Messages in this thread | | | Date | Wed, 22 Jul 2009 15:45:37 +0200 (CEST) | From | Krzysztof Oledzki <> | Subject | Re: Linux 2.6.27.27 |
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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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