[Bug tdep/29405] New: [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[Bug tdep/29405] New: [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

            Bug ID: 29405
           Summary: [gdb,tdep/aarch64] FAIL:
                    gdb.base/step-over-syscall.exp: vfork:
                    displaced=on/off: pc after stepi matches insn addr
                    after syscall
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

On aarch64, I run into:
...
FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=off: pc after stepi
matches insn addr after syscall
FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on: pc after stepi
matches insn addr after syscall
...

In more detail:
...
(gdb) PASS: gdb.base/step-over-syscall.exp: vfork: displaced=off: syscall
number matches
PASS: gdb.base/step-over-syscall.exp: vfork: displaced=off: find syscall insn
in vfork
x/2i $pc^M
=> 0xfffff7e31c0c <vfork+12>:   svc     #0x0^M
   0xfffff7e31c10 <vfork+16>:   cmn     x0, #0xfff^M
(gdb) PASS: gdb.base/step-over-syscall.exp: vfork: displaced=off: pc
before/after syscall instruction
stepi^M
[Detaching after vfork from child process 8493]^M
0x0000fffff7e31c14 in vfork () from /lib64/libc.so.6^M
1: x/i $pc^M
=> 0xfffff7e31c14 <vfork+20>:   b.cs    0xfffff7e31c1c <vfork+28>  // b.hs,
b.nlast^M
(gdb) PASS: gdb.base/step-over-syscall.exp: vfork: displaced=off: stepi vfork
insn
print /x $pc^M
$2 = 0xfffff7e31c14^M
(gdb) PASS: gdb.base/step-over-syscall.exp: vfork: displaced=off: pc after
stepi
FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=off: pc after stepi
matches insn addr after syscall
...

I suppose the test-case expects to land on the cmn insn, but instead it lands
on  the next insn.

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[Bug tdep/29405] [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |luis.machado at arm dot com

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[Bug tdep/29405] [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

--- Comment #1 from Luis Machado <luis.machado at arm dot com> ---
I remember this one, and it's been, AFAICT, kernel and timing-related.
Sometimes it will skip two instructions, other times it won't.

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[Bug tdep/29405] [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

--- Comment #2 from Luis Machado <luis.machado at arm dot com> ---
Where was this reproduced?

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[Bug tdep/29405] [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Luis Machado from comment #2)
> Where was this reproduced?

OS:
...
openSUSE Leap 15.3.  Kernel version 5.3.18-150300.59.49-default.
...

Machine:
...
$ cat /proc/cpuinfo 
processor       : 0
BogoMIPS        : 500.00
Features        : fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid
CPU implementer : 0x41
CPU architecture: 8
CPU variant     : 0x1
CPU part        : 0xd07
CPU revision    : 2
...

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[Bug tdep/29405] [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

Luis Machado <luis.machado at arm dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

--- Comment #4 from Luis Machado <luis.machado at arm dot com> ---
Thanks. I'm currently testing on 5.11 and I don't see it.

I did see this multiple times before on Ubuntu 18.04, if I recall correctly.
And Ubuntu 18.04 ran 5.3.*.

Let's keep this one open for now, but I suspect it is indeed a kernel hiccup or
timing issue that just vanished with newer kernels.

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[Bug tdep/29405] [gdb,tdep/aarch64] FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=on/off: pc after stepi matches insn addr after syscall

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29405

Luis Machado <luis.machado at arm dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |luis.machado at arm dot com

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