// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/strings/safe_sprintf.h"
#include <errno.h>
#include <string.h>
#include <algorithm>
#include <limits>
#include "base/compiler_specific.h"
#include "base/memory/raw_ptr.h"
#include "build/build_config.h"
#if !defined(NDEBUG)
// In debug builds, we use RAW_CHECK() to print useful error messages, if
// SafeSPrintf() is called with broken arguments.
// As our contract promises that SafeSPrintf() can be called from any
// restricted run-time context, it is not actually safe to call logging
// functions from it; and we only ever do so for debug builds and hope for the
// best. We should _never_ call any logging function other than RAW_CHECK(),
// and we should _never_ include any logging code that is active in production
// builds. Most notably, we should not include these logging functions in
// unofficial release builds, even though those builds would otherwise have
// DCHECKS() enabled.
// In other words; please do not remove the #ifdef around this #include.
// Instead, in production builds we opt for returning a degraded result,
// whenever an error is encountered.
// E.g. The broken function call
// SafeSPrintf("errno = %d (%x)", errno, strerror(errno))
// will print something like
// errno = 13, (%x)
// instead of
// errno = 13 (Access denied)
// In most of the anticipated use cases, that's probably the preferred
// behavior.
#include "base/check.h"
#define DEBUG_CHECK RAW_CHECK
#else
#define DEBUG_CHECK(x) \
do { \
if (x) { \
} \
} while (0)
#endif
namespace base::strings {
// The code in this file is extremely careful to be async-signal-safe.
//
// Most obviously, we avoid calling any code that could dynamically allocate
// memory. Doing so would almost certainly result in bugs and dead-locks.
// We also avoid calling any other STL functions that could have unintended
// side-effects involving memory allocation or access to other shared
// resources.
//
// But on top of that, we also avoid calling other library functions, as many
// of them have the side-effect of calling getenv() (in order to deal with
// localization) or accessing errno. The latter sounds benign, but there are
// several execution contexts where it isn't even possible to safely read let
// alone write errno.
//
// The stated design goal of the SafeSPrintf() function is that it can be
// called from any context that can safely call C or C++ code (i.e. anything
// that doesn't require assembly code).
//
// For a brief overview of some but not all of the issues with async-signal-
// safety, refer to:
// http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html
namespace {
const size_t kSSizeMaxConst = ((size_t)(ssize_t)-1) >> 1;
const char kUpCaseHexDigits[] = "0123456789ABCDEF";
const char kDownCaseHexDigits[] = "0123456789abcdef";
} // namespace
#if defined(NDEBUG)
// We would like to define kSSizeMax as std::numeric_limits<ssize_t>::max(),
// but C++ doesn't allow us to do that for constants. Instead, we have to
// use careful casting and shifting. We later use a static_assert to
// verify that this worked correctly.
namespace {
const size_t kSSizeMax = kSSizeMaxConst;
}
#else // defined(NDEBUG)
// For efficiency, we really need kSSizeMax to be a constant. But for unit
// tests, it should be adjustable. This allows us to verify edge cases without
// having to fill the entire available address space. As a compromise, we make
// kSSizeMax adjustable in debug builds, and then only compile that particular
// part of the unit test in debug builds.
namespace {
static size_t kSSizeMax = kSSizeMaxConst;
}
namespace internal {
void SetSafeSPrintfSSizeMaxForTest(size_t max) {
kSSizeMax = max;
}
size_t GetSafeSPrintfSSizeMaxForTest() {
return kSSizeMax;
}
} // namespace internal
#endif // defined(NDEBUG)
namespace {
class Buffer {
public:
// |buffer| is caller-allocated storage that SafeSPrintf() writes to. It
// has |size| bytes of writable storage. It is the caller's responsibility
// to ensure that the buffer is at least one byte in size, so that it fits
// the trailing NUL that will be added by the destructor. The buffer also
// must be smaller or equal to kSSizeMax in size.
Buffer(char* buffer, size_t size) : buffer_(buffer), size_(size - 1) {
// MSVS2013's standard library doesn't mark max() as constexpr yet. cl.exe
// supports static_cast but doesn't really implement constexpr yet so it doesn't
// complain, but clang does.
#if __cplusplus >= 201103 && !(defined(__clang__) && BUILDFLAG(IS_WIN))
static_assert(kSSizeMaxConst ==
static_cast<size_t>(std::numeric_limits<ssize_t>::max()),
"kSSizeMaxConst should be the max value of an ssize_t");
#endif
DEBUG_CHECK(size > 0);
DEBUG_CHECK(size <= kSSizeMax);
}
Buffer(const Buffer&) = delete;
Buffer& operator=(const Buffer&) = delete;
~Buffer() {
// The code calling the constructor guaranteed that there was enough space
// to store a trailing NUL -- and in debug builds, we are actually
// verifying this with DEBUG_CHECK()s in the constructor. So, we can
// always unconditionally write the NUL byte in the destructor. We do not
// need to adjust the count_, as SafeSPrintf() copies snprintf() in not
// including the NUL byte in its return code.
*GetInsertionPoint() = '\000';
}
// Returns true, iff the buffer is filled all the way to |kSSizeMax-1|. The
// caller can now stop adding more data, as GetCount() has reached its
// maximum possible value.
inline bool OutOfAddressableSpace() const {
return count_ == static_cast<size_t>(kSSizeMax - 1);
}
// Returns the number of bytes that would have been emitted to |buffer_|
// if it was sized sufficiently large. This number can be larger than
// |size_|, if the caller provided an insufficiently large output buffer.
// But it will never be bigger than |kSSizeMax-1|.
inline ssize_t GetCount() const {
DEBUG_CHECK(count_ < kSSizeMax);
return static_cast<ssize_t>(count_);
}
// Emits one |ch| character into the |buffer_| and updates the |count_| of
// characters that are currently supposed to be in the buffer.
// Returns "false", iff the buffer was already full.
// N.B. |count_| increases even if no characters have been written. This is
// needed so that GetCount() can return the number of bytes that should
// have been allocated for the |buffer_|.
inline bool Out(char ch) {
if (size_ >= 1 && count_ < size_) {
UNSAFE_TODO(buffer_[count_] = ch);
return IncrementCountByOne();
}
// |count_| still needs to be updated, even if the buffer has been
// filled completely. This allows SafeSPrintf() to return the number of
// bytes that should have been emitted.
IncrementCountByOne();
return false;
}
// Inserts |padding|-|len| bytes worth of padding into the |buffer_|.
// |count_| will also be incremented by the number of bytes that were meant
// to be emitted. The |pad| character is typically either a ' ' space
// or a '0' zero, but other non-NUL values are legal.
// Returns "false", iff the |buffer_| filled up (i.e. |count_|
// overflowed |size_|) at any time during padding.
inline bool Pad(char pad, size_t padding, size_t len) {
DEBUG_CHECK(pad);
DEBUG_CHECK(padding <= kSSizeMax);
for (; padding > len; --padding) {
if (!Out(pad)) {
if (--padding) {
IncrementCount(padding - len);
}
return false;
}
}
return true;
}
// POSIX doesn't define any async-signal-safe function for converting
// an integer to ASCII. Define our own version.
//
// This also gives us the ability to make the function a little more
// powerful and have it deal with |padding|, with truncation, and with
// predicting the length of the untruncated output.
//
// IToASCII() converts an integer |i| to ASCII.
//
// Unlike similar functions in the standard C library, it never appends a
// NUL character. This is left for the caller to do.
//
// While the function signature takes a signed int64_t, the code decides at
// run-time whether to treat the argument as signed (int64_t) or as unsigned
// (uint64_t) based on the value of |sign|.
//
// It supports |base|s 2 through 16. Only a |base| of 10 is allowed to have
// a |sign|. Otherwise, |i| is treated as unsigned.
//
// For bases larger than 10, |upcase| decides whether lower-case or upper-
// case letters should be used to designate digits greater than 10.
//
// Padding can be done with either '0' zeros or ' ' spaces. Padding has to
// be positive and will always be applied to the left of the output.
//
// Prepends a |prefix| to the number (e.g. "0x"). This prefix goes to
// the left of |padding|, if |pad| is '0'; and to the right of |padding|
// if |pad| is ' '.
//
// Returns "false", if the |buffer_| overflowed at any time.
bool IToASCII(bool sign,
bool upcase,
int64_t i,
size_t base,
char pad,
size_t padding,
const char* prefix);
private:
// Increments |count_| by |inc| unless this would cause |count_| to
// overflow |kSSizeMax-1|. Returns "false", iff an overflow was detected;
// it then clamps |count_| to |kSSizeMax-1|.
inline bool IncrementCount(size_t inc) {
// "inc" is either 1 or a "padding" value. Padding is clamped at
// run-time to at most kSSizeMax-1. So, we know that "inc" is always in
// the range 1..kSSizeMax-1.
// This allows us to compute "kSSizeMax - 1 - inc" without incurring any
// integer overflows.
DEBUG_CHECK(inc <= kSSizeMax - 1);
if (count_ > kSSizeMax - 1 - inc) {
count_ = kSSizeMax - 1;
return false;
}
count_ += inc;
return true;
}
// Convenience method for the common case of incrementing |count_| by one.
inline bool IncrementCountByOne() { return IncrementCount(1); }
// Return the current insertion point into the buffer. This is typically
// at |buffer_| + |count_|, but could be before that if truncation
// happened. It always points to one byte past the last byte that was
// successfully placed into the |buffer_|.
inline char* GetInsertionPoint() const {
size_t idx = count_;
if (idx > size_) {
idx = size_;
}
// SAFETY: idx checked against size_ above.
return UNSAFE_BUFFERS(buffer_ + idx);
}
// User-provided buffer that will receive the fully formatted output string.
raw_ptr<char, AllowPtrArithmetic> buffer_;
// Number of bytes that are available in the buffer excluding the trailing
// NUL byte that will be added by the destructor.
const size_t size_;
// Number of bytes that would have been emitted to the buffer, if the buffer
// was sufficiently big. This number always excludes the trailing NUL byte
// and it is guaranteed to never grow bigger than kSSizeMax-1.
size_t count_ = 0;
};
bool Buffer::IToASCII(bool sign,
bool upcase,
int64_t i,
size_t base,
char pad,
size_t padding,
const char* prefix) {
// Sanity check for parameters. None of these should ever fail, but see
// above for the rationale why we can't call CHECK().
DEBUG_CHECK(base >= 2);
DEBUG_CHECK(base <= 16);
DEBUG_CHECK(!sign || base == 10);
DEBUG_CHECK(pad == '0' || pad == ' ');
DEBUG_CHECK(padding <= kSSizeMax);
DEBUG_CHECK(!(sign && prefix && *prefix));
// Handle negative numbers, if the caller indicated that |i| should be
// treated as a signed number; otherwise treat |i| as unsigned (even if the
// MSB is set!)
// Details are tricky, because of limited data-types, but equivalent pseudo-
// code would look like:
// if (sign && i < 0)
// prefix = "-";
// num = abs(i);
size_t minint = 0;
uint64_t num;
if (sign && i < 0) {
prefix = "-";
// Turn our number positive.
if (i == std::numeric_limits<int64_t>::min()) {
// The most negative integer needs special treatment.
minint = 1;
num = static_cast<uint64_t>(-(i + 1));
} else {
// "Normal" negative numbers are easy.
num = static_cast<uint64_t>(-i);
}
} else {
num = static_cast<uint64_t>(i);
}
// If padding with '0' zero, emit the prefix or '-' character now. Otherwise,
// make the prefix accessible in reverse order, so that we can later output
// it right between padding and the number.
// We cannot choose the easier approach of just reversing the number, as that
// fails in situations where we need to truncate numbers that have padding
// and/or prefixes.
const char* reverse_prefix = nullptr;
if (prefix && *prefix) {
if (pad == '0') {
while (*prefix) {
if (padding) {
--padding;
}
UNSAFE_TODO(Out(*prefix++));
}
prefix = nullptr;
} else {
for (reverse_prefix = prefix; *reverse_prefix;
UNSAFE_TODO(++reverse_prefix)) {
}
}
} else {
prefix = nullptr;
}
const size_t prefix_length = static_cast<size_t>(reverse_prefix - prefix);
// Loop until we have converted the entire number. Output at least one
// character (i.e. '0').
size_t start = count_;
size_t discarded = 0;
bool started = false;
do {
// Make sure there is still enough space left in our output buffer.
if (count_ >= size_) {
if (start < size_) {
// It is rare that we need to output a partial number. But if asked
// to do so, we will still make sure we output the correct number of
// leading digits.
// Since we are generating the digits in reverse order, we actually
// have to discard digits in the order that we have already emitted
// them. This is essentially equivalent to:
// memmove(buffer_ + start, buffer_ + start + 1, size_ - start - 1)
// SAFETY: start checked against size_ above.
for (char *move = UNSAFE_BUFFERS(buffer_ + start),
*end = UNSAFE_BUFFERS(buffer_ + size_ - 1);
move < end; UNSAFE_TODO(++move)) {
*move = UNSAFE_TODO(move[1]);
}
++discarded;
--count_;
} else if (count_ - size_ > 1) {
// Need to increment either |count_| or |discarded| to make progress.
// The latter is more efficient, as it eventually triggers fast
// handling of padding. But we have to ensure we don't accidentally
// change the overall state (i.e. switch the state-machine from
// discarding to non-discarding). |count_| needs to always stay
// bigger than |size_|.
--count_;
++discarded;
}
}
// Output the next digit and (if necessary) compensate for the most
// negative integer needing special treatment. This works because,
// no matter the bit width of the integer, the lowest-most decimal
// integer always ends in 2, 4, 6, or 8.
if (!num && started) {
if (reverse_prefix > prefix) {
UNSAFE_TODO(Out(*--reverse_prefix));
} else {
Out(pad);
}
} else {
started = true;
UNSAFE_TODO(Out((upcase ? kUpCaseHexDigits
: kDownCaseHexDigits)[num % base + minint]));
}
minint = 0;
num /= base;
// Add padding, if requested.
if (padding > 0) {
--padding;
// Performance optimization for when we are asked to output excessive
// padding, but our output buffer is limited in size. Even if we output
// a 64bit number in binary, we would never write more than 64 plus
// prefix non-padding characters. So, once this limit has been passed,
// any further state change can be computed arithmetically; we know that
// by this time, our entire final output consists of padding characters
// that have all already been output.
if (discarded > 8 * sizeof(num) + prefix_length) {
IncrementCount(padding);
padding = 0;
}
}
} while (num || padding || (reverse_prefix > prefix));
if (start < size_) {
// Conversion to ASCII actually resulted in the digits being in reverse
// order. We can't easily generate them in forward order, as we can't tell
// the number of characters needed until we are done converting.
// So, now, we reverse the string (except for the possible '-' sign).
// SAFETY: start checked against size_ above.
char* front = UNSAFE_BUFFERS(buffer_ + start);
char* back = GetInsertionPoint();
UNSAFE_TODO({
while (--back > front) {
char ch = *back;
*back = *front;
*front++ = ch;
}
});
}
IncrementCount(discarded);
return !discarded;
}
} // anonymous namespace
namespace internal {
ssize_t SafeSNPrintf(char* buf,
size_t sz,
const char* fmt,
const Arg* args,
const size_t max_args) {
// Make sure that at least one NUL byte can be written, and that the buffer
// never overflows kSSizeMax. Not only does that use up most or all of the
// address space, it also would result in a return code that cannot be
// represented.
if (static_cast<ssize_t>(sz) < 1) {
return -1;
}
sz = std::min(sz, kSSizeMax);
// Iterate over format string and interpret '%' arguments as they are
// encountered.
Buffer buffer(buf, sz);
size_t padding;
char pad;
for (unsigned int cur_arg = 0; *fmt && !buffer.OutOfAddressableSpace();) {
if (UNSAFE_TODO(*fmt++) == '%') {
padding = 0;
pad = ' ';
char ch = UNSAFE_TODO(*fmt++);
format_character_found:
switch (ch) {
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
// Found a width parameter. Convert to an integer value and store in
// "padding". If the leading digit is a zero, change the padding
// character from a space ' ' to a zero '0'.
pad = ch == '0' ? '0' : ' ';
for (;;) {
const size_t digit = static_cast<size_t>(ch - '0');
// The maximum allowed padding fills all the available address
// space and leaves just enough space to insert the trailing NUL.
const size_t max_padding = kSSizeMax - 1;
if (padding > max_padding / 10 ||
10 * padding > max_padding - digit) {
DEBUG_CHECK(padding <= max_padding / 10 &&
10 * padding <= max_padding - digit);
// Integer overflow detected. Skip the rest of the width until
// we find the format character, then do the normal error
// handling.
padding_overflow:
padding = max_padding;
while ((ch = UNSAFE_TODO(*fmt++)) >= '0' && ch <= '9') {
}
if (cur_arg < max_args) {
++cur_arg;
}
goto fail_to_expand;
}
padding = 10 * padding + digit;
if (padding > max_padding) {
// This doesn't happen for "sane" values of kSSizeMax. But once
// kSSizeMax gets smaller than about 10, our earlier range checks
// are incomplete. Unittests do trigger this artificial corner
// case.
DEBUG_CHECK(padding <= max_padding);
goto padding_overflow;
}
ch = UNSAFE_TODO(*fmt++);
if (ch < '0' || ch > '9') {
// Reached the end of the width parameter. This is where the
// format character is found.
goto format_character_found;
}
}
case 'c': { // Output an ASCII character.
// Check that there are arguments left to be inserted.
if (cur_arg >= max_args) {
DEBUG_CHECK(cur_arg < max_args);
goto fail_to_expand;
}
// Check that the argument has the expected type.
const Arg& arg = UNSAFE_TODO(args[cur_arg++]);
if (arg.type != Arg::INT && arg.type != Arg::UINT) {
DEBUG_CHECK(arg.type == Arg::INT || arg.type == Arg::UINT);
goto fail_to_expand;
}
// Apply padding, if needed.
buffer.Pad(' ', padding, 1);
// Convert the argument to an ASCII character and output it.
char as_char = static_cast<char>(arg.integer.i);
if (!as_char) {
goto end_of_output_buffer;
}
buffer.Out(as_char);
break;
}
case 'd': // Output a possibly signed decimal value.
case 'o': // Output an unsigned octal value.
case 'x': // Output an unsigned hexadecimal value.
case 'X':
case 'p': { // Output a pointer value.
// Check that there are arguments left to be inserted.
if (cur_arg >= max_args) {
DEBUG_CHECK(cur_arg < max_args);
goto fail_to_expand;
}
const Arg& arg = UNSAFE_TODO(args[cur_arg++]);
int64_t i;
const char* prefix = nullptr;
if (ch != 'p') {
// Check that the argument has the expected type.
if (arg.type != Arg::INT && arg.type != Arg::UINT) {
DEBUG_CHECK(arg.type == Arg::INT || arg.type == Arg::UINT);
goto fail_to_expand;
}
i = arg.integer.i;
if (ch != 'd') {
// The Arg() constructor automatically performed sign expansion on
// signed parameters. This is great when outputting a %d decimal
// number, but can result in unexpected leading 0xFF bytes when
// outputting a %x hexadecimal number. Mask bits, if necessary.
// We have to do this here, instead of in the Arg() constructor,
// as the Arg() constructor cannot tell whether we will output a
// %d or a %x. Only the latter should experience masking.
if (arg.integer.width < sizeof(int64_t)) {
i &= (1LL << (8 * arg.integer.width)) - 1;
}
}
} else {
// Pointer values require an actual pointer or a string.
if (arg.type == Arg::POINTER) {
i = static_cast<int64_t>(reinterpret_cast<uintptr_t>(arg.ptr));
} else if (arg.type == Arg::STRING) {
i = static_cast<int64_t>(reinterpret_cast<uintptr_t>(arg.str));
} else if (arg.type == Arg::INT &&
arg.integer.width == sizeof(NULL) &&
arg.integer.i == 0) { // Allow C++'s version of NULL
i = 0;
} else {
DEBUG_CHECK(arg.type == Arg::POINTER || arg.type == Arg::STRING);
goto fail_to_expand;
}
// Pointers always include the "0x" prefix.
prefix = "0x";
}
// Use IToASCII() to convert to ASCII representation. For decimal
// numbers, optionally print a sign. For hexadecimal numbers,
// distinguish between upper and lower case. %p addresses are always
// printed as upcase. Supports base 8, 10, and 16. Prints padding
// and/or prefixes, if so requested.
buffer.IToASCII(ch == 'd' && arg.type == Arg::INT, ch != 'x', i,
ch == 'o' ? 8
: ch == 'd' ? 10
: 16,
pad, padding, prefix);
break;
}
case 's': {
// Check that there are arguments left to be inserted.
if (cur_arg >= max_args) {
DEBUG_CHECK(cur_arg < max_args);
goto fail_to_expand;
}
// Check that the argument has the expected type.
const Arg& arg = UNSAFE_TODO(args[cur_arg++]);
const char* s;
if (arg.type == Arg::STRING) {
s = arg.str ? arg.str : "<NULL>";
} else if (arg.type == Arg::INT &&
arg.integer.width == sizeof(NULL) &&
arg.integer.i == 0) { // Allow C++'s version of NULL
s = "<NULL>";
} else {
DEBUG_CHECK(arg.type == Arg::STRING);
goto fail_to_expand;
}
// Apply padding, if needed. This requires us to first check the
// length of the string that we are outputting.
if (padding) {
size_t len = 0;
for (const char* src = s; UNSAFE_TODO(*src++);) {
++len;
}
buffer.Pad(' ', padding, len);
}
// Printing a string involves nothing more than copying it into the
// output buffer and making sure we don't output more bytes than
// available space; Out() takes care of doing that.
for (const char* src = s; *src;) {
buffer.Out(UNSAFE_TODO(*src++));
}
break;
}
case '%':
// Quoted percent '%' character.
goto copy_verbatim;
fail_to_expand:
// C++ gives us tools to do type checking -- something that snprintf()
// could never really do. So, whenever we see arguments that don't
// match up with the format string, we refuse to output them. But
// since we have to be extremely conservative about being async-
// signal-safe, we are limited in the type of error handling that we
// can do in production builds (in debug builds we can use
// DEBUG_CHECK() and hope for the best). So, all we do is pass the
// format string unchanged. That should eventually get the user's
// attention; and in the meantime, it hopefully doesn't lose too much
// data.
default:
// Unknown or unsupported format character. Just copy verbatim to
// output.
buffer.Out('%');
DEBUG_CHECK(ch);
if (!ch) {
goto end_of_format_string;
}
buffer.Out(ch);
break;
}
} else {
copy_verbatim:
buffer.Out(UNSAFE_TODO(fmt[-1]));
}
}
end_of_format_string:
end_of_output_buffer:
return buffer.GetCount();
}
} // namespace internal
ssize_t SafeSNPrintf(char* buf, size_t sz, const char* fmt) {
// Make sure that at least one NUL byte can be written, and that the buffer
// never overflows kSSizeMax. Not only does that use up most or all of the
// address space, it also would result in a return code that cannot be
// represented.
if (static_cast<ssize_t>(sz) < 1) {
return -1;
}
sz = std::min(sz, kSSizeMax);
Buffer buffer(buf, sz);
// In the slow-path, we deal with errors by copying the contents of
// "fmt" unexpanded. This means, if there are no arguments passed, the
// SafeSPrintf() function always degenerates to a version of strncpy() that
// de-duplicates '%' characters.
const char* src = fmt;
UNSAFE_TODO({
for (; *src; ++src) {
buffer.Out(*src);
DEBUG_CHECK(src[0] != '%' || src[1] == '%');
if (src[0] == '%' && src[1] == '%') {
++src;
}
}
});
return buffer.GetCount();
}
} // namespace base::strings