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

[Alexei Starovoitov]

On Fri, Jan 17, 2014 at 1:02 PM, Markus Trippelsdorf
<markus@xxxxxxxxxxxxxxx> wrote:
> On 2014.01.17 at 11:58 -0800, Alexei Starovoitov wrote:
>> On Fri, Jan 17, 2014 at 9:58 AM, Alexei Starovoitov
>> <alexei.starovoitov@xxxxxxxxx> wrote:
>> > On Fri, Jan 17, 2014 at 5:37 AM, Dorau, Lukasz <lukasz.dorau@xxxxxxxxx> 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.

testcase was reduced from drivers/scsi/isci/host.h written back in
2011 (commit b329aff107)
#define for_each_isci_host(id, ihost, pdev) \
        for (id = 0, ihost = to_pci_info(pdev)->hosts[id]; \
             id < ARRAY_SIZE(to_pci_info(pdev)->hosts) && ihost; \
             ihost = to_pci_info(pdev)->hosts[++id])

yes, it does access 3rd element of 2 element array and will read junk.

C standard says the behavior is undefined and comes handy in compiler defense,
but in this case compiler has obviously all the information to make
right decision
instead of misoptimizing the code.
So yes, the loop is erroneous, non-portable, etc, but gcc can be smarter.
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