linux-kernel - Re: [PATCH] [RFC] Initialization of unused function parameters

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Message-ID: <CAG_fn=WEed5NJ8hdrrP_N8aQ_1Ad11VoJgdVxQheo3VfT_xyXQ@mail.gmail.com>
Date:   Tue, 14 Jun 2022 20:07:52 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Evgenii Stepanov <eugenis@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Buka <vitalybuka@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH] [RFC] Initialization of unused function parameters

On Tue, Jun 14, 2022 at 6:48 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Jun 14, 2022 at 7:49 AM Alexander Potapenko <glider@...gle.com> wrote:
> >
> > The bigger question I want to raise here is whether we want to
> > discourage passing uninitialized variables to functions in the kernel
> > altogether.
>
> I'm assuming you mean pass by reference.

No, sorry for being unclear. I mean passing by value.
In the given example the prototype of step_into looks as follows (see
https://elixir.bootlin.com/linux/latest/source/fs/namei.c#L1846):

  static const char *step_into(struct nameidata *nd, int flags, struct
dentry *dentry, struct inode *inode, unsigned seq);

, and the local variables `struct inode *inode` and `unsigned seq` are
being passed to it by value, i.e. in certain cases the struct inode
pointer and the unsigned seq are uninitialized.

Does that change anything?

> Some functions are really fundamentally about initializing things, and
> expect uninitialized allocations.

Agreed, there are a lot of functions around that initialize one struct
or another, they are out of the scope.

> What I think might be a good model is to be able to mark such
> arguments as "must be initialized by callee".

This sounds interesting. In the given example I would suggest that the
call to lookup_fast()
(https://elixir.bootlin.com/linux/latest/source/fs/namei.c#L2016)
should be initializing &inode and &seq, so that it is guaranteed that
they are passed initialized into step_into().

-- 
Alexander Potapenko
Software Engineer

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