Re: linux-next: build failure after merge of the akpm-current tree

[Josh Poimboeuf]

On Wed, Jun 15, 2016 at 11:33:32AM +0200, Paul Bolle wrote:
> On do, 2016-05-05 at 22:44 -0700, Andrew Morton wrote:
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> > Subject: byteswap: try to avoid __builtin_constant_p gcc bug
> > 
> > This is another attempt to avoid a regression in wwn_to_u64() after that
> > started using get_unaligned_be64(), which in turn ran into a bug on
> > gcc-4.9 through 6.1.
> > 
> > The regression got introduced due to the combination of two separate
> > workarounds (e3bde9568d99 ("include/linux/unaligned: force inlining of
> > byteswap operations") and ef3fb2422ffe ("scsi: fc: use get/put_unaligned64
> > for wwn access")) that each try to sidestep distinct problems with gcc
> > behavior (code growth and increased stack usage).  Unfortunately after
> > both have been applied, a more serious gcc bug has been uncovered, leading
> > to incorrect object code that discards part of a function and causes
> > undefined behavior.
> > 
> > As part of this problem is how __builtin_constant_p gets evaluated on an
> > argument passed by reference into an inline function, this avoids the use
> > of __builtin_constant_p() for all architectures that set
> > CONFIG_ARCH_USE_BUILTIN_BSWAP.  Most architectures do not set
> > ARCH_SUPPORTS_OPTIMIZED_INLINING, which means they probably do not suffer
> > from the problem in the qla2xxx driver, but they might still run into it
> > elsewhere.
> > 
> > Both of the original workarounds were only merged in the 4.6 kernel, and
> > the bug that is fixed by this patch should only appear if both are there,
> > so we probably don't need to backport the fix.  On the other hand, it
> > works by simplifying the code path and should not have any negative
> > effects.
> > 
> > [arnd@xxxxxxxx: fix older gcc warnings]
> >   (http://lkml.kernel.org/r/12243652.bxSxEgjgfk@wuerfel)
> > Link: https://lkml.org/lkml/headers/2016/4/12/1103
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70232
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
> > Fixes: e3bde9568d99 ("include/linux/unaligned: force inlining of byteswap operations")
> > Fixes: ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access")
> > Link: http://lkml.kernel.org/r/1780465.XdtPJpi8Tt@wuerfel
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Tested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> # on gcc-5.3
> > Tested-by: Quinn Tran <quinn.tran@xxxxxxxxxx>
> > Cc: Martin Jambor <mjambor@xxxxxxx>
> > Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
> > Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> > Cc: Thomas Graf <tgraf@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Himanshu Madhani <himanshu.madhani@xxxxxxxxxx>
> > Cc: Jan Hubicka <hubicka@xxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> 
> This became commit 7322dd755e7d ("byteswap: try to avoid
> __builtin_constant_p gcc bug"). That commit was included in v4.6-rc7.
> Ever since that rc I see this warning when building for x86_64:
>     net/netfilter/ipvs/ip_vs_sync.c: In function âip_vs_proc_sync_connâ:
>     net/netfilter/ipvs/ip_vs_sync.c:1069:33: warning: âopt.init_seqâ may be used uninitialized in this function [-Wmaybe-uninitialized]
>       struct ip_vs_sync_conn_options opt;
>                                      ^
>     net/netfilter/ipvs/ip_vs_sync.c:1069:33: warning: âopt.deltaâ may be used uninitialized in this function [-Wmaybe-uninitialized]
>     net/netfilter/ipvs/ip_vs_sync.c:1069:33: warning: âopt.previous_deltaâ may be used uninitialized in this function [-Wmaybe-uninitialized]
>     net/netfilter/ipvs/ip_vs_sync.c:1069:33: warning: â*((void *)&opt+12).init_seqâ may be used uninitialized in this function [-Wmaybe-uninitialized]
>     net/netfilter/ipvs/ip_vs_sync.c:1069:33: warning: â*((void *)&opt+12).deltaâ may be used uninitialized in this function [-Wmaybe-uninitialized]
>     net/netfilter/ipvs/ip_vs_sync.c:1069:33: warning: â*((void *)&opt+12).previous_deltaâ may be used uninitialized in this function [-Wmaybe-uninitialized]
> 
> (It doesn't show up when building for 32 bits x86. Perhaps there's an
> int/long mismatch somewhere in the related code.)
> 
> Anyone else seeing this? 
> 
> It looks like a false positive. I can make it disappear by making sure
> ip_vs_proc_seqopt() isn't inlined. But that's not something I dare to
> put into a patch for a false positive. Anyone sitting on a better fix?

Hi Paul,

I agree it looks like a false positive, though the code is a bit
convoluted, so I'm not surprised that gcc might get confused.  How about
initializing opt to 0?

diff --git a/net/netfilter/ipvs/ip_vs_sync.c b/net/netfilter/ipvs/ip_vs_sync.c
index 803001a..f45496c 100644
--- a/net/netfilter/ipvs/ip_vs_sync.c
+++ b/net/netfilter/ipvs/ip_vs_sync.c
@@ -1066,7 +1066,7 @@ static int ip_vs_proc_str(__u8 *p, unsigned int plen, unsigned int *data_len,
  */
 static inline int ip_vs_proc_sync_conn(struct netns_ipvs *ipvs, __u8 *p, __u8 *msg_end)
 {
-	struct ip_vs_sync_conn_options opt;
+	struct ip_vs_sync_conn_options opt = {0};
 	union  ip_vs_sync_conn *s;
 	struct ip_vs_protocol *pp;
 	struct ip_vs_conn_param param;

-- 
Josh