linux-kernel - Re: Why is (2 < 2) true? Is it a gcc bug?

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Date:	Fri, 17 Jan 2014 22:02:01 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	"Dorau, Lukasz" <lukasz.dorau@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	sebastian.riemer@...fitbricks.com, richard.weinberger@...il.com
Subject: Re: Why is (2 < 2) true? Is it a gcc bug?

On 2014.01.17 at 11:58 -0800, Alexei Starovoitov wrote:
> On Fri, Jan 17, 2014 at 9:58 AM, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> > On Fri, Jan 17, 2014 at 5:37 AM, Dorau, Lukasz <lukasz.dorau@...el.com> wrote:
> >> Hi
> >>
> >> My story is very simply...
> >> I applied the following patch:
> >>
> >> diff --git a/drivers/scsi/isci/init.c b/drivers/scsi/isci/init.c
> >> --- a/drivers/scsi/isci/init.c
> >> +++ b/drivers/scsi/isci/init.c
> >> @@ -698,8 +698,11 @@ static int isci_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> >>         if (err)
> >>                 goto err_host_alloc;
> >>
> >> -       for_each_isci_host(i, isci_host, pdev)
> >> +       for_each_isci_host(i, isci_host, pdev) {
> >> +               pr_err("(%d < %d) == %d\n",\
> >> +                      i, SCI_MAX_CONTROLLERS, (i < SCI_MAX_CONTROLLERS));
> >>                 scsi_scan_host(to_shost(isci_host));
> >> +       }
> >>
> >>         return 0;
> >>
> >> --
> >> 1.8.3.1
> >>
> >> Then I issued the command 'modprobe isci' on platform with two SCU controllers (Patsburg D or T chipset)
> >> and received the following, very strange, output:
> >>
> >> (0 < 2) == 1
> >> (1 < 2) == 1
> >> (2 < 2) == 1
> >>
> >> Can anyone explain why (2 < 2) is true? Is it a gcc bug?
> >
> > gcc sees that i < array_size is the same as i < 2 as part of loop condition, so
> > it optimizes (i < sci_max_controllers) into constant 1.
> > and emits printk like:
> >   printk ("\13(%d < %d) == %d\n", i_382, 2, 1);
> >
> >> (The kernel was compiled using gcc version 4.8.2.)
> >
> > it actually looks to be gcc 4.8 bug.
> > Can you try gcc 4.7 ?
> >
> 
> It is interesting GCC 4.8 bug,
> since it seems to expose issues in two compiler passes.
> 
> here is test case:
> 
> struct isci_host;
> struct isci_orom;
> 
> struct isci_pci_info {
>   struct isci_host *hosts[2];
>   struct isci_orom *orom;
> } v = {{(struct isci_host *)1,(struct isci_host *)1}, 0};
> 
> int printf(const char *fmt, ...);
> 
> int isci_pci_probe()
> {
>   int i;
>   struct isci_host *isci_host;
> 
>   for (i = 0, isci_host = v.hosts[i];
>        i < 2 && isci_host;
>        isci_host = v.hosts[++i]) {
>     printf("(%d < %d) == %d\n", i, 2, (i < 2));
>   }
> 
>   return 0;
> }
> 
> int main()
> {
>   isci_pci_probe();
> }
> 
> $ gcc bug.c
> $./a.out
> 0 < 2) == 1
> (1 < 2) == 1
> $ gcc bug.c -O2
> $ ./a.out
> (0 < 2) == 1
> (1 < 2) == 1
> Segmentation fault (core dumped)

Your testcase is invalid:

markus@x4 tmp % clang -fsanitize=undefined -Wall -Wextra -O2 bug.c
markus@x4 tmp % ./a.out
(0 < 2) == 1
(1 < 2) == 1
bug.c:16:20: runtime error: index 2 out of bounds for type 'struct isci_host *[2]'

As Jakub Jelinek said on IRC, changing the loop to e.g.:

  for (i = 0;
       i < 2 && (isci_host = v.hosts[i]);
       i++) {

fixes the issue.

-- 
Markus
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