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

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Date:   Wed, 28 Mar 2018 11:50:09 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Manoj Gupta <manojgupta@...omium.org>,
        linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH] debugfs: Check return value of debugfs_real_fops() for
 NULL

El Wed, Mar 28, 2018 at 08:19:36PM +0200 Greg Kroah-Hartman ha dit:

> On Wed, Mar 28, 2018 at 11:14:56AM -0700, Matthias Kaehlcke wrote:
> > El Wed, Mar 28, 2018 at 08:05:56PM +0200 Greg Kroah-Hartman ha dit:
> > 
> > > 
> > > A: Because it messes up the order in which people normally read text.
> > > Q: Why is top-posting such a bad thing?
> > > A: Top-posting.
> > > Q: What is the most annoying thing in e-mail?
> > > 
> > > A: No.
> > > Q: Should I include quotations after my reply?
> > > 
> > > http://daringfireball.net/2007/07/on_top
> > > 
> > > 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.
> > > 
> > > Then fix the tool, the C code is correct :)
> > > 
> > > > Clang has simply omitted the statement where NULL is returned since
> > > > the pointer was always dereferenced post inlining.
> > > 
> > > Then tell clang not to do that, like we tell gcc not to do that as that
> > > is a foolish thing for a compiler to do when building the kernel.
> > 
> > Thanks all for your input, we'll try to get
> > -fno-delete-null-pointer-checks or a similar flag to be added to
> > clang.
> 
> Wait, clang does not have that?

Nope, clang doesn't currently have such a flag.

> That's crazy, how has this not been hit yet when building the
> kernel?

IIRC this patch was needed to work around the lack of the flag:

commit beaec533fc2701a28a4d667f67c9f59c6e4e0d13
Author: Alexander Potapenko <glider@...gle.com>
Date:   Wed Jul 19 20:27:30 2017 +0200

    llist: clang: introduce member_address_is_nonnull()

    Currently llist_for_each_entry() and llist_for_each_entry_safe() iterate
    until &pos->member != NULL.  But when building the kernel with Clang,
    the compiler assumes &pos->member cannot be NULL if the member's offset
    is greater than 0 (which would be equivalent to the object being
    non-contiguous in memory).  Therefore the loop condition is always true,
    and the loops become infinite.

    To work around this, introduce the member_address_is_nonnull() macro,
    which casts object pointer to uintptr_t, thus letting the member pointer
    to be NULL.

    Signed-off-by: Alexander Potapenko <glider@...gle.com>
    Tested-by: Sodagudi Prasad <psodagud@...eaurora.org>
    Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>


Other than that I am not aware of any known issues.