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

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Message-ID: <20230914135144.udwf3lmhh5zael5e@treble>
Date:   Thu, 14 Sep 2023 15:51:44 +0200
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Kees Cook <keescook@...omium.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 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.

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