[PATCH -tip v8 0/9] kprobes: Kprobes jump optimization support

From: Masami Hiramatsu
Date: Fri Jan 22 2010 - 13:47:57 EST


Here are the patchset of the kprobes jump optimization v8
(a.k.a. Djprobe). This version is just moving onto
2.6.33-rc4-tip. Ingo, I assume its a good timing to
push this code onto -tip tree (maybe developing branch?),
since people can test it with perf-probe.

I've decided to make a separated series of patches of
jump optimization with text_poke_smp() which is
'officially' supported on Intel's processors.
So, this version of patches are just updated against
the latest tip/master, no other updates are included.

I know that int3-bypassing method (text_poke_fixup())
is currently unofficially believed as safe. But we
need to get more official answers from x86 vendors.
Moreover, we need to tweak entry_*.S for preventing
recursive NMI, because int3 inside NMI handler will
unblock NMI blocking. I'd like to push it after this
series of patches are merged.

Anyway, thanks Mathieu and Peter, for helping me to
implement it and organizing discussion points about
int3-bypass XMC!

These patches can be applied on the latest -tip.

Changes in v8:
- Update patches against the latest tip/master.
- Drop text_poke_fixup() related patches.
- Update benchmark results and add jprobes and kprobe(post-handler)

And kprobe stress test didn't found any regressions - from kprobes,
under kvm/x86.

- Support NMI-safe int3-bypassing text_poke.
- Support preemptive kernel (by stack unwinding and checking address).

Jump Optimized Kprobes
o Concept
Kprobes uses the int3 breakpoint instruction on x86 for instrumenting
probes into running kernel. Jump optimization allows kprobes to replace
breakpoint with a jump instruction for reducing probing overhead drastically.

o Performance
An optimized kprobe 5 times faster than a kprobe.

Optimizing probes gains its performance. Usually, a kprobe hit takes
0.5 to 1.0 microseconds to process. On the other hand, a jump optimized
probe hit takes less than 0.1 microseconds (actual number depends on the
processor). Here is a sample overheads.

Intel(R) Xeon(R) CPU E5410 @ 2.33GHz
(without debugging options, with text_poke_smp patch, 2.6.33-rc4-tip+)

x86-32 x86-64
kprobe: 0.80us 0.99us
kprobe+booster: 0.33us 0.43us
kprobe+optimized: 0.05us 0.06us
kprobe(post-handler): 0.81us 1.00us

kretprobe : 1.10us 1.24us
kretprobe+booster: 0.61us 0.68us
kretprobe+optimized: 0.33us 0.30us

jprobe: 1.37us 1.67us
jprobe+booster: 0.80us 1.10us

(booster skips single-stepping, kprobe with post handler
isn't boosted/optimized, and jprobe isn't optimized.)

Note that jump optimization also consumes more memory, but not so much.
It just uses ~200 bytes, so, even if you use ~10,000 probes, it just
consumes a few MB.

o Usage
Set CONFIG_OPTPROBES=y when building a kernel, then all *probes will be
optimized if possible.

Kprobes decodes probed function and checks whether the target instructions
can be optimized(replaced with a jump) safely. If it can't be, Kprobes just
doesn't optimize it.

o Optimization
Before preparing optimization, Kprobes inserts original(user-defined)
kprobe on the specified address. So, even if the kprobe is not
possible to be optimized, it just uses a normal kprobe.

- Safety check
First, Kprobes gets the address of probed function and checks whether the
optimized region, which will be replaced by a jump instruction, does NOT
straddle the function boundary, because if the optimized region reaches the
next function, its caller causes unexpected results.
Next, Kprobes decodes whole body of probed function and checks there is
NO indirect jump, NO instruction which will cause exception by checking
exception_tables (this will jump to fixup code and fixup code jumps into
same function body) and NO near jump which jumps into the optimized region
(except the 1st byte of jump), because if some jump instruction jumps
into the middle of another instruction, it causes unexpected results too.
Kprobes also measures the length of instructions which will be replaced
by a jump instruction, because a jump instruction is longer than 1 byte,
it may replaces multiple instructions, and it checks whether those
instructions can be executed out-of-line.

- Preparing detour code
Then, Kprobes prepares "detour" buffer, which contains exception emulating
code (push/pop registers, call handler), copied instructions(Kprobes copies
instructions which will be replaced by a jump, to the detour buffer), and
a jump which jumps back to the original execution path.

- Pre-optimization
After preparing detour code, Kprobes enqueues the kprobe to optimizing list
and kicks kprobe-optimizer workqueue to optimize it. To wait other optimized
probes, kprobe-optimizer will delay to work.
When the optimized-kprobe is hit before optimization, its handler
changes IP(instruction pointer) to copied code and exits. So, those
copied instructions are executed on the detour buffer.

- Optimization
Kprobe-optimizer doesn't start instruction-replacing soon, it waits
synchronize_sched for safety, because some processors are possible to be
interrupted on the middle of instruction series (2nd or Nth instruction)
which will be replaced by a jump instruction(*).
As you know, synchronize_sched() can ensure that all interruptions which
were executed when synchronize_sched() was called are done, only if
CONFIG_PREEMPT=n. So, this version supports only the kernel with
After that, kprobe-optimizer calls stop_machine() to replace probed-
instructions with a jump instruction by using text_poke_smp().

- Unoptimization
When unregistering, disabling kprobe or being blocked by other kprobe,
an optimized-kprobe will be unoptimized. Before kprobe-optimizer runs,
the kprobe just be dequeued from the optimized list. When the optimization
has been done, it replaces a jump with int3 breakpoint and original code
by using text_poke_smp().

(*)Please imagine that 2nd instruction is interrupted and
optimizer replaces the 2nd instruction with jump *address*
while the interrupt handler is running. When the interrupt
returns to original address, there is no valid instructions
and it causes unexpected result.

(**)This optimization-safety checking may be replaced with stop-machine
method which ksplice is done for supporting CONFIG_PREEMPT=y kernel.

Thank you,


Masami Hiramatsu (9):
kprobes: Add documents of jump optimization
kprobes/x86: Support kprobes jump optimization on x86
x86: Add text_poke_smp for SMP cross modifying code
kprobes/x86: Cleanup save/restore registers
kprobes/x86: Boost probes when reentering
kprobes: Jump optimization sysctl interface
kprobes: Introduce kprobes jump optimization
kprobes: Introduce generic insn_slot framework

Documentation/kprobes.txt | 191 ++++++++++-
arch/Kconfig | 13 +
arch/x86/Kconfig | 1
arch/x86/include/asm/alternative.h | 4
arch/x86/include/asm/kprobes.h | 31 ++
arch/x86/kernel/alternative.c | 60 +++
arch/x86/kernel/kprobes.c | 596 ++++++++++++++++++++++++++++------
include/linux/kprobes.h | 44 +++
kernel/kprobes.c | 626 +++++++++++++++++++++++++++++++-----
kernel/sysctl.c | 12 +
10 files changed, 1373 insertions(+), 205 deletions(-)

Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx
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