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| -rw-r--r-- | Documentation/kprobes.txt | 81 |
1 files changed, 42 insertions, 39 deletions
diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt index 0ea5a0c6e827..2c3b1eae4280 100644 --- a/Documentation/kprobes.txt +++ b/Documentation/kprobes.txt @@ -136,17 +136,20 @@ Kprobes, jprobes, and return probes are implemented on the following architectures: - i386 -- x86_64 (AMD-64, E64MT) +- x86_64 (AMD-64, EM64T) - ppc64 -- ia64 (Support for probes on certain instruction types is still in progress.) +- ia64 (Does not support probes on instruction slot1.) - sparc64 (Return probes not yet implemented.) 3. Configuring Kprobes When configuring the kernel using make menuconfig/xconfig/oldconfig, -ensure that CONFIG_KPROBES is set to "y". Under "Kernel hacking", -look for "Kprobes". You may have to enable "Kernel debugging" -(CONFIG_DEBUG_KERNEL) before you can enable Kprobes. +ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation +Support", look for "Kprobes". + +So that you can load and unload Kprobes-based instrumentation modules, +make sure "Loadable module support" (CONFIG_MODULES) and "Module +unloading" (CONFIG_MODULE_UNLOAD) are set to "y". You may also want to ensure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL are set to "y", since kallsyms_lookup_name() @@ -262,18 +265,18 @@ at any time after the probe has been registered. 5. Kprobes Features and Limitations -As of Linux v2.6.12, Kprobes allows multiple probes at the same -address. Currently, however, there cannot be multiple jprobes on -the same function at the same time. +Kprobes allows multiple probes at the same address. Currently, +however, there cannot be multiple jprobes on the same function at +the same time. In general, you can install a probe anywhere in the kernel. In particular, you can probe interrupt handlers. Known exceptions are discussed in this section. -For obvious reasons, it's a bad idea to install a probe in -the code that implements Kprobes (mostly kernel/kprobes.c and -arch/*/kernel/kprobes.c). A patch in the v2.6.13 timeframe instructs -Kprobes to reject such requests. +The register_*probe functions will return -EINVAL if you attempt +to install a probe in the code that implements Kprobes (mostly +kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such +as do_page_fault and notifier_call_chain). If you install a probe in an inline-able function, Kprobes makes no attempt to chase down all inline instances of the function and @@ -290,18 +293,14 @@ from the accidental ones. Don't drink and probe. Kprobes makes no attempt to prevent probe handlers from stepping on each other -- e.g., probing printk() and then calling printk() from a -probe handler. As of Linux v2.6.12, if a probe handler hits a probe, -that second probe's handlers won't be run in that instance. - -In Linux v2.6.12 and previous versions, Kprobes' data structures are -protected by a single lock that is held during probe registration and -unregistration and while handlers are run. Thus, no two handlers -can run simultaneously. To improve scalability on SMP systems, -this restriction will probably be removed soon, in which case -multiple handlers (or multiple instances of the same handler) may -run concurrently on different CPUs. Code your handlers accordingly. - -Kprobes does not use semaphores or allocate memory except during +probe handler. If a probe handler hits a probe, that second probe's +handlers won't be run in that instance, and the kprobe.nmissed member +of the second probe will be incremented. + +As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of +the same handler) may run concurrently on different CPUs. + +Kprobes does not use mutexes or allocate memory except during registration and unregistration. Probe handlers are run with preemption disabled. Depending on the @@ -316,11 +315,18 @@ address instead of the real return address for kretprobed functions. (As far as we can tell, __builtin_return_address() is used only for instrumentation and error reporting.) -If the number of times a function is called does not match the -number of times it returns, registering a return probe on that -function may produce undesirable results. We have the do_exit() -and do_execve() cases covered. do_fork() is not an issue. We're -unaware of other specific cases where this could be a problem. +If the number of times a function is called does not match the number +of times it returns, registering a return probe on that function may +produce undesirable results. We have the do_exit() case covered. +do_execve() and do_fork() are not an issue. We're unaware of other +specific cases where this could be a problem. + +If, upon entry to or exit from a function, the CPU is running on +a stack other than that of the current task, registering a return +probe on that function may produce undesirable results. For this +reason, Kprobes doesn't support return probes (or kprobes or jprobes) +on the x86_64 version of __switch_to(); the registration functions +return -EINVAL. 6. Probe Overhead @@ -347,14 +353,12 @@ k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 7. TODO -a. SystemTap (http://sourceware.org/systemtap): Work in progress -to provide a simplified programming interface for probe-based -instrumentation. -b. Improved SMP scalability: Currently, work is in progress to handle -multiple kprobes in parallel. -c. Kernel return probes for sparc64. -d. Support for other architectures. -e. User-space probes. +a. SystemTap (http://sourceware.org/systemtap): Provides a simplified +programming interface for probe-based instrumentation. Try it out. +b. Kernel return probes for sparc64. +c. Support for other architectures. +d. User-space probes. +e. Watchpoint probes (which fire on data references). 8. Kprobes Example @@ -411,8 +415,7 @@ int init_module(void) printk("Couldn't find %s to plant kprobe\n", "do_fork"); return -1; } - ret = register_kprobe(&kp); - if (ret < 0) { + if ((ret = register_kprobe(&kp) < 0)) { printk("register_kprobe failed, returned %d\n", ret); return -1; } |
