debugfs vs. device removal

[Omar Sandoval]

Hi,

In the block layer, we abuse sysfs to export some per-device debugging
information. I was looking into moving this to debugfs, but I realized
that debugfs doesn't have a mechanism to ensure that a file associated
with a device is safe to use when the device is removed. 

At a quick glance, HID has some per-device information in debugfs.
However, I don't see any sort of protection against a device being
removed. I was easily able to trigger an oops by reading from
/sys/kernel/debug/hid/*/rdesc in a loop and removing the USB device
(trace attached).

How can I safely export per-device debugging information to debugfs?

Thanks,
Omar
[ 1409.427480] general protection fault: 0000 [#1] PREEMPT SMP
[ 1409.428078] Modules linked in: hid_generic usbhid hid cfg80211 rfkill joydev input_leds led_class mousedev ppdev uhci_hcd evdev mac_hid ehci_pci ehci_hcd psmouse serio_raw atkbd usbcore libps2 pcspkr parport_pc parport i2c_piix4 i6300esb acpi_cpufreq usb_common intel_agp tpm_tis intel_gtt tpm_tis_core qemu_fw_cfg tpm button sch_fq_codel ip_tables x_tables ext4 crc16 jbd2 fscrypto mbcache ata_generic pata_acpi virtio_blk virtio_net ata_piix i8042 libata virtio_pci serio virtio_ring floppy virtio scsi_mod
[ 1409.429869] CPU: 2 PID: 25689 Comm: cat Not tainted 4.8.13-1-ARCH #1
[ 1409.429869] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.1-20161122_114906-anatol 04/01/2014
[ 1409.429869] task: ffff88007ba25b00 task.stack: ffff8800780d8000
[ 1409.429869] RIP: 0010:[<ffffffffa035713c>]  [<ffffffffa035713c>] hid_dump_field+0x1c/0x5e0 [hid]
[ 1409.429869] RSP: 0018:ffff8800780dbc98  EFLAGS: 00010292
[ 1409.429869] RAX: 0000000000000000 RBX: ffff8800374e6b40 RCX: 0000000000000002
[ 1409.429869] RDX: ffff8800374e6b40 RSI: 0000000000000006 RDI: 7273752f00000000
[ 1409.429869] RBP: ffff8800780dbcd8 R08: 0000000000002000 R09: 000000000000000a
[ 1409.429869] R10: ffff880037a65974 R11: ffff880037a6597f R12: ffff8800374e6b40
[ 1409.429869] R13: 7273752f00000000 R14: ffff8800780f7000 R15: ffffffffa036251d
[ 1409.429869] FS:  00007f9824c49480(0000) GS:ffff88007fd00000(0000) knlGS:0000000000000000
[ 1409.429869] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1409.429869] CR2: 000055e0a7b014f7 CR3: 000000007800e000 CR4: 00000000000006e0
[ 1409.429869] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1409.429869] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 1409.429869] Stack:
[ 1409.429869]  ffff8800780dbca8 0000000017a5ac78 000000067b9e1800 0000000000000001
[ 1409.429869]  ffff8800374e6b40 ffff8800780f7020 ffff8800780f7000 ffffffffa036251d
[ 1409.429869]  ffff8800780dbd28 ffffffffa0357804 0000000000000000 ffffffffa03626e4
[ 1409.429869] Call Trace:
[ 1409.429869]  [<ffffffffa0357804>] hid_dump_device+0x104/0x160 [hid]
[ 1409.429869]  [<ffffffffa03578dd>] hid_debug_rdesc_show+0x7d/0x1d0 [hid]
[ 1409.429869]  [<ffffffff8122d940>] ? seq_buf_alloc+0x20/0x40
[ 1409.429869]  [<ffffffff8122dd42>] seq_read+0x102/0x3c0
[ 1409.429869]  [<ffffffff81299460>] full_proxy_read+0x60/0xa0
[ 1409.429869]  [<ffffffff81208667>] __vfs_read+0x37/0x130
[ 1409.429869]  [<ffffffff812b5013>] ? security_file_permission+0xa3/0xc0
[ 1409.429869]  [<ffffffff81209416>] vfs_read+0x96/0x130
[ 1409.429869]  [<ffffffff8120a925>] SyS_read+0x55/0xc0
[ 1409.429869]  [<ffffffff81067747>] ? trace_do_page_fault+0x37/0xf0
[ 1409.429869]  [<ffffffff815f8032>] entry_SYSCALL_64_fastpath+0x1a/0xa4
[ 1409.429869] Code: 36 a0 48 63 f0 e8 85 27 fb e0 e9 0d fe ff ff 0f 1f 44 00 00 55 48 89 e5 41 5741 56 41 55 41 54 49 89 fd 53 48 89 d3 48 83 ec 18 <44> 8b 07 89 75 d4 45 85 c0 0f 85 69 05 00 00 41 8b 7d 04 85 ff
[ 1409.429869] RIP  [<ffffffffa035713c>] hid_dump_field+0x1c/0x5e0 [hid]
[ 1409.429869]  RSP <ffff8800780dbc98>
[ 1409.473184] ---[ end trace 451c7935d39fca87 ]---