do page fault in atomic bug on arm

[Russell King - ARM Linux]

On Fri, Nov 24, 2017 at 07:27:00PM +0000, Russell King - ARM Linux wrote:
> Adding Tixy, as he's more knowledgable in this area - I suggest
> someone knowledgable in this area runs
> 
> 	ftracetest test.d/kprobe/multiple_kprobes.tc
> 
> and fixes these bugs... also running the entire ftracetest suite
> would probably also be a very good idea.

There's some other stupidities as well:

trace_kprobe: Inserting kprobe at __error_lpae+0
trace_kprobe: Inserting kprobe at str_lpae+0
trace_kprobe: Inserting kprobe at __error_p+0
trace_kprobe: Inserting kprobe at str_p1+0
trace_kprobe: Inserting kprobe at str_p2+0
trace_kprobe: Could not insert probe at str_p2+0: -22

str_lpae: .asciz "\nError: Kernel with LPAE support, but CPU does not support LPAE.\n"
str_p1: .asciz  "\nError: unrecognized/unsupported processor variant (0x"
str_p2: .asciz  ").\n"

So we successfully placed a kprobe on some ASCII strings, which are
used prior to the kernel being properly up and running, which means
we have to use relative references to these strings, and relative
references to strings in other sections is not simple.

More worrying:

trace_kprobe: Inserting kprobe at __turn_mmu_on+0
trace_kprobe: Inserting kprobe at __idmap_text_start+0
trace_kprobe: Inserting kprobe at __turn_mmu_on_end+0
...
trace_kprobe: Inserting kprobe at __idmap_text_end+0
...
trace_kprobe: Inserting kprobe at secondary_startup+0
trace_kprobe: Inserting kprobe at secondary_startup_arm+0
trace_kprobe: Inserting kprobe at __secondary_switched+0
trace_kprobe: Inserting kprobe at __secondary_data+0
trace_kprobe: Inserting kprobe at __enable_mmu+0
trace_kprobe: Inserting kprobe at __do_fixup_smp_on_up+0
trace_kprobe: Inserting kprobe at __fixup_a_pv_table+0
trace_kprobe: Inserting kprobe at fixup_pv_table+0
trace_kprobe: Inserting kprobe at __lookup_processor_type+0
trace_kprobe: Inserting kprobe at __lookup_processor_type_data+0

These are definitely a no-no for tracing, because they're part of the
early startup code for the kernel when no stacks are available.

Some of these can't live in the special "do not use kprobes here"
section as they need to be in other sections (like .idmap) because
they need to be part of the identity-mapped code.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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