# The Unsafe Buffers Clang plugin. Our compiler contains a [plugin](../tools/clang/plugins/UnsafeBuffersPlugin.cpp) which reports places in the code where unsafe buffer operations are present. [TOC] ## Preventing OOB by removing Unsafe Buffers operations. Out-of-bounds (OOB) security bugs commonly happen through C-style pointers which have no bounds information associated with them. We can prevent such bugs by always using C++ containers. Furthermore, the plugin will warn warn about unsafe pointer usage that should be converted to containers. When an unsafe usage is detected, Clang prints a warning similar to ``` error: unsafe buffer access [-Werror,-Wunsafe-buffer-usage] ``` and directs developers to this file for more information. Several common [Techniques](#container_based-ecosystem) for fixing these issues are presented later in this document. Clang documentation includes a guide to working with unsafe-buffer-usage warnings here: https://clang.llvm.org/docs/SafeBuffers.html ## Preventing OOB by removing unsafe libc calls. OOB bugs also commonly happen through C-style library calls such as memcpy() and memset() where the programmer is responsible for specifying an (unchecked) length. In order to encourage safer alternatives, the plugin can warn about unsafe calls which should be converted to safer C++ alternatives. When an unsafe libc call is detected, Clang prints a warning similar to ``` error: function 'memcpy' is unsafe [-Werror,-Wunsafe-buffer-usage-in-libc-call] ``` ## Unsafe buffer warning suppressions Because the Chromium codebase is not yet compliant with these warnings, there are mechanisms to opt out code on a directory, file, or per-occurence basis. By default, all files are checked for unsafe-buffer-usage. ### Opting out entire directories Entire directories are opted out of unsafe buffer usage warnings through the [`//build/config/unsafe_buffers_paths.txt`](../build/config/unsafe_buffers_paths.txt) file. As work progresses, directories will be removed from this list, and non-compliant files marked on a per-file basis as below. Early results indicate that often 85%+ of files in a directory already happen to be compliant, so file-by-file suppression allows this code to be subject to enforcement. This mechanism opts directories out of all warning categories (unsafe buffers and unsafe libc calls). #### Syntax of Unsafe Buffer Paths file Note: Paths should be written as relative to the root of the source tree with unix-style path separators. Directory prefixes should end with `/`, such as `base/`. Empty lines are ignored. The `#` character introduces a comment until the end of the line. Lines starting with `.` declare which checks are to be enforced, as a comma-separated list of values. Currently allowed values are `buffers`, `libc`, and `unique_ptr`. All other lines specify which paths are to be included/excluded. Lines that begin with `-` are immediately followed by path prefixes that will *not* be checked for unsafe-buffer-usage. They are known to do unsafe things and should be changed to use constructs like base::span or containers like base::HeapArray and std::vector instead. See https://crbug.com/40285824 Lines that begin with `+` are immediately followed by path prefixes that will be checked for unsafe-buffer-usage. These have no such usage (or all such usage is annotated), and are protected against new unsafe pointer behaviour by the compiler. Generally, `+` lines are used to enable checks for sub-directories of a path that has previously disabled checks (with a `-` line). If a file matches both a `-` and `+` line, the longest matching prefix takes precedence. #### Removing directories from Unsafe Buffers Paths file The recommended process for removing a `-dir/` line from this file is: 1. Remove the `-dir/` line from this paths file. Possibly add some subdirectories now needed to reduce scope, like `-dir/sub_dir/`. 2. Add `#pragma allow_unsafe_buffers` to every file in the directory that now has a compilation error (see the next section). ### Opting out individual files Individual files are opted out of unsafe pointer usage warnings though the use of the following snippet, which is to be placed immediately following the copyright header in a source file. ``` #ifdef UNSAFE_BUFFERS_BUILD // TODO(crbug.com/ABC): Remove this and convert code to safer constructs. #pragma allow_unsafe_buffers #endif ``` The above mechanism also suppress unsafe libc call warnings in addition to the unsafe buffer warnings. To prevent back-sliding on files which have been made safe with respect to unsafe buffers, there is now a per-file pragma which suppresses the libc warnings while still enforcing the unsafe buffer warnings. ``` #ifdef UNSAFE_BUFFERS_BUILD // TODO(crbug.com/ABC): Remove libc calls to fix these warnings. #pragma allow_unsafe_libc_calls #endif ``` An initial set of files containing allow_unsafe_libc_calls has been uploaded; please keep these in place until the pending libc enforcement is enabled for Chromium. #### Removing pragmas from individual files. The recommended process for removing pragmas from individual files is: 1. 1. Remove the pragma from the file. 2. Use the compiler warnings now generated to identify the individual expressions to suppress (see the next section). ### Opting out individual expressions Individual expressions or blocks of code are opted out by using the `UNSAFE_BUFFERS()` macro as defined in [`//base/compiler_specific.h`](../base/compiler_specific.h) file. These should be rare once a project is fully converted, except perhaps when working with C-style external APIs. These must always be accompanied by a `// SAFETY:` comment explaining in detail how the code has been evaluated to be safe for all possible input. Code introducing `UNSAFE_BUFFERS()` macro invocations without corresponding `// SAFETY:` comment should be summarily rejected during code review. To allow for incremental conversion, code can be temporarily opted out by using the `UNSAFE_TODO()` macro. This provides the same functionality as the `UNSAFE_BUFFERS()` macro, but allows easier searching for code in need of revision. This mechanism opts expressions out of all warning categories (unsafe buffers and unsafe libc calls). #### Removing UNSAFE_TODO() from individual expressions We seek to convert code to use owning containers like HeapArray and vector, as explained in the next section. ## Container-based ecosystem Containers may be owning types or view types. The common owning containers that us contiguous storage are `std::vector`, `std::string`, `base::HeapArray`, `std::array`. Their common view types are `base::span`, `std::string_view`, `base::cstring_view`. Other owning containers include maps, sets, deques, etc. These are not compatible with `base::span` as they are not contiguous and generally do not have an associated view type at this time. We are using `base::span` instead of `std::span` in order to provide a type that can do more than the standard type. We also have other types and functions to work with ranges and spans instead of unbounded pointers and iterators. The common conversions to spans are: - `base::span` replaces `T* ptr, size_t size`. - `base::span` replaces `T (&ptr)[N]` (a reference to a compile-time-sized array). - `base::raw_span` replaces `base::span` (and `T* ptr, size_t size`) for class fields. ### Span construction - `base::span()` makes a span, deducing the type and size, from any contiguous range. It can also take explicit begin/end or data/size pairs. - `base::to_fixed_extent()` makes a fixed-size span from a dynamic one. - `base::as_bytes()` and `base::as_chars()` convert a span’s inner type to `uint8_t` or `char` respectively, making a byte-span or char-span. - `base::span_from_ref()` and `base::byte_span_from_ref()` make a span, or byte-span, from a single object. - `base::as_byte_span()` and `base::as_writable_byte_span()` make a byte-span from any contiguous range. #### Padding bytes Note that if the type contains padding bytes that were not somehow explicitly initialized, this can create reads of uninitialized memory. Conversion to a byte-span is most commonly used for spans of primitive types, such as going from `char` (such as in `std::string`) or `uint32_t` (in a `std::vector`) to `unit8_t`. ### Dynamic read/write of a span - `base::SpanReader` reads heterogeneous values from a (typically, byte-) span in a dynamic manner. - `base::SpanWriter` writes heterogeneous values into a (typically, byte-) span in a dynamic manner. ### Values to/from byte spans In [`//base/numerics/byte_conversions.h`](../base/numerics/byte_conversions.h) we have conversions between byte-arrays and big/little endian integers or floats. For example (and there are many other variations): - `base::U32FromBigEndian` converts from a big-endian byte-span to an unsigned 32-bit integer. - `base::U32FromLittleEndian` converts from a little-endian byte-span to an unsigned - `base::U32ToBigEndian` converts from an integer to a big-endian-encoded 4-byte-array. - `base::U32ToLittleEndian` converts from an integer to a little-endian-encoded 4-byte-array. ### Heap-allocated arrays - `base::HeapArray` replaces `std::unique_ptr` and places the bounds of the array inside the `HeapArray` which makes it a bounds-safe range. ### Copying and filling arrays - `base::span::copy_from(span)` replaces `memcpy` and `memmove`, and verifies that the source and destination spans have the same size instead of writing out of bounds. It lowers to the same code as `memmove` when possible. - Note `std::ranges::copy` is not bounds-safe (though its name sounds like it should be). - `std::ranges::fill` replaces `memset` and works with a range so you don't need explicit bounds. ### String pointers A common form of pointer is `const char*` which is used (sometimes) to represent a NUL-terminated string. The standard library gives us two types to replace `char*`, which allow us to know the bounds of the character array and work with the string as a range: - `std::string` owns a NUL-terminated string. - `std::string_view` is a view of a non-NUL-terminated string. What’s missing is a view of a string that is guaranteed to be NUL-terminated so that you can call `.c_str()` to generate a `const char*` suitable for C APIs. - `base::cstring_view` is a view of a NUL-terminated string. This avoids the need to construct a `std::string` in order to ensure a terminating NUL is present. Use this as a view type whenever your code bottoms out in a C API that needs NUL-terminated string pointer. ### Use of std::array. The clang plugin is very particular about indexing a C-style array (e.g. `int arr[100]`) with a variable index. Often these issues can be resolved by replacing this with `std::array arr`, which provides safe indexed operations. In particular, new code should prefer to use the `std::array` mechanism. For arrays where the size is determined by the compiler (e.g. `int arr[] = { 1, 3, 5 };`), the `std::to_array()` helper function should be used along with the `auto` keyword: `auto arr = std::to_array({1, 3, 5});` ### Avoid reinterpret_cast Casts to bytes are common and can be handled as follows. #### Writing to a byte span A common idiom in older code is to write into a byte array by casting the array into a pointer to a larger type (such as `uint32_t` or `float`) and then writing through that pointer. This can result in Undefined Behaviour and violates the rules of the C++ abstract machine. Instead, keep the byte array as a `base::span`, and write to it directly by chunking it up into pieces of the size you want to write. Using `take_first()` (good for repeated modifications and loops): ```cc void write_floats(base::span out, float f1, float f2) { // Write `f1` into `out`'s prefix, moving `out` forward. out.take_first<4>().copy_from(base::byte_span_from_ref(f1)); // Write `f2` into `out`'s new prefix (after `f1`). out.copy_prefix_from(base::byte_span_from_ref(f2)); } ``` Using `split_at()` (good when there are exactly two pieces): ```cc void write_floats(base::span out, float f1, float f2) { // Split `out` into a prefix to write `f1` into, and a remainder. auto [write_f1, rem] = out.split_at<4>(); // Write `f1` into the prefix portion, `write_f1`. write_f1.copy_from(base::byte_span_from_ref(f1)); // Write `f2` into the beginning of the remainder. rem.copy_prefix_from(base::byte_span_from_ref(f2)); } ``` Using `SpanWriter` and endian-aware `FloatToLittleEndian()` (good when non-fatal APIs are desired): ```cc void write_floats(base::span out, float f1, float f2) { auto writer = base::SpanWriter(out); CHECK(writer.Write(base::FloatToLittleEndian(f1))); CHECK(writer.Write(base::FloatToLittleEndian(f2))); } ``` Writing big-endian, with `SpanWriter` and endian-aware `U32ToBigEndian()`: ```cc void write_values(base::span out, uint32_t i1, uint32_t i2) { auto writer = base::SpanWriter(out); CHECK(writer.Write(base::U32ToBigEndian(i1))); // SpanWriter has a built-in shortcut to do the same thing. CHECK(writer.WriteU32BigEndian(i2)); // Verify we wrote to the whole output. We can put a size parameter in the // `out` span to push this check to compile-time when it's a constant. CHECK_EQ(writer.remaining(), 0u); } ``` Writing an array to a byte span with `copy_from()`: ```cc void write_floats(base::span out, std::vector floats) { base::span byte_floats = base::as_byte_span(floats); // Or use copy_from() if you want to CHECK at runtime that all of `out` has // been written to. out.copy_prefix_from(byte_floats); } ``` #### Reading from a byte span Instead of turning a `span` into a pointer of a larger type, which can cause Undefined Behaviour, read values out of the byte span and convert each one as a value (not as a pointer). Using `take_first()` and endian-aware conversion `FloatFromLittleEndian`: ```cc void read_floats(base::span in, float& f1, float& f2) { f1 = base::FloatFromLittleEndian(in.take_first<4>()); f2 = base::FloatFromLittleEndian(in.take_first<4>()); } ``` Using `SpanReader` and endian-aware `U32FromBigEndian()`: ```cc void read_values(base::span in, int& i1, int& i2, int& i3) { auto reader = base::SpanReader(in); i1 = base::U32FromBigEndian(*reader.Read<4>()); i2 = base::U32FromBigEndian(*reader.Read<4>()); // SpanReader has a built-in shortcut to do the same thing. CHECK(reader.ReadU32BigEndian(i3)); // Verify we read the whole input. We can put a size parameter in the `in` // span to push this check to compile-time when it's a constant. CHECK_EQ(reader.remaining(), 0u); } ``` ## Patterns for spanification Most pointer issues ought to be resolved by converting to containers. In particular, one common conversion is to replace `T*` pointers with `base::span` in a process known as spanification, since most pointers are unowned references into an array (or vector). The appropriate replacement for the pointer is [`base::span`](../base/containers/span.h). ### Copying arrays (`memcpy`) You have: ```cc uint8_t array1[12]; uint8_t array2[16]; uint64_t array3[2]; memcpy(array1, array2 + 8, 4); memcpy(array1 + 4, array3, 8); ``` Spanified: ```cc uint8_t array1[12]; uint8_t array2[16]; uint64_t array3[2]; base::span span1(array1); span1.take_first<4>().copy_from(base::span(array2).subspan<8, 4>()); span1.copy_from(base::as_byte_span(array3).first<8>()); // Use `split_at()` to ensure `array1` is fully written. auto [from2, from3] = base::span(array1).split_at<4>(); from2.copy_from(base::span(array2).subspan<8, 4>()); from3.copy_from(base::as_byte_span(array3).first<8>()); ``` ### Zeroing arrays (`memset`) `std::ranges::fill` works on any range/container and won't write out of bounds. Converting arbitrary types into a byte array (through `base::as_writable_byte_span`) is only valid when the type holds trivial types such as primitives. Unlike `memset`, a constructed object can be given as the value to use as in `std::ranges_fill(container, Object())`. You have: ```cc uint8_t array1[12]; uint64_t array2[2]; Object array3[4]; memset(array1, 0, 12); memset(array2, 0, 2 * sizeof(uint64_t)); memset(array3, 0, 4 * sizeof(Object)); ``` Spanified: ```cc uint8_t array1[12]; uint64_t array2[2]; Object array3[4]; std::ranges::fill(array1, 0); std::ranges::fill(array2, 0); std::ranges::fill(base::as_writable_byte_span(array3), 0); ``` ### Comparing arrays (`memcmp`) You have: ```cc uint8_t array1[12] = {}; uint8_t array2[12] = {}; bool eq = memcmp(array1, array2, sizeof(array1)) == 0; bool less = memcmp(array1, array2, sizeof(array1)) < 0; // In tests. for (size_t i = 0; i < sizeof(array1); ++i) { SCOPED_TRACE(i); EXPECT_EQ(array1[i], array2[i]); } ``` Spanified: ```cc uint8_t array1[12] = {}; uint8_t array2[12] = {}; // If one side is a span, the other will convert to span too. bool eq = base::span(array1) == array2; bool less = base::span(array1) < array2; // In tests. EXPECT_EQ(base::span(array1), array2); ``` ### Copying array into an integer You have: ```cc uint8_t array[44] = {}; uint32_t v1; memcpy(&v1, array, sizeof(v1)); // Front. uint64_t v2; memcpy(&v2, array + 6, sizeof(v2)); // Middle. ``` Spanified: ```cc #include "base/numerics/byte_conversions.h" ... uint8_t array[44] = {}; uint32_t v1 = base::U32FromLittleEndian(base::span(array).first<4u>()); // Front. uint64_t v2 = base::U64FromLittleEndian(base::span(array).subspan<6u, 8u>()); // Middle. ``` ### Copy an array into an integer via cast Note: This pattern is prone to more UB than out-of-bounds access. It is UB to cast pointers if the result is not aligned, so these cases are often buggy or were only correct due to subtle assumptions on buffer alignment. The spanified version avoids this pitfalls. It has no alignment requirement. You have: ```cc uint8_t array[44] = {}; uint32_t v1 = *reinterpret_cast(array); // Front. uint64_t v2 = *reinterpret_cast(array + 16); // Middle. ``` Spanified: ```cc #include "base/numerics/byte_conversions.h" ... uint8_t array[44] = {}; uint32_t v1 = base::U32FromLittleEndian(base::span(array).first<4u>()); // Front. uint64_t v2 = base::U64FromLittleEndian(base::span(array).subspan<16u, 8u>()); // Middle. ``` ### Making a byte array (`span`) from a string (or any array/range) You have: ```cc std::string str = "hello world"; func_with_const_ptr_size(reinterpret_cast(str.data()), str.size()); func_with_mut_ptr_size(reinterpret_cast(str.data()), str.size()); ``` Spanified: ```cc std::string str = "hello world"; base::span bytes = base::as_byte_span(str); func_with_const_ptr_size(bytes.data(), bytes.size()); base::span mut_bytes = base::as_writable_byte_span(str); func_with_mut_ptr_size(mut_bytes.data(), mut_bytes.size()); // Replace pointer and size with a span, though. func_with_const_span(base::as_byte_span(str)); func_with_mut_span(base::as_writable_byte_span(str)); ``` ### Making a byte array (`span`) from an object You have: ```cc uint8_t array[8]; uint64_t val; two_byte_arrays(array, reinterpret_cast(&val)); ``` Spanified: ```cc uint8_t array[8]; uint64_t val; base::span val_span = base::byte_span_from_ref(val); two_byte_arrays(array, val_span.data()); // Replace an unbounded pointer a span, though. two_byte_spans(base::span(array), base::byte_span_from_ref(val)); ``` ### Avoid std::next() for silencing warnings, use ranges When we convert `pointer + index` to `std::next(pointer, index)` we silence the `Wunsafe-buffer-usage` warning by pushing the unsafe pointer arithmetic into the `std::next()` function in a system header, but we have the same unsafety. `std::next()` does no additional bounds checking. Instead of using `std::next()`, rewrite away from using pointers (or iterators) entirely by using ranges. `span()` allows us to take a subset of a contiguous range without having to use iterators that we move with arithmetic or `std::next()`. Likewise, `std::advance()` can silence the warning but does not add any safety to the pointer arithmetic and should be avoided. Instead of using pointer/iterator arithmetic: ```cc // Unsafe buffers warning on the unchecked arithmetic. auto it = std::find(vec.begin() + offset, vec.end(), 20); // No warning... But has the same security risk! auto it = std::find(std::next(vec.begin(), offset), vec.end(), 20); ``` Use a range, with `span()` providing a view of a subset of the range: ```cc auto it = std::ranges::find(base::span(vec).subspan(offset), 20); ``` ### Identifying span bounds-checking The bounds-checking that is built into the span operations is often elided by the compiler at build time. Even when they remain, the cost of a branch not taken is typically negligible on any modern CPU. However, given a situation where "spans are slow", the checks that remain can be identified via warnings at compile time. The GN args that control this are: ``` is_debug = false # Must run an optimized build treat_warnings_as_errors = false # Numerous warnings abound. enable_check_elision_warning = true ``` Such a build will produce voluminous output as these checks can't always be elided, but investigation will often reveal subtle bugs. Here's a case study: ```cc for (size_t i = 0; i < data.size();) { uint8_t length = data[i]; if (i + length >= data.size()) { return nullptr; } texts_.emplace_back( base::as_string_view(base::as_chars(data.subspan(i + 1, length)))); i += length + 1; } ``` This produced the chain of warnings: ``` warning: call to 'base::check_not_elided()' declared with 'warning' attribute: check not elided [-Wattribute-warning] 1379 | SPAN_BOUNDS_CHECK(size_type{offset} <= size() && | ^ note: expanded from macro 'SPAN_BOUNDS_CHECK' 294 | base::check_not_elided(); \ | ^ note: called by function 'subspan' note: inlined by function 'Create' 246 | base::as_string_view(base::as_chars(data.subspan(i + 1, length)))); | ^ ``` Take a moment and ponder about why the logic above might not be good enough to avoid a bounds check in subspan(). Hint: The compiler can't know that data.size() isn't enormous. Hint: Hence i + length might overflow and not return early. Hint: Hence i + 1 might not overflow and be out of bounds. This is of course fixed by: ```cc if (length >= data.size() - i) { return nullptr; } ``` since we know i < data.size(), data.size() - i can't underflow. Tip: AI models are good at explaining these findings. ### Functions with array pointer parameters Functions that receive a pointer argument into an array may read or write out of bounds of that array if subsequent manual size calculations are incorrect. Such functions should be avoided if possible, or marked with the `UNSAFE_BUFFER_USAGE` attribute macro otherwise. This macro propagates to their callers that they must be called from inside an `UNSAFE_BUFFERS()` region (along with a corresponding safety comment explaining how the caller knows the call will be safe). The same is true for functions that accept an iterator instead of a range type. Some examples of each are `memcpy()` and `std::copy()`. Again, calling such functions is unsafe and should be avoided. Replace such functions with an API built on base::span or other range types which prevents any chance of OOB memory access. For instance, replace `memcpy()`, `std::copy()` and `std::ranges::copy()` with `base::span::copy_from()`. And replace `memset()` with `std::ranges::fill()`. ### Aligned memory An aligned heap allocation can be constructed into a `base::HeapArray` through the `base::AlignedUninit(size, alignment)` function in `//base/memory/aligned_memory.h`. It will allocate space for `size` many `T` objects aligned to `alignment`, and return a `base::AlignedHeapArray` which is a `base::HeapArray` with an appropriate deleter. Note that the returned memory is uninitialized. ```cc base::AlignedHeapArray array = base::AlignedUninit(size, alignment); ``` Some containers are built on top of buffers of `char`s that are aligned for some other `T` in order to manage the lifetimes of objects in the buffer through in-place construction (`std::construct_at`) and destruction. While the memory is allocated and destroyed as `char*`, it is accessed as `T*`. The `base::AlignedUninitCharArray(size, alignment)` function in `//base/memory/aligned_memory.h` handles this by returning both: - A `base::AlignedHeapArray` that will not call destructors on anything in its buffer. - A `base::span` that points to all of the (not-yet-created) objects in the `AlignedHeapArray`. This span can be used to construct `T` objects in place in the buffer, and the caller is responsible for destroying them as well. ```cc auto [a, s] = base::AlignedUninitCharArray(size, alignment); base::AlignedHeapArray array = std::move(a); base::span span = s; ```