linux-kernel - Re: [PATCH v4 2/2] uaccess: rust: add UserSliceReader::strcpy_into

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Message-Id: <DABBSL9WTE9P.DX3I6CFIYH7P@kernel.org>
Date: Sun, 01 Jun 2025 18:09:26 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Andrew Morton"
 <akpm@...ux-foundation.org>, "Alexander Viro" <viro@...iv.linux.org.uk>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>, "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>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] uaccess: rust: add
 UserSliceReader::strcpy_into_buf

On Sat May 31, 2025 at 11:09 PM CEST, Alice Ryhl wrote:
> On Sat, May 31, 2025 at 10:38 PM Benno Lossin <lossin@...nel.org> wrote:
>> Maybe I misunderstood the code, but if you do this:
>>
>>     let slice = UserSlice::new(ptr, 1024);
>>     let mut buf = [0; 42];
>>     let s = slice.strcpy_into_buf(&mut buf)?;
>>
>> Then it will read 42 characters from userspace and (if there was no nul
>> byte) overwrite the last character with `\0`. If we now do
>>
>>     let mut buf2 = [0; 42];
>>     let s2 = slice.strcpy_into_buf(&mut buf2)?;
>>
>> Then that will continue the read at index 42, but effectively one
>> character will get skipped.
>>
>> (Now it's not possible to call `strcpy_into_buf` multiple times, but I
>> see no real reason why it isn't a `&mut self` method. Also a user could
>> call `clone_reader` and then manually `skip` 42 bytes. Although they
>> might only skip 41 bytes, since that's the length of the CStr. But that
>> runs into the problem that if there was a `\0` at index 41, then
>> repeated uses of the pattern above will yield empty strings.)
>
> I removed the ability to call it multiple times to avoid dealing with
> this kind of question. You may submit a follow-up patch to change it
> if you have a use-case.

I don't have a use-case, but we should document this behavior somewhere
especially since the ability to only call this function once guarantees
the correctness.

---
Cheers,
Benno