> On Oct 6, 2023, at 4:01 PM, Martin Uecker <uec...@tugraz.at> wrote:
>
> Am Freitag, dem 06.10.2023 um 06:50 -0400 schrieb Siddhesh Poyarekar:
>> On 2023-10-06 01:11, Martin Uecker wrote:
>>> Am Donnerstag, dem 05.10.2023 um 15:35 -0700 schrieb Kees Cook:
>>>> On Thu, Oct 05, 2023 at 04:08:52PM -0400, Siddhesh Poyarekar wrote:
>>>>> 2. How would you handle signedness of the size field? The size gets
>>>>> converted to sizetype everywhere it is used and overflows/underflows may
>>>>> produce interesting results. Do you want to limit the types to unsigned
>>>>> or
>>>>> do you want to add a disclaimer in the docs? The former seems like the
>>>>> *right* thing to do given that it is a new feature; best to enforce the
>>>>> cleaner habit at the outset.
>>>>
>>>> The Linux kernel has a lot of "int" counters, so the goal is to catch
>>>> negative offsets just like too-large offsets at runtime with the sanitizer
>>>> and report 0 for __bdos. Refactoring all these to be unsigned is going
>>>> to take time since at least some of them use the negative values as
>>>> special values unrelated to array indexing. :(
>>>>
>>>> So, perhaps if unsigned counters are worth enforcing, can this be a
>>>> separate warning the kernel can turn off initially?
>>>>
>>>
>>> I think unsigned counters are much more problematic than signed ones
>>> because wraparound errors are more difficult to find.
>>>
>>> With unsigned you could potentially diagnose wraparound, but only if we
>>> add -fsanitize=unsigned-overflow *and* add mechanism to mark intentional
>>> wraparound *and* everybody adds this annotation after carefully screening
>>> their code *and* rewriting all operations such as (counter - 3) + 5
>>> where the wraparound in the intermediate expression is harmless.
>>>
>>> For this reason, I do not think we should ever enforce some rule that
>>> the counter has to be unsigned.
>>>
>>> What we could do, is detect *storing* negative values into the
>>> counter at run-time using UBSan. (but if negative values are
>>> used for special cases, one also should be able to turn this
>>> off).
>>
>> All of the object size detection relies on object sizes being sizetype.
>> The closest we could do with that is detect (sz != SIZE_MAX && sz >
>> size_t / 2), since allocators typically cannot allocate more than
>> SIZE_MAX / 2.
>
> I was talking about the counter in:
>
> struct {
> int counter;
> char buf[] __counted_by__((counter))
> };
>
> which could be checked to be positive either when stored to or
> when buf is used.
>
> And yes, we could also check the size of buf. Not sure what is
> done for VLAs now, but I guess it could be similar.
>
For VLAs, the bounds expression could be both signed or unsigned.
But we have added a sanitizer option -fsanitize=vla-bound to catch the cases
when the size of the VLA is not positive.
For example:
opc@qinzhao-ol8u3-x86 Martin]$ cat t3.c
#include <stdio.h>
size_t foo(int m)
{
char t[m];
return sizeof(t);
}
int main()
{
printf ("the sizeof flexm is %lu \n", foo(-100000000));
return 0;
}
[opc@qinzhao-ol8u3-x86 Martin]$ sh t
/home/opc/Install/latest-d/bin/gcc -fsanitize=undefined -O2 -Wall -Wpedantic
t3.c
t3.c:4:8: runtime error: variable length array bound evaluates to non-positive
value -100000000
the sizeof flexm is 18446744073609551616
We can do the same thing for “counted_by”. i.e:
1. No specification for signed or unsigned for counted_by field.
2. Add an sanitizer option -fsanitize=counted-by-bound to catch the cases when
the size of the counted-by is not positive.
Is this good enough?
Qing
> Best,
> Martin
>
>
>
>>
>> Sid
