The probe function is supposed to return NULL on failure (as we can see in
kobj_lookup: kobj = probe(dev, index, data); ... if (kobj) return kobj;

However, in loop and brd, it returns negative error from ERR_PTR.

This causes a crash if we simulate disk allocation failure and run
less -f /dev/loop0 because the negative number is interpreted as a pointer:

BUG: unable to handle kernel NULL pointer dereference at 00000000000002b4
IP: [<ffffffff8118b188>] __blkdev_get+0x28/0x450
PGD 23c677067 PUD 23d6d1067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: loop hpfs nvidia(PO) ip6table_filter ip6_tables uvesafb cfbcopyarea cfbimgblt cfbfillrect fbcon font bitblit fbcon_rotate fbcon_cw fbcon_ud fbcon_ccw softcursor fb fbdev msr ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_conntrack_ipv4 nf_defrag_ipv4 xt_state ipt_REJECT xt_tcpudp iptable_filter ip_tables x_tables bridge stp llc tun ipv6 cpufreq_stats cpufreq_ondemand cpufreq_userspace cpufreq_powersave cpufreq_conservative hid_generic spadfs usbhid hid fuse raid0 snd_usb_audio snd_pcm_oss snd_mixer_oss md_mod snd_pcm snd_timer snd_page_alloc snd_hwdep snd_usbmidi_lib dmi_sysfs snd_rawmidi nf_nat_ftp nf_nat nf_conntrack_ftp nf_conntrack snd soundcore lm85 hwmon_vid ohci_hcd ehci_pci ehci_hcd serverworks sata_svw libata acpi_cpufreq freq_table mperf ide_core usbcore kvm_amd kvm tg3 i2c_piix4 libphy microcode e100 usb_common ptp skge i2c_core pcspkr k10temp evdev floppy hwmon pps_core mii rtc_cmos button processor unix [last unloaded: nvidia]
CPU: 1 PID: 6831 Comm: less Tainted: P        W  O 3.10.15-devel #18
Hardware name: empty empty/S3992-E, BIOS 'V1.06   ' 06/09/2009
task: ffff880203cc6bc0 ti: ffff88023e47c000 task.ti: ffff88023e47c000
RIP: 0010:[<ffffffff8118b188>]  [<ffffffff8118b188>] __blkdev_get+0x28/0x450
RSP: 0018:ffff88023e47dbd8  EFLAGS: 00010286
RAX: ffffffffffffff74 RBX: ffffffffffffff74 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000001
RBP: ffff88023e47dc18 R08: 0000000000000002 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88023f519658
R13: ffffffff8118c300 R14: 0000000000000000 R15: ffff88023f519640
FS:  00007f2070bf7700(0000) GS:ffff880247400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000002b4 CR3: 000000023da1d000 CR4: 00000000000007e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Stack:
 0000000000000002 0000001d00000000 000000003e47dc50 ffff88023f519640
 ffff88043d5bb668 ffffffff8118c300 ffff88023d683550 ffff88023e47de60
 ffff88023e47dc98 ffffffff8118c10d 0000001d81605698 0000000000000292
Call Trace:
 [<ffffffff8118c300>] ? blkdev_get_by_dev+0x60/0x60
 [<ffffffff8118c10d>] blkdev_get+0x1dd/0x370
 [<ffffffff8118c300>] ? blkdev_get_by_dev+0x60/0x60
 [<ffffffff813cea6c>] ? _raw_spin_unlock+0x2c/0x50
 [<ffffffff8118c300>] ? blkdev_get_by_dev+0x60/0x60
 [<ffffffff8118c365>] blkdev_open+0x65/0x80
 [<ffffffff8114d12e>] do_dentry_open.isra.18+0x23e/0x2f0
 [<ffffffff8114d214>] finish_open+0x34/0x50
 [<ffffffff8115e122>] do_last.isra.62+0x2d2/0xc50
 [<ffffffff8115eb58>] path_openat.isra.63+0xb8/0x4d0
 [<ffffffff81115a8e>] ? might_fault+0x4e/0xa0
 [<ffffffff8115f4f0>] do_filp_open+0x40/0x90
 [<ffffffff813cea6c>] ? _raw_spin_unlock+0x2c/0x50
 [<ffffffff8116db85>] ? __alloc_fd+0xa5/0x1f0
 [<ffffffff8114e45f>] do_sys_open+0xef/0x1d0
 [<ffffffff8114e559>] SyS_open+0x19/0x20
 [<ffffffff813cff16>] system_call_fastpath+0x1a/0x1f
Code: 44 00 00 55 48 89 e5 41 57 49 89 ff 41 56 41 89 d6 41 55 41 54 4c 8d 67 18 53 48 83 ec 18 89 75 cc e9 f2 00 00 00 0f 1f 44 00 00 <48> 8b 80 40 03 00 00 48 89 df 4c 8b 68 58 e8 d5
a4 07 00 44 89
RIP  [<ffffffff8118b188>] __blkdev_get+0x28/0x450
 RSP <ffff88023e47dbd8>
CR2: 00000000000002b4
---[ end trace bb7f32dbf02398dc ]---

The brd change should be backported to stable kernels starting with 2.6.25.
The loop change should be backported to stable kernels starting with 2.6.22.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org	# 2.6.22+
Signed-off-by: Jens Axboe <axboe@kernel.dk>