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

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Message-ID: <CAG_fn=UPoM3bafwu6inGPMjg1bPw3HSFM_KrE_hen_MN3fu2vA@mail.gmail.com>
Date:   Tue, 14 Jun 2022 22:19:53 +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 8:31 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Jun 14, 2022 at 11:08 AM Alexander Potapenko <glider@...gle.com> wrote:
> >
> > On Tue, Jun 14, 2022 at 6:48 PM Linus Torvalds
> > >
> > > I'm assuming you mean pass by reference.
> >
> > No, sorry for being unclear. I mean passing by value.
>
> Pass-by-value most definitely should warn about uninitialized variables.
>
> > 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.
>
> Then those cases should warn. No question about it.

What about the cases where these uninitialized values are never used
in the callee?
step_into() is one of the instances from the kernel, but here is a
distilled example from https://godbolt.org/z/s1oPve6d4:

================
char *kmalloc(int size);

char *kmalloc_or_not(int flag, int size, char *p) {
  if (flag)
    return kmalloc(size);
  else
    return p;
}

char global[16];

char *p(int flag) {
  char *c;
  int size;
  if (flag)
    return kmalloc_or_not(1, 4, c);
  else
    return kmalloc_or_not(0, size, global);
}
================

In this example `size` is passed into kmalloc_or_not() initialized,
however it is never used, so the code probably has defined behavior.
In this particular case Clang's -Winitialized is able to notice that
`size` is uninitialized, but in more complex cases it cannot.

> I assume the only reason they don't warn right now is that the
> compiler doesn't see that they are uninitialized, possibly due to some
> earlier pass-by-reference use.

That's right, and here is where dynamic analysis comes to the rescue.
So should we let KMSAN catch such cases and consider them bugs^W
smelly code patterns that need to be fixed?

>
>              Linus

-- 
Alexander Potapenko
Software Engineer

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