Subject: [PATCH 5/5] arm64: Add uprobe support

From: [email-masked] (Pratyush Anand)
Date: Wed, 21 Sep 2016 16:30:47 +0530

Hi Catalin,

Thanks for your review.

On 20/09/2016:05:59:46 PM, Catalin Marinas wrote:
> Hi Pratyush,
> 
> On Tue, Aug 02, 2016 at 11:00:09AM +0530, Pratyush Anand wrote:
> > --- a/arch/arm64/include/asm/debug-monitors.h
> > +++ b/arch/arm64/include/asm/debug-monitors.h
> > @@ -70,6 +70,9 @@
> >  #define BRK64_ESR_MASK		0xFFFF
> >  #define BRK64_ESR_KPROBES	0x0004
> >  #define BRK64_OPCODE_KPROBES	(AARCH64_BREAK_MON | (BRK64_ESR_KPROBES << 5))
> > +/* uprobes BRK opcodes with ESR encoding  */
> > +#define BRK64_ESR_UPROBES	0x0008
> > +#define BRK64_OPCODE_UPROBES	(AARCH64_BREAK_MON | (BRK64_ESR_UPROBES << 5))
> 
> You can use 0x0005 as immediate, we don't need individual bits here.

OK. will define BRK64_ESR_UPROBES as 0x0005

> 
> > --- a/arch/arm64/include/asm/probes.h
> > +++ b/arch/arm64/include/asm/probes.h
> > @@ -35,4 +35,8 @@ struct arch_specific_insn {
> >  };
> >  #endif
> >  
> > +#ifdef CONFIG_UPROBES
> > +typedef u32 uprobe_opcode_t;
> > +#endif
> 
> We don't need #ifdef around this typedef. Also, all the other
> architectures seem to have this typedef in asm/uprobes.h.

kprobe_opcode_t was defined here, so I defined it here as well. But yes, it
would be good to move it to asm/uprobes.h to keep in sync with other arch.

> 
> > --- a/arch/arm64/include/asm/ptrace.h
> > +++ b/arch/arm64/include/asm/ptrace.h
> > @@ -217,6 +217,14 @@ int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task);
> >  
> >  #include <asm-generic/ptrace.h>
> >  
> > +#define procedure_link_pointer(regs)	((regs)->regs[30])
> > +
> > +static inline void procedure_link_pointer_set(struct pt_regs *regs,
> > +					   unsigned long val)
> > +{
> > +	procedure_link_pointer(regs) = val;
> > +}
> 
> Do you need these macro and function here? It seems that they are only
> used in arch_uretprobe_hijack_return_addr(), so you can just expand them
> in there, no need for additional definitions.

I introduced it to keep sync with other register usage like
instruction_pointer_set().
I have used it in uprobe code only, but I think, we can have a patch to modify
following code, which can use them as well.

arch/arm64/kernel/probes/kprobes.c:     ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
arch/arm64/kernel/probes/kprobes.c:     regs->regs[30] = (long)&kretprobe_trampoline;
arch/arm64/kernel/process.c:            lr = regs->regs[30];
arch/arm64/kernel/signal.c:     __put_user_error(regs->regs[30], &sf->lr, err);
arch/arm64/kernel/signal.c:     regs->regs[30] = (unsigned long)sigtramp;

> 
> > --- a/arch/arm64/include/asm/thread_info.h
> > +++ b/arch/arm64/include/asm/thread_info.h
> > @@ -109,6 +109,7 @@ static inline struct thread_info *current_thread_info(void)
> >  #define TIF_NEED_RESCHED	1
> >  #define TIF_NOTIFY_RESUME	2	/* callback before returning to user */
> >  #define TIF_FOREIGN_FPSTATE	3	/* CPU's FP state is not current's */
> > +#define TIF_UPROBE		5	/* uprobe breakpoint or singlestep */
> 
> Nitpick: you can just use 4 until we cover this gap.

Hummm..as stated in commit log, Shi Yang suggested to define TIF_UPROBE as 5 in
stead of 4, since 4 has already been used in -rt kernel. May be, I can put a
comment in code as well.
Or, keep it 4 and -rt kernel will change their definitions. I am OK with both.
let me know.

> 
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/uprobes.h
> > @@ -0,0 +1,37 @@
> > +/*
> > + * Copyright (C) 2014-2015 Pratyush Anand <[email-masked]>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +
> > +#ifndef _ASM_UPROBES_H
> > +#define _ASM_UPROBES_H
> > +
> > +#include <asm/debug-monitors.h>
> > +#include <asm/insn.h>
> > +#include <asm/probes.h>
> > +
> > +#define MAX_UINSN_BYTES		AARCH64_INSN_SIZE
> > +
> > +#define UPROBE_SWBP_INSN	BRK64_OPCODE_UPROBES
> > +#define UPROBE_SWBP_INSN_SIZE	4
> 
> Nitpick: for consistency, just use AARCH64_INSN_SIZE.

OK

> 
> > +#define UPROBE_XOL_SLOT_BYTES	MAX_UINSN_BYTES
> > +
> > +struct arch_uprobe_task {
> > +	unsigned long saved_fault_code;
> > +};
> > +
> > +struct arch_uprobe {
> > +	union {
> > +		u8 insn[MAX_UINSN_BYTES];
> > +		u8 ixol[MAX_UINSN_BYTES];
> > +	};
> > +	struct arch_probe_insn api;
> > +	bool simulate;
> > +};
> > +
> > +extern void flush_uprobe_xol_access(struct page *page, unsigned long uaddr,
> > +		void *kaddr, unsigned long len);
> 
> I don't think we need this. It doesn't seem to be used anywhere other
> than the arm64 uprobes.c file. So I would rather expose
> sync_icache_aliases(), similarly to __sync_icache_dcache().

OK.

> 
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -688,7 +688,8 @@ ret_fast_syscall:
> >  	ldr	x1, [tsk, #TI_FLAGS]		// re-check for syscall tracing
> >  	and	x2, x1, #_TIF_SYSCALL_WORK
> >  	cbnz	x2, ret_fast_syscall_trace
> > -	and	x2, x1, #_TIF_WORK_MASK
> > +	mov     x2, #_TIF_WORK_MASK
> > +	and     x2, x1, x2
> 
> Is this needed because _TIF_WORK_MASK cannot work as an immediate value
> to 'and'? We could reorder the TIF bits, they are not exposed to user to
> have ABI implications.

_TIF_WORK_MASK is defined as follows:

#define _TIF_WORK_MASK          (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
                                  _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE | \
                                  _TIF_UPROBE)
Re-ordering will not help, because 0-3 have already been used by previous
definitions. Only way to use immediate value could be if, TIF_UPROBE is defined
as 4. 

> 
> >  	cbnz	x2, work_pending
> >  	enable_step_tsk x1, x2
> >  	kernel_exit 0
> > @@ -718,7 +719,8 @@ work_resched:
> >  ret_to_user:
> >  	disable_irq				// disable interrupts
> >  	ldr	x1, [tsk, #TI_FLAGS]
> > -	and	x2, x1, #_TIF_WORK_MASK
> > +	mov     x2, #_TIF_WORK_MASK
> > +	and     x2, x1, x2
> >  	cbnz	x2, work_pending
> >  	enable_step_tsk x1, x2
> >  	kernel_exit 0
> 
> Same here.
> 
> > --- /dev/null
> > +++ b/arch/arm64/kernel/probes/uprobes.c
> > @@ -0,0 +1,227 @@
> > +/*
> > + * Copyright (C) 2014-2015 Pratyush Anand <[email-masked]>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +#include <linux/highmem.h>
> > +#include <linux/ptrace.h>
> > +#include <linux/uprobes.h>
> > +
> > +#include "decode-insn.h"
> > +
> > +#define UPROBE_INV_FAULT_CODE	UINT_MAX
> > +
> > +void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
> > +		void *src, unsigned long len)
> > +{
> > +	void *xol_page_kaddr = kmap_atomic(page);
> > +	void *dst = xol_page_kaddr + (vaddr & ~PAGE_MASK);
> > +
> > +	preempt_disable();
> 
> kmap_atomic() already disabled preemption.

Yes..will remove.

> 
> > +
> > +	/* Initialize the slot */
> > +	memcpy(dst, src, len);
> > +
> > +	/* flush caches (dcache/icache) */
> > +	flush_uprobe_xol_access(page, vaddr, dst, len);
> 
> Just use sync_icache_aliases() here (once exposed in an arm64 header).

OK.

> 
> > +
> > +	preempt_enable();
> > +
> > +	kunmap_atomic(xol_page_kaddr);
> > +}
> > +
> > +unsigned long uprobe_get_swbp_addr(struct pt_regs *regs)
> > +{
> > +	return instruction_pointer(regs);
> > +}
> > +
> > +int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm,
> > +		unsigned long addr)
> > +{
> > +	probe_opcode_t insn;
> > +
> > +	/* TODO: Currently we do not support AARCH32 instruction probing */
> 
> Is there a way to check (not necessarily in this file) that we don't
> probe 32-bit tasks?

- Well, I do not have complete idea about it that, how it can be done. I think
  we can not check that just by looking a single bit in an instruction.
  My understanding is that, we can only know about it when we are executing the
  instruction, by reading pstate, but that would not be useful for uprobe
  instruction analysis.
  
  I hope, instruction encoding for aarch32 and aarch64 are different, and by
  analyzing for all types of aarch32 instructions, we will be able to decide
  that whether instruction is 32 bit trace-able or not.  Accordingly, we can use
  either BRK or BKPT instruction for breakpoint generation.

> 
> > +	if (!IS_ALIGNED(addr, AARCH64_INSN_SIZE))
> > +		return -EINVAL;
> > +
> > +	insn = *(probe_opcode_t *)(&auprobe->insn[0]);
> > +
> > +	switch (arm_probe_decode_insn(insn, &auprobe->api)) {
> > +	case INSN_REJECTED:
> > +		return -EINVAL;
> > +
> > +	case INSN_GOOD_NO_SLOT:
> > +		auprobe->simulate = true;
> > +		break;
> > +
> > +	case INSN_GOOD:
> > +	default:
> > +		break;
> 
> Nitpick, we don't need case INSN_GOOD as well since default would cover
> it (that's unless you want to change default to BUG()).

we will not have other than these 3, so need to introduce BUG(). I will remove
INSN_GOOD.

> 
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +int arch_uprobe_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> > +{
> > +	struct uprobe_task *utask = current->utask;
> > +
> > +	/* saved fault code is restored in post_xol */
> > +	utask->autask.saved_fault_code = current->thread.fault_code;
> 
> Does the current->thread.fault_code has any meaning here? We use it in
> the arm64 code when delivering a signal to user but I don't think that's
> the case here, we are handling a breakpoint instruction and there isn't
> anything that set the fault_code.

Correct, they will just having garbage values. will remove.

> 
> > +
> > +	/* An invalid fault code between pre/post xol event */
> > +	current->thread.fault_code = UPROBE_INV_FAULT_CODE;
> > +
> > +	/* Instruction point to execute ol */
> > +	instruction_pointer_set(regs, utask->xol_vaddr);
> > +
> > +	user_enable_single_step(current);
> > +
> > +	return 0;
> > +}
> > +
> > +int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> > +{
> > +	struct uprobe_task *utask = current->utask;
> > +
> > +	WARN_ON_ONCE(current->thread.fault_code != UPROBE_INV_FAULT_CODE);
> > +
> > +	/* restore fault code */
> > +	current->thread.fault_code = utask->autask.saved_fault_code;
> 
> Same here, I don't think this needs restoring if it wasn't meaningful in
> the first place.

OK.

~Pratyush