Re: [PATCH v2] arm64: Improve kprobes test for atomic sequence

[Masami Hiramatsu]

On Tue,  6 Sep 2016 13:54:59 -0400
David Long <dave.long@xxxxxxxxxx> wrote:

> From: "David A. Long" <dave.long@xxxxxxxxxx>
> 
> Kprobes searches backwards a finite number of instructions to determine if
> there is an attempt to probe a load/store exclusive sequence. It stops when
> it hits the maximum number of instructions or a load or store exclusive.
> However this means it can run up past the beginning of the function and
> start looking at literal constants. This has been shown to cause a false
> positive and blocks insertion of the probe. To fix this, further limit the
> backwards search to stop if it hits a symbol address from kallsyms. The
> presumption is that this is the entry point to this code (particularly for
> the common case of placing probes at the beginning of functions).
> 
> This also improves efficiency by not searching code that is not part of the
> function. There may be some possibility that the label might not denote the
> entry path to the probed instruction but the likelihood seems low and this
> is just another example of how the kprobes user really needs to be
> careful about what they are doing.

Hi Dave,

kallsyms_lookup_size_offset() part looks good to me. By the way,
is there any reason we'll check the _text and module's base address
boundary? 
I think those are already searced by kallsyms_lookup_size_offset(),
so you don't need to check those. If the address is not found by
kallsyms_lookup_size_offset(), that address maybe out-of-text.

Thank you,
> 
> Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/probes/decode-insn.c | 48 +++++++++++++++++++++++-----------
>  1 file changed, 33 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
> index 37e47a9..356ee52 100644
> --- a/arch/arm64/kernel/probes/decode-insn.c
> +++ b/arch/arm64/kernel/probes/decode-insn.c
> @@ -16,6 +16,7 @@
>  #include <linux/kernel.h>
>  #include <linux/kprobes.h>
>  #include <linux/module.h>
> +#include <linux/kallsyms.h>
>  #include <asm/kprobes.h>
>  #include <asm/insn.h>
>  #include <asm/sections.h>
> @@ -122,7 +123,7 @@ arm_probe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi)
>  static bool __kprobes
>  is_probed_address_atomic(kprobe_opcode_t *scan_start, kprobe_opcode_t *scan_end)
>  {
> -	while (scan_start > scan_end) {
> +	while (scan_start >= scan_end) {
>  		/*
>  		 * atomic region starts from exclusive load and ends with
>  		 * exclusive store.
> @@ -144,26 +145,43 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
>  	kprobe_opcode_t insn = le32_to_cpu(*addr);
>  	kprobe_opcode_t *scan_start = addr - 1;
>  	kprobe_opcode_t *scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;
> +	unsigned long size = 0, offset = 0;
>  #if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
>  	struct module *mod;
>  #endif
>  
> -	if (addr >= (kprobe_opcode_t *)_text &&
> -	    scan_end < (kprobe_opcode_t *)_text)
> -		scan_end = (kprobe_opcode_t *)_text;
> +	/*
> +	 * If there's a symbol defined in front of and near enough to
> +	 * the probe address assume it is the entry point to this
> +	 * code and use it to further limit how far back we search
> +	 * when determining if we're in an atomic sequence.
> +	 */
> +	if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset))
> +		if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t)))
> +			scan_end = addr - (offset / sizeof(kprobe_opcode_t));
> +
> +	if (scan_end <= scan_start) {
> +		if (addr >= (kprobe_opcode_t *)_text &&
> +		    scan_end < (kprobe_opcode_t *)_text)
> +			scan_end = (kprobe_opcode_t *)_text;
>  #if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
> -	else {
> -		preempt_disable();
> -		mod = __module_address((unsigned long)addr);
> -		if (mod && within_module_init((unsigned long)addr, mod) &&
> -			!within_module_init((unsigned long)scan_end, mod))
> -			scan_end = (kprobe_opcode_t *)mod->init_layout.base;
> -		else if (mod && within_module_core((unsigned long)addr, mod) &&
> -			!within_module_core((unsigned long)scan_end, mod))
> -			scan_end = (kprobe_opcode_t *)mod->core_layout.base;
> -		preempt_enable();
> -	}
> +		else {
> +			preempt_disable();
> +			mod = __module_address((unsigned long)addr);
> +			if (mod &&
> +			    within_module_init((unsigned long)addr, mod) &&
> +			    !within_module_init((unsigned long)scan_end, mod))
> +				scan_end =
> +				    (kprobe_opcode_t *)mod->init_layout.base;
> +			else if (mod &&
> +			    within_module_core((unsigned long)addr, mod) &&
> +			    !within_module_core((unsigned long)scan_end, mod))
> +				scan_end =
> +				    (kprobe_opcode_t *)mod->core_layout.base;
> +			preempt_enable();
> +		}
>  #endif
> +	}
>  	decoded = arm_probe_decode_insn(insn, asi);
>  
>  	if (decoded == INSN_REJECTED ||
> -- 
> 2.5.0
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>