linux-kernel - Re: linux-next: Tree for Sep 12 (bcachefs, objtool)

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Date:   Thu, 14 Sep 2023 09:51:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: linux-next: Tree for Sep 12 (bcachefs, objtool)

On Thu, Sep 14, 2023 at 03:51:44PM +0200, Josh Poimboeuf wrote:
> On Wed, Sep 13, 2023 at 06:01:42PM -0700, Kees Cook wrote:
> > > +++ b/fs/bcachefs/buckets.h
> > > @@ -180,7 +180,7 @@ static inline u64 bch2_dev_buckets_reserved(struct bch_dev *ca, enum bch_waterma
> > >  
> > >  	switch (watermark) {
> > >  	case BCH_WATERMARK_NR:
> > > -		unreachable();
> > > +		BUG();
> > 
> > Linus gets really upset about new BUG() usage (takes out the entire
> > system):
> > https://docs.kernel.org/process/deprecated.html#bug-and-bug-on
> > 
> > It'd be nicer to actually handle the impossible case. (WARN and return
> > 0?)
> 
> Sure, see below.

Looks good to me; thanks!

-Kees

> 
> BTW, I'm about to go off grid for 1.5 weeks, so there will be no v3
> coming from me anytime soon :-)
> 
> ---8<---
> 
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> Subject: [PATCH v2] bcachefs: Remove undefined behavior in bch2_dev_buckets_reserved()
> 
> In general it's a good idea to avoid using bare unreachable() because it
> introduces undefined behavior in compiled code.  In this case it even
> confuses GCC into emitting an empty unused
> bch2_dev_buckets_reserved.part.0() function.
> 
> Use WARN_ON(1) instead, which is nice and defined.  While in theory it
> should never trigger, if something were to go awry and the
> BCH_WATERMARK_NR case were to actually hit, the failure mode is more
> robust.
> 
> Fixes the following warnings:
> 
>   vmlinux.o: warning: objtool: bch2_bucket_alloc_trans() falls through to next function bch2_reset_alloc_cursors()
>   vmlinux.o: warning: objtool: bch2_dev_buckets_reserved.part.0() is missing an ELF size annotation
> 
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  fs/bcachefs/buckets.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/bcachefs/buckets.h b/fs/bcachefs/buckets.h
> index f192809f50cf..211f054bf83d 100644
> --- a/fs/bcachefs/buckets.h
> +++ b/fs/bcachefs/buckets.h
> @@ -180,7 +180,8 @@ static inline u64 bch2_dev_buckets_reserved(struct bch_dev *ca, enum bch_waterma
>  
>  	switch (watermark) {
>  	case BCH_WATERMARK_NR:
> -		unreachable();
> +		WARN_ON(1);
> +		break;
>  	case BCH_WATERMARK_stripe:
>  		reserved += ca->mi.nbuckets >> 6;
>  		fallthrough;
> -- 
> 2.41.0
> 

-- 
Kees Cook