[Virtio-fs] Problem using gdb/lldb on a binary residing on a DAX-enabled volume

[Sergio Lopez]

Hi,

I've noticed that trying to use gdb/lldb on any binary residing on a
DAX-enabled virtio-fs volume leads to a SIGSEGV in userspace...

[root at fedora ~]# mount -t virtiofs -o dax,context=system_u:object_r:root_t:s0 /dev/root /mnt/virtio-fs
[root at fedora ~]# gdb /mnt/virtio-fs/true 
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Reading symbols from /mnt/virtio-fs/true...
Missing separate debuginfo for /mnt/virtio-fs/true
Try: dnf --enablerepo='*debug*' install /usr/lib/debug/.build-id/25/47d5e7307ccc0f5a75e5990afdc5e15293ccc1.debug
Reading symbols from .gnu_debugdata for /mnt/virtio-fs/true...
(No debugging symbols found in .gnu_debugdata for /mnt/virtio-fs/true)
(gdb) break main
Breakpoint 1 at 0x14c0
(gdb) r
Starting program: /mnt/virtio-fs/true 
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.33-5.fc34.x86_64

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555960 in _start ()


... while the kernel drops a warning:


[   39.962629] ------------[ cut here ]------------
[   39.963854] WARNING: CPU: 1 PID: 718 at mm/memory.c:2666 wp_page_copy+0x565/0x5d0
[   39.965736] Modules linked in: virtio_net net_failover i2c_i801 failover i2c_smbus joydev drm ip_tablew
[   39.969040] CPU: 1 PID: 718 Comm: gdb Not tainted 5.11.12-300.fc34.x86_64 #1
[   39.970800] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebui4
[   39.973535] RIP: 0010:wp_page_copy+0x565/0x5d0
[   39.974675] Code: 89 7b 48 48 8b 43 30 49 39 07 75 76 48 8b 7c 24 08 48 8b 74 24 10 ba 00 10 00 00 e8 5
[   39.979165] RSP: 0018:ffffa3aa40cb7bc0 EFLAGS: 00010206
[   39.980483] RAX: 0000000000001000 RBX: ffffa3aa40cb7c60 RCX: 0000000000001000
[   39.982244] RDX: 0000000000001000 RSI: 0000555555555000 RDI: ffff898117a9f000
[   39.984015] RBP: ffffe934045ea7c0 R08: ffffe934041c7f68 R09: 0000000000000001
[   39.985780] R10: 0000000000117a9f R11: ffff89810825dd20 R12: ffff898105829770
[   39.987552] R13: 0000000000000000 R14: ffff898106c11dc0 R15: 0000000000000001
[   39.989324] FS:  00007f9691a91080(0000) GS:ffff898763a40000(0000) knlGS:0000000000000000
[   39.991319] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   39.992760] CR2: 0000555555555000 CR3: 000000010422a000 CR4: 00000000000006e0
[   39.994535] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   39.996312] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   39.998086] Call Trace:
[   39.998766]  handle_mm_fault+0x113f/0x1970
[   39.999828]  __get_user_pages+0x28c/0x7f0
[   40.000868]  __get_user_pages_remote+0xd4/0x320
[   40.002028]  __access_remote_vm+0x106/0x370
[   40.003099]  ptrace_access_vm+0x9d/0xd0
[   40.004100]  ptrace_request+0x560/0x620
[   40.005097]  arch_ptrace+0x13d/0x3b0
[   40.006039]  __x64_sys_ptrace+0x82/0x120
[   40.007050]  do_syscall_64+0x33/0x40
[   40.008002]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   40.009283] RIP: 0033:0x7f9694ce2aae
[   40.010220] Code: 70 41 83 f8 03 c7 44 24 10 08 00 00 00 48 89 44 24 18 48 8d 44 24 30 8b 70 08 4c 0f 4
[   40.014714] RSP: 002b:00007ffe0cffb4e0 EFLAGS: 00000202 ORIG_RAX: 0000000000000065
[   40.016577] RAX: ffffffffffffffda RBX: 00005555555554c0 RCX: 00007f9694ce2aae
[   40.018331] RDX: 00005555555554c0 RSI: 00000000000002ef RDI: 0000000000000005
[   40.020092] RBP: 0000000000000001 R08: 0000000000000004 R09: 00005555555554c0
[   40.021855] R10: 55415641fa1e0fcc R11: 0000000000000202 R12: 0000000000000001
[   40.023612] R13: 00007f9691a90848 R14: 00000000000002ef R15: 0000000000000000
[   40.025364] ---[ end trace 2273d62ee1978eb4 ]---


Seems like DAX breaks something in the ptrace_access_vm path. On a
volume without DAX works fine.

The guest is a Fedora with a 5.11.12-300.fc34.x86_64 kernel.

Sergio.
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