linux-kernel - Re: [PATCH 9/9] ARM: add uprobes support

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Date:	Mon, 15 Oct 2012 18:31:47 +0100
From:	Dave Martin <dave.martin@...aro.org>
To:	Rabin Vincent <rabin@....in>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>, oleg@...hat.com
Subject: Re: [PATCH 9/9] ARM: add uprobes support

On Sun, Oct 14, 2012 at 09:23:13PM +0200, Rabin Vincent wrote:
> Add basic uprobes support for ARM.
> 
> perf probe --exec and SystemTap's userspace probing work.  The ARM
> kprobes test code has also been run in a userspace harness to test the
> uprobe instruction decoding.
> 
> Caveats:
> 
>  - Thumb is not supported
>  - XOL abort/trap handling is not implemented

[...]

> diff --git a/arch/arm/kernel/uprobes.c b/arch/arm/kernel/uprobes.c
> new file mode 100644
> index 0000000..f25a4af
> --- /dev/null
> +++ b/arch/arm/kernel/uprobes.c

[...]

> +bool is_swbp_insn(uprobe_opcode_t *insn)
> +{
> +	return (__mem_to_opcode_arm(*insn) & 0x0fffffff) == UPROBE_SWBP_INSN;

You should take care not to match any instruction whose top bits are
0xF0000000.  That could be some completely different instruction.

[...]

> +static int uprobe_trap_handler(struct pt_regs *regs, unsigned int instr)
> +{
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	if ((instr & 0x0fffffff) == UPROBE_SWBP_INSN)

Is the check unnecessary here?  I think the same comparison will
happen as a result of evaluating the associated undef_hook.

However, as above you must still check for and reject cases where
(instr & 0xF0000000) == 0xF0000000.

[...]

> +static struct undef_hook uprobes_arm_break_hook = {
> +	.instr_mask	= 0x0fffffff,
> +	.instr_val	= UPROBE_SWBP_INSN,
> +	.cpsr_mask	= MODE_MASK,
> +	.cpsr_val	= USR_MODE,
> +	.fn		= uprobe_trap_handler,
> +};
> +
> +static struct undef_hook uprobes_arm_ss_hook = {
> +	.instr_mask	= 0x0fffffff,
> +	.instr_val	= UPROBE_SS_INSN,
> +	.cpsr_mask	= MODE_MASK,
> +	.cpsr_val	= USR_MODE,
> +	.fn		= uprobe_trap_handler,
> +};
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