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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m noticing an unexpected difference between targets when I hit a dbgtrap in the debugger. Consider this simple llvm function:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">define void @do_break() {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> entry:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> call void @llvm.debugtrap()<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> ret void<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">}</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If I compile that with llc and use lldb to launch a program that calls it, on x86_64 linux (Ubuntu 18.04), here’s what I see at the stop:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">Process 130404 stopped<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">* thread #1, name = 'doit', stop reason = signal SIGTRAP<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> frame #0: 0x0000000000400541 doit`do_break at stub.ll:2:1<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">(lldb) disas<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">doit`do_break:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> 0x400540 <+0>: int3
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">-> 0x400541 <+1>: retq
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">(lldb) register read rip<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> rip = 0x0000000000400541 doit`do_break + 1 at stub.ll:2:1<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Note that rip is reported as pointing to the next instruction after the int3. If I ‘continue’ from there, the program continues doing whatever was after the debugtrap.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If I follow the same steps on aarch64 (also Ubuntu 18.04), I see this:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">Process 21586 stopped<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">* thread #1, name = 'doit', stop reason = signal SIGTRAP<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> frame #0: 0x00000000004005dc doit`do_break at stub.ll:1:1<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">(lldb) disas<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">doit`do_break:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">-> 0x4005dc <+0>: brk #0x1<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> 0x4005e0 <+4>: ret
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New"">(lldb) register read pc<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"Courier New""> pc = 0x00000000004005dc doit`do_break at stub.ll:1:1<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Note that here, pc is reported as pointing at the ‘brk’ instruction itself. If I ‘continue’ from there, I immediately find myself stopped back at the same point, ad infinitum.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">From what I can tell, GDB also “gets stuck” when it hits this instruction on aarch64 (and also doesn’t on x86_64).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m wondering what to make of this / where’s the “bug”.<o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1">Should llvm use a different lowering for dbgtrap on aarch64-linux? I don’t think so, it seems to be standard<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1">Should the system signal handler be reporting an incremented pc in the context struct when it hits brk?<o:p></o:p></li><ul style="margin-top:0in" type="circle">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level2 lfo1">And even if so, what should the workaround be for systems without such a fix?<o:p></o:p></li></ul>
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1">Should lldb (and gdb for that matter) somehow recognize this case, and increment pc when stopping or resuming at a brk?<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1">Is this just unsupported, is continuing past a debugtrap UB or otherwise disallowed?<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’d appreciate any insights here.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">-Joseph<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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