Add missing // SAFETY comments

[ojeda]

We do not allow new unsafe comments to enter the codebase without a // SAFETY comment. Nowadays, we use Clippy's undocumented_unsafe_blocks to check for that. However, some // SAFETY: TODO. comments remain (please see the rust-next branch). The issue is about cleaning those up.

It is not needed to fix all the comments at once (in fact, a given area/module/file should be cleaned independently), so this is a set of "good first issues". Please note that some of the comments may be easier or harder to figure out than others.

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This requires submitting a proper patch to the LKML and the Rust for Linux mailing list. Please recall to test your changes (including generating the documentation if changed, running the Rust doctests if changed, etc.), to use a proper title for the commit, to sign your commit under the Developer's Certificate of Origin and to add a Suggested-by: tag and a Link: tag to this issue. Please see https://rust-for-linux.com/contributing for details.

[nbdd0121]

First of all, remove the unsafe keyword and compile again so that the compiler tells you 1) what subexpressions need a block (sometimes there is more than one) and 2) what is the reason unsafe is needed (e.g. the simplest case being a FFI call).

There is an allow-by-default unsafe_code lint which is designed for auditing unsafe usage.

[LeSeulArtichaut]

Here's a Python script to find undocumented unsafe blocks, borrowing logic from rust-lang/rust's tidy checks:

import os


def check_safety_comments(path):
    with open(path, 'r') as f:
        last_safety_comment = False
        for i, line in enumerate(f):
            if 'unsafe {' in line and not line.lstrip().startswith('//') and not last_safety_comment:
                print(f"{path}:{i + 1}: {line.strip()}")
            if '// SAFETY' in line:
                last_safety_comment = True
            elif not line.lstrip().startswith('//') and line.strip() != '':
                last_safety_comment = False


rust_dir = os.path.dirname(os.path.realpath(__file__))
kernel_dir = os.path.join(rust_dir, 'kernel')
for dirpath, _, filenames in os.walk(kernel_dir):
    for filename in filenames:
        if filename.endswith('.rs'):
            check_safety_comments(os.path.join(dirpath, filename))

If placed in the rust/ folder, it will check every file in the kernel crate.