ring inherited C, C++, and assembly language code from BoringSSL, and the style guidelines for that code are in the second section of this document.
ring Style Guide (for code not in crypto/)
ring usually follows the Rust Guidelines, but there are some differences and ring adds additional guidelines.
In general, import modules, not non-module items, e.g. use core, not
use core::mem::size_of_val. This means that the uses of such functions must
be qualified: core::mem::size_of_val(x), not size_of_val(x). Exceptions may
be made for things that are very annoying to qualify; for example, we usually
use super::input::* or use super::input::Input because writing things like
input::Input is highly annoying.
In general, avoid nesting modules and avoid exporting any non-module items from
the main ring crate. Instead, prefer a flat module structure that is one
level deep. Thus, for example, we have ring::digest::SHA256 but not
ring::SHA256 or ring::digest::sha256::SHA256 or ring::digest::sha2::SHA256.
Sometimes it is useful to break up a module's source code into multiple files.
In this case, it is useful to make use of the Rust visibility rule where a
submodule can use non-public items defined in the enclosing module. In that
case, it is OK to use nested submodules. The nested submodules must be
non-public (mod x, not pub mod x) and the enclosing module must re-export,
using pub use submodule::x, the items that are intended to be public. This
way, the implementation details that drove the choice to use nested submodules
do not affect the public API.
Generally the Rust Guidelines for submodules are followed in ring. However,
the Rust Guidelines (and rustc/cargo) require (by default) that when a module
x has submodules, the module must be in a x/mod.rs. This would result in
many files named mod.rs, which would make navigating through the source code
more difficult and confusing. Instead, use #[path = "x/x.rs"] pub mod x; (all
on one line, which is an exception to the rule that attributes should each be
on their own line) so that every module's filename is unique. Example:
#[path = "good_example/good_example.rs"] pub mod good_example;
Note that this is only necessary when the module has submodules.
Use Result<T, ()> as the return type for functions that may fail. Never use
Option<T> or bool or other types as return values to indicate failure.
Result is used because it is annotated #[must_use], so the Rust compiler
will not let callers silently ignore the return value of functions that return
Results.
ring functions generally do not report error codes for a variety of reasons;
when they fail, they only report that they fail. If a function only needs to
return a boolean indicator that it succeeded or failed, use Result<(), ()> as
the return type.
If an external function (e.g. part of the Rust standard library) returns
Option<T> to indicate failure, use ok_or(()) to map it to Result<T, ()>.
When the last statement x in a function is already the same Result<T, ()>
type that the function returns, just make that statement the return expression;
that is, write x, not let result = try!(x); Ok(result).
Use the early-return-on-failure pattern by wrapping calls to functions that may
fail with try!(). Do not use Result::or_else, Result::and, etc. to chain
together strings of potentially-failing operations.
// The return type is of the form `Result<_, ()>`, not `Option<_>` or something
// else.
fn good_example(x: u32, y: u32) -> Result<u32, ()> {
// * `ok_or` is used to map `Option<u32>` to `Result<u32, ()>` here.
// * `try!` is used to return early on failure.
let sum = try!(x.checked_add(y).ok_or(()));
// Early return is used.
try!(foo(sum));
// `try!()` isn't used when the last statement is already of the form
// `Result<_, ()>`.
bar(sum)
}When creating a slice from the start of a indexable value, use x[..n], not
x[0..n]. Similarly, use x[n..], not x[n..x.len()] for creating a slice
from a specific point to the end of the value.
When copying and filling arrays and slices, use the functions in ring::polyfill when possible.
Avoid using the as operator. When using as seems necessary, see if there is
already a safer function for doing the conversion in
ring::polyfill. If not, add one to ring::polyfill.
The C code generally uses the C int type as a return value, where 1 indicates
success and 0 indicates failure. Sometimes the C code has functions that return
pointers, and a NULL pointer indicates failure. The module
ring::bssl contains some utilities for mapping these return
values to Result<(), ()> and Result<*mut T, ()>, respectively. They should
be used as in the following example (note the placement of unsafe):
fn foo() -> Result<(), ()> {
try!(bssl::map_result(unsafe {
unsafe_fn2(when, the, entire, thing, does, not, fit, on, a, single,
line)
}));
try!(bssl::map_result(unsafe {
unsafe_fn1() // Use the same style even when the call fits on one line.
}));
let ptr = try!(bssl::map_ptr_result(unsafe {
unsafe_fn_returning_pointer()
}));
// The return value of `foo` will be the mapped result of calling
// `unsafe_fn3`.
bssl::map_result(unsafe {
unsafe_fn3()
})
}In general, prefer using unsigned types over signed types, and prefer using
checked arithmetic (e.g. x.checked_add(y), x.checked_mul(y), etc.) over
unchecked arithmetic. Prefer using checked arithmetic over explicit bounds
checks. Example:
fn good_example(a: u64, b: u64) -> Result<u64, ()> {
let n = a.checked_add(b).ok_or(());
}
fn bad_example(a: u64, b: u64) -> Result<u64, ()> {
if usize::max_value() - a > b {
return Err(());
}
Ok(a + b)
}In general, avoid using unsafe whenever it is practical to do so. The ring
developers chose to use Rust because of the goodness of the safe subset; stuff
that requires unsafe is generally better off being written in C or assembly
language code. Generally, this means that unsafe is only used to call
functions written in C or assembly language. Even if your goal is to replace C
and/or assembly language code with Rust code, don't be afraid to leave, or even
add, C code to avoid adding a load of unsafe Rust code.
In particular, prefer references and indexing (which is checked at runtime) to pointers and pointer arithmetic. Example:
fn good_example(x: &[u8], n: usize) {
unsafe {
unsafe_fn(x[n..].as_ptr()) // The compiler inserts bounds checks for us.
}
}
fn bad_example(x: &[u8], n: usize) {
unsafe {
// If we do things this way, the compiler won't do bounds checking for
// us. Also, since `offset` takes an `isize`, we have to do a cast from
// `usize` to `isize` which is potentially unsafe because an `isize`
// cannot hold every positive value of `usize`.
unsafe_fn(x.as_ptr().offset(n as isize))
}
}When you must use unsafe, minimize the scope of unsafe. Example:
fn good_example() {
unsafe { unsafe_fn(); }
safe_fn();
unsafe { unsafe_fn(); }
}
fn bad_example() {
unsafe {
unsafe_fn();
safe_fn(); // No safe statements allowed in an unsafe block.
unsafe_fn();
}
}But, don't go overboard:
fn ok_example(x: &[u8], n: usize) {
unsafe {
unsafe_fn1(x[n]); // `x[n]` is a safe expression
}
}
fn bad_example(x: &[u8], n: usize) {
let x_n = x[n]; // This is going overboard.
unsafe {
unsafe_fn1(x_n);
}
}BoringSSL usually follows the Google C++ style guide, The rest of this section describes differences and clarifications on top of the base guide.
As a derivative of OpenSSL, BoringSSL contains a lot of legacy code that does not follow this style guide. Particularly where public API is concerned, balance consistency within a module with the benefits of a given rule. Module-wide deviations on naming should be respected while integer and return value conventions take precedence over consistency.
Modules from OpenSSL's legacy ASN.1 and X.509 stack are retained for
compatibility and left largely unmodified. To ease importing patches from
upstream, they match OpenSSL's new indentation style. For Emacs,
doc/openssl-c-indent.el from OpenSSL may be helpful in this.
The majority of the project is in C, so C++-specific rules in the Google style guide do not apply. Support for C99 features depends on our target platforms. Typically, Chromium's target MSVC is the most restrictive.
Variable declarations in the middle of a function are allowed.
Comments should be /* C-style */ for consistency.
When declaration pointer types, * should be placed next to the variable
name, not the type. So
uint8_t *ptr;
not
uint8_t* ptr;
Rather than malloc() and free(), use the wrappers OPENSSL_malloc()
and OPENSSL_free(). Use the standard C assert() function freely.
For new constants, prefer enums when the values are sequential and typed
constants for flags. If adding values to an existing set of #defines,
continue with #define.
Single-statement blocks are not allowed. All conditions and loops must use braces:
if (foo) {
do_something();
}
not
if (foo)
do_something();
Prefer using explicitly-sized integers where appropriate rather than
generic C ones. For instance, to represent a byte, use uint8_t, not
unsigned char. Likewise, represent a two-byte field as uint16_t, not
unsigned short.
Sizes are represented as size_t.
Within a struct that is retained across the lifetime of an SSL
connection, if bounds of a size are known and it's easy, use a smaller
integer type like uint8_t. This is a "free" connection footprint
optimization for servers. Don't make code significantly more complex for
it, and do still check the bounds when passing in and out of the
struct. This narrowing should not propagate to local variables and
function parameters.
When doing arithmetic, account for overflow conditions.
Except with platform APIs, do not use ssize_t. MSVC lacks it, and
prefer out-of-band error signaling for size_t (see Return values).
Follow Google naming conventions in C++ files. In C files, use the following naming conventions for consistency with existing OpenSSL and C styles:
Define structs with typedef named TYPE_NAME. The corresponding struct
should be named struct type_name_st.
Name public functions as MODULE_function_name, unless the module
already uses a different naming scheme for legacy reasons. The module
name should be a type name if the function is a method of a particular
type.
Some types are allocated within the library while others are initialized
into a struct allocated by the caller, often on the stack. Name these
functions TYPE_NAME_new/TYPE_NAME_free and
TYPE_NAME_init/TYPE_NAME_cleanup, respectively. All TYPE_NAME_free
functions must do nothing on NULL input.
If a variable is the length of a pointer value, it has the suffix
_len. An output parameter is named out or has an out_ prefix. For
instance, For instance:
uint8_t *out,
size_t *out_len,
const uint8_t *in,
size_t in_len,
Name public headers like include/openssl/evp.h with header guards like
OPENSSL_HEADER_EVP_H. Name internal headers like
crypto/ec/internal.h with header guards like
OPENSSL_HEADER_EC_INTERNAL_H.
Name enums like enum unix_hacker_t. For instance:
enum should_free_handshake_buffer_t {
free_handshake_buffer,
dont_free_handshake_buffer,
};
As even malloc may fail in BoringSSL, the vast majority of functions
will have a failure case. Functions should return int with one on
success and zero on error. Do not overload the return value to both
signal success/failure and output an integer. For example:
OPENSSL_EXPORT int CBS_get_u16(CBS *cbs, uint16_t *out);
If a function needs more than a true/false result code, define an enum rather than arbitrarily assigning meaning to int values.
If a function outputs a pointer to an object on success and there are no
other outputs, return the pointer directly and NULL on error.
Where not constrained by legacy code, parameter order should be:
- context parameters
- output parameters
- input parameters
For example,
/* CBB_add_asn sets |*out_contents| to a |CBB| into which the contents of an
* ASN.1 object can be written. The |tag| argument will be used as the tag for
* the object. It returns one on success or zero on error. */
OPENSSL_EXPORT int CBB_add_asn1(CBB *cbb, CBB *out_contents, uint8_t tag);
All public symbols must have a documentation comment in their header file. The style is based on that of Go. The first sentence begins with the symbol name, optionally prefixed with "A" or "An". Apart from the initial mention of symbol, references to other symbols or parameter names should be surrounded by |pipes|.
Documentation should be concise but completely describe the exposed behavior of the function. Pay special note to success/failure behaviors and caller obligations on object lifetimes. If this sacrifices conciseness, consider simplifying the function's behavior.
/* EVP_DigestVerifyUpdate appends |len| bytes from |data| to the data which
* will be verified by |EVP_DigestVerifyFinal|. It returns one on success and
* zero otherwise. */
OPENSSL_EXPORT int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *data,
size_t len);
Explicitly mention any surprising edge cases or deviations from common return value patterns in legacy functions.
/* RSA_private_encrypt encrypts |flen| bytes from |from| with the private key in
* |rsa| and writes the encrypted data to |to|. The |to| buffer must have at
* least |RSA_size| bytes of space. It returns the number of bytes written, or
* -1 on error. The |padding| argument must be one of the |RSA_*_PADDING|
* values. If in doubt, |RSA_PKCS1_PADDING| is the most common.
*
* WARNING: this function is dangerous because it breaks the usual return value
* convention. Use |RSA_sign_raw| instead. */
OPENSSL_EXPORT int RSA_private_encrypt(int flen, const uint8_t *from,
uint8_t *to, RSA *rsa, int padding);
Document private functions in their internal.h header or, if static,
where defined.