linux-kernel - Re: [PATCH v2 1/4] lib: introduce copy_struct_from

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Message-ID: <20190927010736.gy3vvvkjhwlybosj@yavin.dot.cyphar.com>
Date:   Fri, 27 Sep 2019 11:07:36 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        libc-alpha@...rceware.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] lib: introduce copy_struct_from_user() helper

On 2019-09-26, Christian Brauner <christian.brauner@...ntu.com> wrote:
> On Thu, Sep 26, 2019 at 01:03:29AM +0200, Aleksa Sarai wrote:
> > +int is_zeroed_user(const void __user *from, size_t size)
> > +{
> > +	unsigned long val;
> > +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> > +
> > +	if (unlikely(!size))
> > +		return true;
> 
> You're returning "true" and another implicit boolean with (val == 0)
> down below but -EFAULT in other places. But that function is int
> is_zeroed_user() Would probably be good if you either switch to bool
> is_zeroed_user() as the name suggests or rename the function and have
> it return an int everywhere.

I just checked, and in C11 (and presumably in older specs) it is
guaranteed that "true" and "false" from <stdbool.h> have the values 1
and 0 (respectively) [§7.18]. So this is perfectly well-defined.

Personally, I think it's more readable to have:

  if (unlikely(size == 0))
    return true;
  /* ... */
  return (val == 0);

compared to:

  if (unlikely(size == 0))
    return 1;
  /* ... */
  return val ? 0 : 1;

But I will change the function name (to check_zeroed_user) to make it
clearer that it isn't returning a boolean and that you need to check for
negative returns.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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