rcu: dump vmalloc memory info safely

In the Linux kernel, the following vulnerability has been resolved:

rcu: dump vmalloc memory info safely

Currently, for double invoke call_rcu(), will dump rcu_head objects memory

info, if the objects is not allocated from the slab allocator, the

vmalloc_dump_obj() will be invoke and the vmap_area_lock spinlock need to

be held, since the call_rcu() can be invoked in interrupt context,

therefore, there is a possibility of spinlock deadlock scenarios.

And in Preempt-RT kernel, the rcutorture test also trigger the following

lockdep warning:

BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48

in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 1, name: swapper/0

preempt_count: 1, expected: 0

RCU nest depth: 1, expected: 1

3 locks held by swapper/0/1:

#0: ffffffffb534ee80 (fullstop_mutex){+.+.}-{4:4}, at: torture_init_begin+0x24/0xa0

#1: ffffffffb5307940 (rcu_read_lock){....}-{1:3}, at: rcu_torture_init+0x1ec7/0x2370

#2: ffffffffb536af40 (vmap_area_lock){+.+.}-{3:3}, at: find_vmap_area+0x1f/0x70

irq event stamp: 565512

hardirqs last enabled at (565511): [<ffffffffb379b138>] __call_rcu_common+0x218/0x940

hardirqs last disabled at (565512): [<ffffffffb5804262>] rcu_torture_init+0x20b2/0x2370

softirqs last enabled at (399112): [<ffffffffb36b2586>] __local_bh_enable_ip+0x126/0x170

softirqs last disabled at (399106): [<ffffffffb43fef59>] inet_register_protosw+0x9/0x1d0

Preemption disabled at:

[<ffffffffb58040c3>] rcu_torture_init+0x1f13/0x2370

CPU: 0 PID: 1 Comm: swapper/0 Tainted: G W 6.5.0-rc4-rt2-yocto-preempt-rt+ #15

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014

Call Trace:

<TASK>

dump_stack_lvl+0x68/0xb0

dump_stack+0x14/0x20

__might_resched+0x1aa/0x280

? __pfx_rcu_torture_err_cb+0x10/0x10

rt_spin_lock+0x53/0x130

? find_vmap_area+0x1f/0x70

find_vmap_area+0x1f/0x70

vmalloc_dump_obj+0x20/0x60

mem_dump_obj+0x22/0x90

__call_rcu_common+0x5bf/0x940

? debug_smp_processor_id+0x1b/0x30

call_rcu_hurry+0x14/0x20

rcu_torture_init+0x1f82/0x2370

? __pfx_rcu_torture_leak_cb+0x10/0x10

? __pfx_rcu_torture_leak_cb+0x10/0x10

? __pfx_rcu_torture_init+0x10/0x10

do_one_initcall+0x6c/0x300

? debug_smp_processor_id+0x1b/0x30

kernel_init_freeable+0x2b9/0x540

? __pfx_kernel_init+0x10/0x10

kernel_init+0x1f/0x150

ret_from_fork+0x40/0x50

? __pfx_kernel_init+0x10/0x10

ret_from_fork_asm+0x1b/0x30

</TASK>

The previous patch fixes this by using the deadlock-safe best-effort

version of find_vm_area. However, in case of failure print the fact that

the pointer was a vmalloc pointer so that we print at least something.