delve golang debugger can't read vvar on arm64

[Dan Bungert]

Also documented upstream:
https://github.com/go-delve/delve/issues/2630

In version 1.6.1 of the delve golang debugger, a 'dump' feature was added.
https://github.com/go-delve/delve/pull/2173

Since that time, arm64 autopkgtest of delve has been failing like so:

https://autopkgtest.ubuntu.com/results/autopkgtest-impish/impish/arm64/d/delve/20210707_003312_d3699@/log.gz
=== RUN TestDump
    proc_test.go:5117: testing normal dump
    proc_test.go:5029: bad DumpState &proc.DumpState{Mutex:sync.Mutex{state:0, sema:0x0}, Dumping:false, AllDone:true, Canceled:false, DoneChan:(chan struct {})(nil), ThreadsDone:5, ThreadsTotal:5, MemDone:0x657b000, MemTotal:0x657d000, Err:error(nil)}
--- FAIL: TestDump (0.98s)

The test fails becaue MemDone != MemTotal.
The decision on what to dump or not is based on smaps.
The failing item is vvar.
err when attempting to read is
error(syscall.Errno) golang.org/x/sys/unix.EIO (5)

Notably however, this test passes in debian CI. (kernel 4.19)
Also, running on older series such as bionic/focal does pass.

vvar section from vm with kernel 5.4 (test pass):
ffff924a4000-ffff924a5000 r--p 00000000 00:00 0 [vvar]
Size: 4 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Rss: 0 kB
Pss: 0 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
Referenced: 0 kB
Anonymous: 0 kB
LazyFree: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
FilePmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
Locked: 0 kB
THPeligible: 0
VmFlags: rd mr de

vvar from failing case
ffffac144000-ffffac146000 r--p 00000000 00:00 0 [vvar]
Size: 8 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Rss: 0 kB
Pss: 0 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
Referenced: 0 kB
Anonymous: 0 kB
LazyFree: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
FilePmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
Locked: 0 kB
THPeligible: 0
VmFlags: rd mr pf de

Two noticible differences
1) 4kB vs 8kB size
2) Failing case adds the "pf" flag ("pure PFN range")

Open questions:
1) Would we expect that a debugger could read vvar on arm64?
2) Any thoughts on why this seems to work on older kernels?
3) Do we believe this to be a kernel bug,
   or should the delve debugger not expect to read vvar?
4) Is it sensible for a debugger to attempt to read things with the "pf"
   flag? Note that the "dd" ("do not include area into core dump")
   flag is not set.

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: linux-image-5.11.0-20-generic 5.11.0-20.21+21.10.1
ProcVersionSignature: Ubuntu 5.11.0-20.21+21.10.1-generic 5.11.21
Uname: Linux 5.11.0-20-generic aarch64
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Jul 29 21:21 seq
 crw-rw---- 1 root audio 116, 33 Jul 29 21:21 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.20.11-0ubuntu67
Architecture: arm64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CasperMD5CheckResult: pass
Date: Thu Jul 29 21:34:34 2021
InstallationDate: Installed on 2021-07-29 (0 days ago)
InstallationMedia: Ubuntu-Server 21.10 "Impish Indri" - Alpha arm64 (20210729)
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lsusb:
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Lsusb-t:
 /: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/15p, 5000M
 /: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/15p, 480M
MachineType: QEMU QEMU Virtual Machine
PciMultimedia:

ProcEnviron:
 TERM=screen-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
ProcFB:

ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-5.11.0-20-generic root=/dev/mapper/username--vg-username--lv ro
RelatedPackageVersions:
 linux-restricted-modules-5.11.0-20-generic N/A
 linux-backports-modules-5.11.0-20-generic N/A
 linux-firmware 1.199
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 02/06/2015
dmi.bios.release: 0.0
dmi.bios.vendor: EFI Development Kit II / OVMF
dmi.bios.version: 0.0.0
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: virt-5.2
dmi.modalias: dmi:bvnEFIDevelopmentKitII/OVMF:bvr0.0.0:bd02/06/2015:br0.0:svnQEMU:pnQEMUVirtualMachine:pvrvirt-5.2:cvnQEMU:ct1:cvrvirt-5.2:
dmi.product.name: QEMU Virtual Machine
dmi.product.version: virt-5.2
dmi.sys.vendor: QEMU

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This change was made by a bot.

[Dan Bungert (dbungert)]

Should the 'dd' flag be set in this case?

[Shengjing Zhu (zhsj)]

Fixed in v1.8.0

[Oibaf (oibaf)]

Fixed in delve.