(第一版:致力于让我们研究调试Linux内核代码不再痛苦) 是获取任何一个对象，对象的来源可以是动态事件(kprobe_event/uprobe_event) 或者静态事件(tracepoint)设置的参数，然后跟踪这个对象穿过的所有函数。
目前到了第四版，演变成首先获取一个对象，然后动态获取对象的值，举一个例子，对于函数bio_add_page():
For the function bio_add_page():int bio_add_page(struct bio *bio, struct page *page,unsigned int len, unsigned int offset)
probe 函数bio_add_page(), 获取这个函数的第一个参数，这个参数就称为一个object，这个object就是struct bio的指针。
echo 'p bio_add_page arg1=$arg1' >> ./kprobe_events然后接下来关键的一步是，根据以上object, 可以动态输出这个object结构中某个成员的值:
find the offset of the bi_size in struct bio:$ gdb vmlinux(gdb) p &(((struct bio *)0)->bi_iter.bi_size)$1 = (unsigned int *) 0x28echo 'objtrace:0x28(arg1):u32:1 if comm == "cat"' > ./events/kprobes/ \p_bio_add_page_0/trigger
由于以上输出到ring buffer的数据会比较多，可以设置起始和结束跟踪函数:
dd if=/dev/zero of=/test.txt bs=4k count=3cd /sys/kernel/debug/tracing/echo 0 > ./tracing_onecho 'p read_pages' >> ./kprobe_eventsecho 'p blk_account_io_start' >> ./kprobe_eventsecho 'traceon if comm == "cat"' > ./events/kprobes/p_read_pages_0/triggerecho 'traceoff if comm == "cat"' > ./events/kprobes/p_blk_account_io_start_0/triggerecho 'p bio_add_page arg1=$arg1' >> ./kprobe_eventsecho 'objtrace:0x28(arg1):u32:1 if comm == "cat"' > ./events/kprobes/p_bio_add_page_0/triggercat /test.txt > /dev/nullcat ./trace
最后输出的数据:
可以动态抓到bio->bi_iter.bi_size 值的变化。
cat-95 [000] ..... 1.412065: _raw_spin_unlock_irqrestore <-event_triggers_call object:0xffff888108af6328 value:0x0cat-95 [000] ..... 1.412066: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x0cat-95 [000] ..... 1.412066: __bio_add_page <-bio_add_page object:0xffff888108af6328 value:0x0cat-95 [000] ..... 1.412066: rcu_read_unlock_strict <-xa_load object:0xffff888108af6328 value:0x1000cat-95 [000] ..... 1.412066: bio_add_page <-ext4_mpage_readpages object:0xffff888108af6328 value:0x1000cat-95 [000] ..... 1.412066: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x1000cat-95 [000] ..... 1.412067: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x1000cat-95 [000] ..... 1.412067: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x1000cat-95 [000] ..... 1.412067: __bio_add_page <-bio_add_page object:0xffff888108af6328 value:0x1000cat-95 [000] ..... 1.412067: rcu_read_unlock_strict <-xa_load object:0xffff888108af6328 value:0x2000cat-95 [000] ..... 1.412067: bio_add_page <-ext4_mpage_readpages object:0xffff888108af6328 value:0x2000cat-95 [000] ..... 1.412067: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x2000cat-95 [000] ..... 1.412067: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x2000cat-95 [000] ..... 1.412067: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x2000cat-95 [000] ..... 1.412068: submit_bio <-ext4_mpage_readpages object:0xffff888108af6328 value:0x3000cat-95 [000] ..... 1.412068: submit_bio_noacct <-ext4_mpage_readpages object:0xffff888108af6328 value:0x3000....
第四版发过去之后, 由于Linux5.15刚刚发布，进入到了linux5.16的合并窗口，maintainer最近两周没有时间review,还专门解释了一下，nice(+0).
查看[PATCH v4 1/2] trace: Add trace any kernel object
点击阅读原文.
我在阅码场平台发布过tracing视频课程，欢迎扫码订阅:
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