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From: Alexander Potapenko <glider@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Evgenii Stepanov <eugenis@google.com>,
	Kees Cook <keescook@chromium.org>, Marco Elver <elver@google.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vitaly Buka <vitalybuka@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-toolchains <linux-toolchains@vger.kernel.org>
Subject: Re: [PATCH] [RFC] Initialization of unused function parameters
Date: Tue, 14 Jun 2022 22:19:53 +0200	[thread overview]
Message-ID: <CAG_fn=UPoM3bafwu6inGPMjg1bPw3HSFM_KrE_hen_MN3fu2vA@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=whjz3wO8zD+itoerphWem+JZz4uS3myf6u1Wd6epGRgmQ@mail.gmail.com>

On Tue, Jun 14, 2022 at 8:31 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Tue, Jun 14, 2022 at 11:08 AM Alexander Potapenko <glider@google.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
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  reply	other threads:[~2022-06-14 20:20 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-14 14:48 [PATCH] [RFC] Initialization of unused function parameters Alexander Potapenko
2022-06-14 16:48 ` Linus Torvalds
2022-06-14 17:11   ` Nick Desaulniers
2022-06-14 17:24     ` Linus Torvalds
2022-06-14 18:08       ` Nick Desaulniers
2022-06-14 22:27         ` Peter Zijlstra
2022-06-14 18:07   ` Alexander Potapenko
2022-06-14 18:30     ` Linus Torvalds
2022-06-14 20:19       ` Alexander Potapenko [this message]
2022-06-14 20:43         ` Linus Torvalds
2022-06-14 21:40         ` Segher Boessenkool
2022-06-14 22:08           ` Evgenii Stepanov
2022-06-15  8:30           ` Alexander Potapenko
2022-06-15 16:46             ` Segher Boessenkool

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