On Wed, 7 Sep 2016 10:12:14 -0400
David Long <dave.l...@linaro.org> wrote:
> On 09/07/2016 01:52 AM, Masami Hiramatsu wrote:
> > On Tue, 6 Sep 2016 13:54:59 -0400
> > David Long <dave.l...@linaro.org> wrote:
> >
> >> From: "David A. Long" <dave.l...@linaro.org>
> >>
> >> 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?
Would you have any case where the address is in kernel_text but kallsyms_lookup
failed to find symbol? IMHO, even if there is, we should reject probing on
such "nowhere" address.
Thank you,
>
> > Thank you,
> >>
> >> Signed-off-by: David A. Long <dave.l...@linaro.org>
> >> ---
> >> 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
>
>
--
Masami Hiramatsu <mhira...@kernel.org>
