Re: [PATCH v4 15/16] ARM: add uprobes support

From: David Long
Date: Tue Jan 21 2014 - 11:51:36 EST

On 12/20/13 13:34, Jon Medhurst (Tixy) wrote:
On Sun, 2013-12-15 at 23:08 -0500, David Long wrote:
From: "David A. Long" <dave.long@xxxxxxxxxx>

Using Rabin Vincent's ARM uprobes patches as a base, enable uprobes
support on ARM.


- Thumb is not supported
- XOL abort/trap handling is not implemented

I shall repeat my comment from version one of the patch...

What are the consequences of this, e.g. is it possible for a probe to
get stuck in an infinite loop of faulting? I hope there are no integrity
issues for the kernel itself.

Would be good if someone familiar with uprobes working could answer

Testing shows it does indeed get stuck continuously trapping. The process is killable. Fortunately all the infrastructure is already there for fixing this. I've patched the code to detect the trap and allow it to be processed, using the powerpc uprobes code as a model. The changes required are fairly small and entirely in the architecture-specific code.

As mentioned before, thumb support is a follow-on project.

I've a few other comments...


+const union decode_item uprobes_probes_actions[] = {
+ [PROBES_EMULATE_NONE] {.handler = probes_simulate_nop},

There is a missing '=' in the line above. Interesting that GCC doesn't
complain (I tried compiling this patch and it didn't).

That is indeed odd. I have fixed it (my code, not the compiler).


+bool arch_uprobe_skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
+ void *addr;

'addr' is not used so this line can be deleted

+ probes_opcode_t opcode;
+ if (!auprobe->simulate)
+ return false;
+ addr = (void *) regs->ARM_pc;

and so can this line ^^^


+ opcode = __mem_to_opcode_arm(*(unsigned int *) auprobe->insn);
+ auprobe->asi.insn_singlestep(opcode, &auprobe->asi, regs);
+ return true;


[rest of patch snipped]


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