| Message ID | 20240620071617.61040-1-syq@gcc.gnu.org |
|---|---|
| State | New |
| Headers | show |
| Series | testsuite/ubsan/overflow-div-3.c: Use SIGTRAP for MIPS | expand |
On Thu, 20 Jun 2024, YunQiang Su wrote: > The DIV instructions of MIPS won't be trapped themself if the divisor > is zero. The compiler will emit a conditional trap instruct for it. > So the signal will be SIGTRAP instead of SIGFPE. It's an OS kernel bug if you get SIGTRAP for integer division by zero, so if a test case fails, it rightfully does. Maciej
On Thu, Jun 20, 2024 at 12:57 PM Maciej W. Rozycki <macro@orcam.me.uk> wrote: > > On Thu, 20 Jun 2024, YunQiang Su wrote: > > > The DIV instructions of MIPS won't be trapped themself if the divisor > > is zero. The compiler will emit a conditional trap instruct for it. > > So the signal will be SIGTRAP instead of SIGFPE. > > It's an OS kernel bug if you get SIGTRAP for integer division by zero, so > if a test case fails, it rightfully does. MIPS doesn't trap on integer division by zero, instead it's a GCC bug that we emit a conditional trapping instruction. So I think the fix is fine and accurate. Richard. > Maciej
On Thu, 20 Jun 2024, Richard Biener wrote: > > > The DIV instructions of MIPS won't be trapped themself if the divisor > > > is zero. The compiler will emit a conditional trap instruct for it. > > > So the signal will be SIGTRAP instead of SIGFPE. > > > > It's an OS kernel bug if you get SIGTRAP for integer division by zero, so > > if a test case fails, it rightfully does. > > MIPS doesn't trap on integer division by zero, instead it's a GCC bug that > we emit a conditional trapping instruction. So I think the fix is fine and > accurate. Then GCC emits the wrong trap instruction, wherever it comes from and whatever has caused it. The correct ones for integer division by zero (and FWIW overflow; and also the corresponding BREAK variants for MIPS I compatibility) have been designated in the ABI decades ago, documented in MIPS architecture handbooks, including printed ones, and they are supposed to cause the OS kernel to deliver SIGFPE (with the code correctly set) rather than SIGTRAP. It's been like this for 20+ years now, used to work last time I checked (but it may have been a decade ago, possibly when the microMIPS port was added, which sadly needlessly repeated IRIX compatibility breakage here), and suddenly requiring a SIGTRAP here would be a divergence from the ABI and a functional regression. In fact I fixed bugs in this area in the Linux kernel myself, beacuse indeed it used to send SIGTRAP incorrectly, but again it was 20+ years ago. FAOD GAS gets the trap encoding right in output from the DIV, MULO, REM, etc. macros in the MIPS assembly dialect. Maciej
> > Then GCC emits the wrong trap instruction, wherever it comes from and > whatever has caused it. The correct ones for integer division by zero Thanks so much. It is not the bug of Linux kernel or GCC. It is a bug of me ;) and qemu. Qemu didn't pass the code of TEQ correctly; and I haven't run this test on real hardware.
On Fri, 21 Jun 2024, YunQiang Su wrote: > > Then GCC emits the wrong trap instruction, wherever it comes from and > > whatever has caused it. The correct ones for integer division by zero > > Thanks so much. It is not the bug of Linux kernel or GCC. > It is a bug of me ;) and qemu. > > Qemu didn't pass the code of TEQ correctly; and I haven't run this test on > real hardware. QEMU is a simulator only and has bugs or other discrepancies from real hardware. Especially the user emulation mode combines issues with actual instruction simulation *and* the OS interface. Therefore you can use QEMU as a valuable tool in the course of making your changes, but you need to always verify the final result with real hardware before submitting it upstream. Also submissions need to include details as to how the problem addressed has been reproduced. Otherwise this is just producing noise and wasting people's time. Maciej
diff --git a/gcc/testsuite/c-c++-common/ubsan/overflow-div-3.c b/gcc/testsuite/c-c++-common/ubsan/overflow-div-3.c index 479dffb0304..eef58aca832 100644 --- a/gcc/testsuite/c-c++-common/ubsan/overflow-div-3.c +++ b/gcc/testsuite/c-c++-common/ubsan/overflow-div-3.c @@ -7,6 +7,11 @@ #include <stdlib.h> int cnt; +#ifdef __mips + int sig = SIGTRAP; +#else + int sig = SIGFPE; +#endif __attribute__((noipa)) int foo (int x, int y) @@ -30,7 +35,7 @@ main (void) sigemptyset (&s.sa_mask); s.sa_handler = handler; s.sa_flags = 0; - sigaction (SIGFPE, &s, NULL); + sigaction (sig, &s, NULL); volatile int a = foo (42, 0); cnt++; volatile int b = foo (INT_MIN, -1);