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id = 1184
title = "Extra SIGTRAP when breakpoint + watchpoint occur on same instruction"
state = "opened"
created_at = "2022-08-31T18:52:40.414Z"
closed_at = "n/a"
labels = ["GDB", "accel: TCG"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/1184"
host-os = "Ubuntu 18"
host-arch = "x86, but likely platform-agnostic"
qemu-version = "7.1.0"
guest-os = "x86 multiboot stub"
guest-arch = "x86, but likely platform-agnostic"
description = """If a breakpoint and watchpoint occur on the same instruction in TCG, gdb receives a breakpoint notification, a watchpoint notification, and then a SIGTRAP not corresponding to any set breakpoint/watchpoint."""
reproduce = """Start QEMU via:
```
./qemu-system-i386 -display none -accel tcg -kernel kernel.elf -s -S
```
Here's the gdb session:
```
(gdb) file kernel.elf
Reading symbols from kernel.elf...done.
(gdb) tar rem :1234
Remote debugging using :1234
0x0000fff0 in ?? ()
(gdb) b _start
Breakpoint 1 at 0x10000c: file kernel.s, line 17.
(gdb) c
Continuing.
Breakpoint 1, _start () at kernel.s:17
17 mov eax, 3
(gdb) b bp
Breakpoint 2 at 0x100011: file kernel.s, line 20.
(gdb) watch *(int*)&value
Hardware watchpoint 3: *(int*)&value
(gdb) c
Continuing.
Breakpoint 2, bp () at kernel.s:20
20 mov dword ptr value, eax
(gdb) c
Continuing.
Hardware watchpoint 3: *(int*)&value
Old value = 0
New value = 3
done () at kernel.s:23
23 jmp done
(gdb) c
Continuing.
Program received signal SIGTRAP, Trace/breakpoint trap.
done () at kernel.s:23
23 jmp done
```"""
additional = """This patch fixes it by disabling the extra debug interrupt if the CPU is already singlestepping, but I'm not certain it's the 'correct' fix?
```patch
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -894,7 +894,9 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
* trigger after the current instruction.
*/
qemu_mutex_lock_iothread();
- cpu_interrupt(cpu, CPU_INTERRUPT_DEBUG);
+ if ((cpu->singlestep_enabled & SSTEP_NOIRQ) == 0) {
+ cpu_interrupt(cpu, CPU_INTERRUPT_DEBUG);
+ }
qemu_mutex_unlock_iothread();
return;
}
```"""
|