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

[David Long]

On 09/07/2016 01:52 AM, Masami Hiramatsu wrote:
> 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.

CONFIG KPROBES does currently select CONFIG_KALLSYMS, but is it wise for 
this code to depend on that?  Perhaps the text boundary checking should 
be moved under an else clause for the case of 
kallsyms_lookup_size_offset() failing?

> 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

Thanks,
-dl