[Bug 1850258] Re: First breakpoint at AVX instruction with memory operand causes SIGSEGV when tring to continue execution

[Pauli]

A bit more debugging I see that signal comes from kernel (si_code=0x80)
but it claims null pointer reference. I don't understand how it could be
a null pointer .... I would need to have better understanding what
happens in gdb and kernel to trigger the SIGSEGV.

I found out also that workaround is to have breakpoint but disable it
after stopping on the problematic instruction. If breakpoint isn't
active then execution continues without issues. Only issues is that I
don't know full set of instructions which actually trigger this issue. I
have had issues with some other memory referencing VEX coded
instructions. But I also have examples of memory reference instructions
which don't trigger the bug.

I attached updated test2.cc which has now signal handling dumping
siginfo.

Reading symbols from ./test2...
(gdb) b main
Breakpoint 1 at 0x650: file test2.cc, line 41.
(gdb) r
Starting program: /home/coren/project/test2 

Breakpoint 1, main () at test2.cc:41
41	    asm("\tvmovdqa %1, %0\n" : "=x" (bar) : "xm" (foo));
(gdb) disassemble 
Dump of assembler code for function main():
=> 0x0000555555554650 <+0>:	vmovdqa 0x19d8(%rip),%xmm0        # 0x555555556030 <foo>
   0x0000555555554658 <+8>:	vmovd  %xmm0,%eax
   0x000055555555465c <+12>:	retq   
End of assembler dump.
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
main () at test2.cc:41
41	    asm("\tvmovdqa %1, %0\n" : "=x" (bar) : "xm" (foo));
(gdb) 
Continuing.
sig: 11, ctx: 0x7fffffffd740
si_signo: 11, si_erron: 0, si_code: 128
si_addr: (nil), si_addr_lsb: 0, si_pid: 0, si_uid: 0

Breakpoint 1, main () at test2.cc:41
41	    asm("\tvmovdqa %1, %0\n" : "=x" (bar) : "xm" (foo));
(gdb) 
Continuing.

Program received signal SIGSEGV, Segmentation fault.
main () at test2.cc:41
41	    asm("\tvmovdqa %1, %0\n" : "=x" (bar) : "xm" (foo));
(gdb) 
Continuing.
sig: 11, ctx: 0x7fffffffd740
si_signo: 11, si_erron: 0, si_code: 128
si_addr: (nil), si_addr_lsb: 0, si_pid: 0, si_uid: 0

Breakpoint 1, main () at test2.cc:41
41	    asm("\tvmovdqa %1, %0\n" : "=x" (bar) : "xm" (foo));
(gdb) dis 1
(gdb) c
Continuing.
[Inferior 1 (process 9091) exited with code 01]


** Attachment added: "test2.cc (A test case with signal handler returning)"
   https://bugs.launchpad.net/ubuntu/+source/gdb/+bug/1850258/+attachment/5301182/+files/test2.cc

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https://bugs.launchpad.net/bugs/1850258

Title:
  First breakpoint at AVX instruction with memory operand causes SIGSEGV
  when tring to continue execution

Status in gdb package in Ubuntu:
  New

Bug description:
  I noticed random looking SIGSEGV to application when trying to
  continue execution after first breakpoint. I now seem to have narrowed
  the issue to SIMD instruction with memory operand as first breakpoint
  location. I haven't managed to figure out why the SIGSEGV is delivered
  to the debugger application.

  It is important have first breakpoint exactly at a problematic
  instructions. If I first break on a different instruction then later
  breakpoints won't reproduce that crash

  I haven't tested if this is a hardware specific issue.

  I managed to write a simple test case which reproduces the crash if
  breakpoint is set. I attached the test.cc which includes compilation
  and testing instructions. test.cc is supposed to generate a simple
  main function like:

  Dump of assembler code for function main():
  => 0x0000555555554520 <+0>:	vmovdqa 0x1af8(%rip),%xmm0        # 0x555555556020 <foo>
     0x0000555555554528 <+8>:	vmovd  %xmm0,%eax
     0x000055555555452c <+12>:	retq

  I set breakpoint with:
  b main

  Then either continue or stepping causes SIGSEGV to the debugged
  application.

  This was happening already with disco. I only now figured out enough
  details to make a simple test case which is worth a bug report.

  ProblemType: Bug
  DistroRelease: Ubuntu 19.10
  Package: gdb 8.3-0ubuntu1
  ProcVersionSignature: Ubuntu 5.3.0-19.20-generic 5.3.1
  Uname: Linux 5.3.0-19-generic x86_64
  NonfreeKernelModules: nvidia_modeset nvidia
  ApportVersion: 2.20.11-0ubuntu8
  Architecture: amd64
  CurrentDesktop: GNOME
  Date: Tue Oct 29 09:44:52 2019
  InstallationDate: Installed on 2037-12-25 (-6632 days ago)
  InstallationMedia: Lubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
  SourcePackage: gdb
  UpgradeStatus: Upgraded to eoan on 2019-10-27 (1 days ago)

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