linux-kernel - Re: [PATCH] debugfs: Check return value of debugfs_real

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Date:   Wed, 28 Mar 2018 10:34:54 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Manoj Gupta <manojgupta@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] debugfs: Check return value of debugfs_real_fops() for
 NULL

On Wed, Mar 28, 2018 at 10:30:51AM -0500, Josh Poimboeuf wrote:
> On Wed, Mar 28, 2018 at 07:47:53AM -0700, Manoj Gupta wrote:
> > Please note that there is nothing wrong in the generated code, just
> > that it confuses objtool.
> > Clang has simply omitted the statement where NULL is returned since
> > the pointer was always dereferenced post inlining.
> 
> ... but returning NULL would be far more sane than falling through to
> the next function.

Or, as the case may be, oopsing at the point of failure.


> > Note that GCC will also remove the NULL pointers if it knows that the
> > pointer is dereferenced.
> > Here is an example.
> > 
> > void null_check(int *P) {
> >   int deref = *P;
> >   if (P == 0) // GCC won't check the condition.
> >     return;
> >   *P = 4;
> > }
> > 
> > Compiling with gcc -O2 gives:
> >         movl    $4, (%rdi)
> >         ret
> 
> This is why we use -fno-delete-null-pointer-checks.

-- 
Josh