespace internal {
+// Helpers that use C++ overloading, templates, and specializations to deduce
+// and record type information from function arguments. This allows us to
+// later write a type-safe version of snprintf().
+
+struct Arg {
+  enum Type { INT, UINT, STRING, POINTER };
+
+  // Any integer-like value.
+  Arg(signed char c)        : i(c), width(sizeof(char)),      type(INT)  { }
+  Arg(unsigned char c)      : i(c), width(sizeof(char)),      type(UINT) { }
+  Arg(signed short j)       : i(j), width(sizeof(short)),     type(INT)  { }
+  Arg(unsigned short j)     : i(j), width(sizeof(short)),     type(UINT) { }
+  Arg(signed int j)         : i(j), width(sizeof(int)),       type(INT)  { }
+  Arg(unsigned int j)       : i(j), width(sizeof(int)),       type(UINT) { }
+  Arg(signed long j)        : i(j), width(sizeof(long)),      type(INT)  { }
+  Arg(unsigned long j)      : i(j), width(sizeof(long)),      type(UINT) { }
+  Arg(signed long long j)   : i(j), width(sizeof(long long)), type(INT)  { }
+  Arg(unsigned long long j) : i(j), width(sizeof(long long)), type(UINT) { }
+
+  // A C-style text string.
+  Arg(const char* s) : str(s), type(STRING) { }
+  Arg(char* s)       : str(s), type(STRING) { }
+
+  // Any pointer value that can be cast to a "void*".
+  template<class T> Arg(T* p) : ptr((void*)p), type(POINTER) { }
+
+  union {
+    // An integer-like value.
+    struct {
+      int64_t       i;
+      unsigned char width;
+    };
+
+    // A C-style text string.
+    const char* str;
+
+    // A pointer to an arbitrary object.
+    const void* ptr;
+  };
+  const enum Type type;
+};
+
+// This is the internal function that performs the actual formatting of
+// an snprintf()-style format string.
+BASE_EXPORT ssize_t SafeSNPrintf(char* buf, size_t sz, const char* fmt,
+                                 const Arg* args, size_t max_args);
+
+#if !defined(NDEBUG)
+// In debug builds, allow unit tests to artificially lower the kSSizeMax
+// constant that is used as a hard upper-bound for all buffers. In normal
+// use, this constant should always be std::numeric_limits<ssize_t>::max().
+BASE_EXPORT void SetSafeSPrintfSSizeMaxForTest(size_t max);
+BASE_EXPORT size_t GetSafeSPrintfSSizeMaxForTest();
+#endif
+
+}  // namespace internal
+
+#if __cplusplus >= 201103  // C++11
+
+template<typename... Args>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt, Args... args) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { args... };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N, typename... Args>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt, Args... args) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { args... };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+#else  // Pre-C++11
+
+// TODO(markus): C++11 has a much more concise and readable solution for
+//   expressing what we are doing here. Delete the fall-back code for older
+//   compilers as soon as we have fully switched to C++11.
+
+template<class T0, class T1, class T2, class T3, class T4,
+         class T5, class T6, class T7, class T8, class T9>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                     T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N,
+         class T0, class T1, class T2, class T3, class T4,
+         class T5, class T6, class T7, class T8, class T9>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt,
+                    T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                    T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2, class T3, class T4,
+         class T5, class T6, class T7, class T8>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                     T5 arg5, T6 arg6, T7 arg7, T8 arg8) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N,
+         class T0, class T1, class T2, class T3, class T4, class T5,
+         class T6, class T7, class T8>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt,
+                    T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                    T5 arg5, T6 arg6, T7 arg7, T8 arg8) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2, class T3, class T4, class T5,
+         class T6, class T7>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                     T5 arg5, T6 arg6, T7 arg7) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N,
+         class T0, class T1, class T2, class T3, class T4, class T5,
+         class T6, class T7>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt,
+                    T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                    T5 arg5, T6 arg6, T7 arg7) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2, class T3, class T4, class T5,
+         class T6>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,
+                     T5 arg5, T6 arg6) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N,
+         class T0, class T1, class T2, class T3, class T4, class T5,
+         class T6>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt,
+                    T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5,
+                    T6 arg6) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = {
+    arg0, arg1, arg2, arg3, arg4, arg5, arg6
+  };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2, class T3, class T4, class T5>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2, arg3, arg4, arg5 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N,
+         class T0, class T1, class T2, class T3, class T4, class T5>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt,
+                    T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2, arg3, arg4, arg5 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2, class T3, class T4>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2, arg3, arg4 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N, class T0, class T1, class T2, class T3, class T4>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt, T0 arg0, T1 arg1,
+                    T2 arg2, T3 arg3, T4 arg4) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2, arg3, arg4 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2, class T3>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2, T3 arg3) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2, arg3 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N, class T0, class T1, class T2, class T3>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt,
+                    T0 arg0, T1 arg1, T2 arg2, T3 arg3) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2, arg3 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1, class T2>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt,
+                     T0 arg0, T1 arg1, T2 arg2) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N, class T0, class T1, class T2>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt, T0 arg0, T1 arg1,
+                    T2 arg2) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1, arg2 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0, class T1>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt, T0 arg0, T1 arg1) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N, class T0, class T1>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt, T0 arg0, T1 arg1) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0, arg1 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<class T0>
+ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt, T0 arg0) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+
+template<size_t N, class T0>
+ssize_t SafeSPrintf(char (&buf)[N], const char* fmt, T0 arg0) {
+  // Use Arg() object to record type information and then copy arguments to an
+  // array to make it easier to iterate over them.
+  const internal::Arg arg_array[] = { arg0 };
+  return internal::SafeSNPrintf(buf, N, fmt, arg_array, arraysize(arg_array));
+}
+#endif
+
+// Fast-path when we don't actually need to substitute any arguments.
+BASE_EXPORT ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt);
+template<size_t N>
+inline ssize_t SafeSPrintf(char (&buf)[N], const char* fmt) {
+  return SafeSNPrintf(buf, N, fmt);
+}
+
+}  // namespace strings
+}  // namespace base
+
+#endif  // BASE_STRINGS_SAFE_SPRINTF_H_