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SubjectRe: Linux 2.6.27.27


On Wed, 22 Jul 2009, Krzysztof Oledzki wrote:

>
>
> On Tue, 21 Jul 2009, Linus Torvalds wrote:
>
>>
>> [ Added Ian Lance Taylor to the cc, he knows the background, and unlike me
>> is competent with gcc. ]
>>
>> On Tue, 21 Jul 2009, Troy Moure wrote:
>>>
>>> I think I've found something interesting. Look at the the code generated
>>> for edid_checksum() in driver/video/fbmon.c. This is what I see for the
>>> -fno-strict-overflow kernel:
>>
>> Ooh.
>>
>> Bingo. You're 100% right, and you definitely found it (of course, there
>> may be _other_ cases like this, but that's certainly _one_ of the
>> problems, and probably the only one).
>>
>> Just out of curiosity, how did you find it? Now that I know where to look,
>> it's very obvious in the assembler diffs, but I didn't notice it until you
>> pointed it out just because there is so _much_ of the diffs...
>>
>> And yes, that's very much a compiler bug. And I also bet it's very easily
>> fixed.
>>
>> The code in question is this loop:
>>
>> #define EDID_LENGTH 128
>>
>> unsigned char i, ...
>>
>> for (i = 0; i < EDID_LENGTH; i++) {
>> csum += edid[i];
>> all_null |= edid[i];
>> }
>>
>> and gcc -fno-strict-overflow has apparently decided that that is an
>> infinite loop, even though it clearly is not. So then the stupid and buggy
>> compiler will compile that loop (and the whole rest of the function) to
>> the "optimized" version that is just
>>
>> loop:
>> jmp loop;
>>
>> I even bet I know why: it looks at "unsigned char", and sees that it is an
>> 8-bit variable, and then it looks at "i < EDID_LENGTH" and sees that it is
>> a _signed_ comparison (it's signed because the C type rules mean that
>> 'unsigned char' will be extended to 'int' in an expression), and then it
>> decides that in a signed comparison an 8-bit entry is always going to be
>> smaller than 128.
>>
>> Anyway, I bet we can work around the compiler bug by just changing the
>> type of "i" from "unsigned char" to be a plain "int".
>>
>> Krzysztof? Mind testing that?
>>
>> Ian? This is Linux 2.6.27.27 compiled with gcc-4.2.4. I'm not seeing the
>> bug in the gcc I have on my machine (gcc-4.4.0), but the bug is very clear
>> (once you _find_ it, which was the problem) in the binaries that Krzysztof
>> posted. They're still at:
>>
>> http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fno-strict-overflow.bz2
>> (Hangs)
>> http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fwrapv.bz2
>> (OK)
>> http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fnone.bz2
>> (OK)
>>
>> and you can clearly see the 'edid_checksum' miscompilation in the objdump
>> disassembly.
>
> BTW: here is a simple testcase for this bug:
>
> --- fno-strict-overflow-fixed-bug.c ---
> #include <stdio.h>
>
> int main() {
>
> unsigned char i;
>
> for (i = 0; i < 128; i++)
> printf("loop %u\n", i);
>
> return 0;
> }
> --- cut here ---

Or this better one (no infinite loop):

--- cut here ---
#include <stdio.h>
int main() {
unsigned char i, j=0;

for (i = 0; i <= 127; i++) {
if (!i && j++) {
printf("Buggy GCC\n");
return 1;
}
}
printf("GCC is OK\n");

return 0;
}
--- cut here ---
> The code should be compiled with:
> cc -o fno-strict-overflow-fixed-bug -Os -fno-strict-overflow
> fno-strict-overflow-fixed-bug.c
> or:
> cc -o fno-strict-overflow-fixed-bug -O2 -fno-strict-overflow
> fno-strict-overflow-fixed-bug.c
>
> This bug does not exist with -O1 or if the loop is controlled by "i < 127" or
> "i < 129".
>
> So, we should make sure there is no
> unsigned char i; (...) for (i = 0; i < 128; i++)
> somewhere inside the kernel.


Best regards,

Krzysztof Olędzki
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