All of lore.kernel.org
 help / color / mirror / Atom feed
From: Arnd Bergmann <arnd@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Tom Rix <trix@redhat.com>,
	Zheng Zengkai <zhengzengkai@huawei.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	David Howells <dhowells@redhat.com>,
	Hulk Robot <hulkci@huawei.com>,
	linux-afs@lists.infradead.org,
	Marc Dionne <marc.dionne@auristor.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] afs: fix no return statement in function returning non-void
Date: Fri, 18 Jun 2021 17:23:02 +0200	[thread overview]
Message-ID: <CAK8P3a3L6B9HXsOXSu9_c6pz1kN91Vig6EPsetLuYVW=M72XaQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wgW6yfsLUtepANX2PVkADR_7WDzk05YVhtw1ZBmDEGT2Q@mail.gmail.com>

On Thu, Jun 17, 2021 at 12:51 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, Jun 16, 2021 at 9:22 AM Tom Rix <trix@redhat.com> wrote:
> >
> > to fix, add an unreachable() to the generic BUG()
> >
> > diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> > index f152b9bb916f..b250e06d7de2 100644
> > --- a/include/asm-generic/bug.h
> > +++ b/include/asm-generic/bug.h
> > @@ -177,7 +177,10 @@ void __warn(const char *file, int line, void
> > *caller, unsigned taint,
> >
> >   #else /* !CONFIG_BUG */
> >   #ifndef HAVE_ARCH_BUG
> > -#define BUG() do {} while (1)
> > +#define BUG() do {                                             \
> > +               do {} while (1);                                \
> > +               unreachable();                                  \
> > +       } while (0)
> >   #endif
>
> I'm a bit surprised that the compiler doesn't make that code after an
> infinite loop automatically be marked "unreachable". But at the same I
> can imagine the compiler doing some checks without doing real flow
> analysis, and doing "oh, that conditional branch is unconditional".
>
> So this patch at least makes sense to me and I have no objections to
> it, even if it makes me go "silly compiler, we shouldn't have to tell
> you this".
>
> So Ack from me on this.

I've tried to figure out what the compiler is trying to do here, and it's
still weird. When I saw the patch posted, I misread it as having just
unreachable() without the loop, and that would have been bad
because it triggers undefined behavior.

What I found is a minimal test case of

static int f(void)
{
   do {} while (1);
}

to trigger the warning with any version of gcc (not clang), but none of
these other variations cause a warning:

 // not static -> no warning!
int f(void)
{
   do {} while (1);
}

// some return statement anywhere in the function, no warning
static int f(int i)
{
  if (i)
      return 0;
   do {} while (1);
}

// annotated as never returning, as discussed in this thread
static int __attribute__((noreturn)) f(void)
{
   do {} while (1);
}

// unreachable annotation, as suggested by Tom
static int f(void)
{
   do {} while (1);
   __builtin_unreachable();
}

The last three are obviously intentional, as the warning is only for functions
that can *never* return but lack an annotation. I have no idea why the warning
is only for static functions though.

All my randconfig builds for arm/arm64/x86 missed this problem since those
architectures have a custom BUG() implementation with an inline asm.
I've taken them out now and found only two other instances of the issue so far:
arbitrary_virt_to_machine() and ppc64 get_hugepd_cache_index(). My preference
would be to annotate these as __noreturn, but change to the asm-generic
BUG() is probably better.

        Arnd

  reply	other threads:[~2021-06-18 15:27 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-15 11:55 David Howells
2021-06-15 12:03 ` David Howells
2021-06-15 14:49 ` Linus Torvalds
2021-06-15 23:58   ` Randy Dunlap
2021-06-16  0:32     ` Linus Torvalds
2021-06-16  1:38       ` Randy Dunlap
2021-06-16  2:19         ` Randy Dunlap
2021-06-16  3:15         ` Zheng Zengkai
2021-06-16 12:56           ` Tom Rix
2021-06-16 14:34             ` Linus Torvalds
2021-06-16 16:22               ` Tom Rix
2021-06-16 16:29                 ` Linus Torvalds
2021-06-18 15:23                   ` Arnd Bergmann [this message]
2021-06-16 13:41           ` David Howells
  -- strict thread matches above, loose matches on Subject: below --
2021-03-27 12:16 Zheng Zengkai
2021-03-31  2:32 ` Zheng Zengkai
2021-04-08 14:06 ` David Howells
2021-04-19 22:31   ` Randy Dunlap
2021-05-27 19:48     ` Randy Dunlap

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAK8P3a3L6B9HXsOXSu9_c6pz1kN91Vig6EPsetLuYVW=M72XaQ@mail.gmail.com' \
    --to=arnd@kernel.org \
    --cc=dhowells@redhat.com \
    --cc=hulkci@huawei.com \
    --cc=linux-afs@lists.infradead.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=marc.dionne@auristor.com \
    --cc=rdunlap@infradead.org \
    --cc=torvalds@linux-foundation.org \
    --cc=trix@redhat.com \
    --cc=zhengzengkai@huawei.com \
    --subject='Re: [PATCH] afs: fix no return statement in function returning non-void' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.