Date: Tue, 3 Nov 2020 14:45:36 +0300 From: xtouqh@hotmail.com To: Konstantin Belousov <kostikbel@gmail.com> Cc: Hans Petter Selasky <hps@selasky.org>, hackers@freebsd.org Subject: Re: KASSERT(val != 0) not triggering in linux_errno.c reading outside of array Message-ID: <AM0PR06MB39866FEEB7CBA44F12247283C0110@AM0PR06MB3986.eurprd06.prod.outlook.com> In-Reply-To: <20201103111613.GP2654@kib.kiev.ua> References: <AM0PR06MB39867F0AE62A737D2EE596F5C0110@AM0PR06MB3986.eurprd06.prod.outlook.com> <77d2eef0-9cc8-aa39-6d28-a7fb41e233ac@selasky.org> <AM0PR06MB398671E9C991E0A1B4223660C0110@AM0PR06MB3986.eurprd06.prod.outlook.com> <c87d3aef-7a66-48bb-ca9e-fe4ae4e9f1d3@selasky.org> <AM0PR06MB3986D1AA1E59725F4D677727C0110@AM0PR06MB3986.eurprd06.prod.outlook.com> <20201103111613.GP2654@kib.kiev.ua>
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Konstantin Belousov wrote:
> On Tue, Nov 03, 2020 at 01:17:51PM +0300, xtouqh@hotmail.com wrote:
>> Hans Petter Selasky wrote:
>>> Hi,
>>>
>>> Should:
>>>
>>> for (i = 1; i < sizeof(linux_errtbl); i++) {
>>>
>>> Be:
>>>
>>> for (i = 1; i < sizeof(linux_errtbl)/sizeof(linux_errtbl[0]);
>>> i++) {
>>>
>>>
>>> Or:
>>>
>>> for (i = 1; i < (int)nitems(linux_errtbl); i++) {
>>
>> That's right, and I added the same comment in the review. My question is
>> different though -- with the issue present, KASSERT() should have triggered
>> (there are 0 values with incorrect indexes, added printf() confirms that)
>> exposing the bug, but it does not -- WHY? -- I just want to understand
>> what's going on.
>
> I think this is a poster child for the current undefined behaviour treatment
> by compilers. You are accessing beyond array last element, and compiler can
> prove it, so it allowed to do anything.
Makes sense, thank you. Just for the record, if I compile userland test
case with -O2, it behaves the same; previously I was building without
explicitly specifying optimization level, and it did "work".
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