linux-kernel - Re: [PATCH 04/10] rust: Add bindings for reading device properties

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <D8WAMBXFOO2O.1Q3YT6FC05EKO@buenzli.dev>
Date: Wed, 02 Apr 2025 18:28:17 +0200
From: "Remo Senekowitsch" <remo@...nzli.dev>
To: "Rob Herring" <robh@...nel.org>
Cc: "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>, "Daniel Scally"
 <djrscally@...il.com>, "Heikki Krogerus" <heikki.krogerus@...ux.intel.com>,
 "Sakari Ailus" <sakari.ailus@...ux.intel.com>, "Dirk Behme"
 <dirk.behme@...bosch.com>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
 "Danilo Krummrich" <dakr@...nel.org>, "Saravana Kannan"
 <saravanak@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 <linux-kernel@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH 04/10] rust: Add bindings for reading device properties

On Wed Mar 26, 2025 at 10:27 PM CET, Rob Herring wrote:
> On Wed, Mar 26, 2025 at 06:13:43PM +0100, Remo Senekowitsch wrote:
>> +    pub fn property_read_string(&self, name: &CStr) -> Result<CString> {
>> +        let mut str = core::ptr::null_mut();
>> +        let pstr: *mut _ = &mut str;
>> +
>> +        // SAFETY: `name` is non-null and null-terminated. `self.as_raw` is
>> +        // valid because `self` is valid.
>> +        let ret = unsafe {
>> +            bindings::fwnode_property_read_string(self.as_raw(), name.as_char_ptr(), pstr as _)
>> +        };
>> +        to_result(ret)?;
>> +
>> +        // SAFETY: `pstr` contains a non-null ptr on success
>> +        let str = unsafe { CStr::from_char_ptr(*pstr) };
>> +        Ok(str.try_into()?)
>> +    }
>
> There's a problem with the C version of this function that I'd like to 
> not repeat in Rust especially since ownership is clear. 
>
> The issue is that we never know when the returned string is no longer 
> needed. For DT overlays, we need to be able free the string when/if an 
> overlay is removed. Though overlays are somewhat orthogonal to this. 
> It's really just when the property's node refcount goes to 0 that the 
> property value could be freed.
>
> So this function should probably return a copy of the string that the 
> caller owns.

I think the function already does that. The variable `str` is of type `&CStr`,
so the expression `str.try_into()?` on the last line calls the implementation
of `impl TryFrom<&CStr> for CString`, which is documented here:
https://rust.docs.kernel.org/kernel/str/struct.CString.html#impl-TryFrom%3C%26CStr%3E-for-CString

And looking at the source, it shows that the `CString` is created with a new
owned memory allocation:
https://rust.docs.kernel.org/src/kernel/str.rs.html#878-890

Remo