Re: Kernel oops caused by signed divide

[Yonghong Song <yonghong.song@linux.dev>]

On 9/10/24 11:25 AM, Alexei Starovoitov wrote:
> On Tue, Sep 10, 2024 at 11:02 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>>
>> On 9/10/24 8:21 AM, Alexei Starovoitov wrote:
>>> On Tue, Sep 10, 2024 at 7:21 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>>>> On 9/9/24 10:29 AM, Alexei Starovoitov wrote:
>>>>> On Mon, Sep 9, 2024 at 10:21 AM Zac Ecob <zacecob@protonmail.com> wrote:
>>>>>> Hello,
>>>>>>
>>>>>> I recently received a kernel 'oops' about a divide error.
>>>>>> After some research, it seems that the 'div64_s64' function used for the 'MOD'/'REM' instructions boils down to an 'idiv'.
>>>>>>
>>>>>> The 'dividend' is set to INT64_MIN, and the 'divisor' to -1, then because of two's complement, there is no corresponding positive value, causing the error (at least to my understanding).
>>>>>>
>>>>>>
>>>>>> Apologies if this is already known / not a relevant concern.
>>>>> Thanks for the report. This is a new issue.
>>>>>
>>>>> Yonghong,
>>>>>
>>>>> it's related to the new signed div insn.
>>>>> It sounds like we need to update chk_and_div[] part of
>>>>> the verifier to account for signed div differently.
>>>> In verifier, we have
>>>>      /* [R,W]x div 0 -> 0 */
>>>>      /* [R,W]x mod 0 -> [R,W]x */
>>> the verifier is doing what hw does. In this case this is arm64 behavior.
>> Okay, I see. I tried on a arm64 machine it indeed hehaves like the above.
>>
>> # uname -a
>> Linux ... #1 SMP PREEMPT_DYNAMIC Thu Aug  1 06:58:32 PDT 2024 aarch64 aarch64 aarch64 GNU/Linux
>> # cat t2.c
>> #include <stdio.h>
>> #include <limits.h>
>> int main(void) {
>>     volatile long long a = 5;
>>     volatile long long b = 0;
>>     printf("a/b = %lld\n", a/b);
>>     return 0;
>> }
>> # cat t3.c
>> #include <stdio.h>
>> #include <limits.h>
>> int main(void) {
>>     volatile long long a = 5;
>>     volatile long long b = 0;
>>     printf("a%%b = %lld\n", a%b);
>>     return 0;
>> }
>> # gcc -O2 t2.c && ./a.out
>> a/b = 0
>> # gcc -O2 t3.c && ./a.out
>> a%b = 5
>>
>> on arm64, clang18 compiled binary has the same result
>>
>> # clang -O2 t2.c && ./a.out
>> a/b = 0
>> # clang -O2 t3.c && ./a.out
>> a%b = 5
>>
>> The same source code, compiled on x86_64 with -O2 as well,
>> it generates:
>>     Floating point exception (core dumped)
>>
>>>> What the value for
>>>>      Rx_a sdiv Rx_b -> ?
>>>> where Rx_a = INT64_MIN and Rx_b = -1?
>>> Why does it matter what Rx_a contains ?
>> It does matter. See below:
>>
>> on arm64:
>>
>> # cat t1.c
>> #include <stdio.h>
>> #include <limits.h>
>> int main(void) {
>>     volatile long long a = LLONG_MIN;
>>     volatile long long b = -1;
>>     printf("a/b = %lld\n", a/b);
>>     return 0;
>> }
>> # clang -O2 t1.c && ./a.out
>> a/b = -9223372036854775808
>> # gcc -O2 t1.c && ./a.out
>> a/b = -9223372036854775808
>>
>> So the result of a/b is LLONG_MIN
>>
>> The same code will cause exception on x86_64:
>>
>> $ uname -a
>> Linux ... #1 SMP Wed Jun  5 06:21:21 PDT 2024 x86_64 x86_64 x86_64 GNU/Linux
>> [yhs@devvm1513.prn0 ~]$ gcc -O2 t1.c && ./a.out
>> Floating point exception (core dumped)
>> [yhs@devvm1513.prn0 ~]$ clang -O2 t1.c && ./a.out
>> Floating point exception (core dumped)
>>
>> So this is what we care about.
>>
>> So I guess we can follow arm64 result too.
>>
>>> What cpus do in this case?
>> See above. arm64 produces *some* result while x64 cause exception.
>> We do need to special handle for LLONG_MIN/(-1) case.
> My point about Rx_a that idiv will cause out-of-range exception
> for many other values than Rx_a == INT64_MIN.
> I'm not sure that divisor -1 is the only such case either.
> Probably is, since intuitively -2 and all other divisors should fit fine.
> So the check likely needs Rx_b == -1 and a check for high bit in Rx_a ?

Looks like only Rx_a == INT64_MIN may cause the problem.
All other Rx_a numbers (from INT64_MIN+1 to INT64_MAX)
should be okay. Some selective testing below on x64 host:

$ cat t5.c
#include <stdio.h>
#include <limits.h>

unsigned long long res;
int main(void) {
   volatile long long a;
   long long i;
   for (i = LLONG_MIN + 1; i <= LLONG_MIN + 100; i++) {
     volatile long long b = -1;
     a = i;
     res += (unsigned long long)(a/b);
   }
   for (i = LLONG_MAX - 100; i <= LLONG_MAX - 1; i++) {
     volatile long long b = -1;
     a = i;
     res += (unsigned long long)(a/b);
   }
   printf("res = %llx\n", res);
   return 0;
}
$ gcc -O2 t5.c && ./a.out
res = 64

So I think it should be okay if the range is from LLONG_MIN + 1
to LLONG_MAX - 1.

Now for LLONG_MAX/(-1)

$ cat t6.c
#include <stdio.h>
#include <limits.h>
int main(void) {
   volatile long long a = LLONG_MAX;
   volatile long long b = -1;
   printf("a/b = %lld\n", a/b);
   return 0;
}
$ gcc -O2 t6.c && ./a.out
a/b = -9223372036854775807

It is okay too. So I think LLONG_MIN/(-1) is the only case
we should take care of.