Linux内核patch发到第四版[trace: Add trace any kernel object]

Posted 2021-12-05 宋宝华

(第一版:致力于让我们研究调试Linux内核代码不再痛苦

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 *) 0x28


echo '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=3
cd /sys/kernel/debug/tracing/
echo 0 > ./tracing_on
echo 'p read_pages' >> ./kprobe_events
echo 'p blk_account_io_start' >> ./kprobe_events
echo 'traceon if comm == "cat"' > ./events/kprobes/p_read_pages_0/trigger
echo 'traceoff if comm == "cat"' > ./events/kprobes/p_blk_account_io_start_0/trigger
echo 'p bio_add_page arg1=$arg1' >> ./kprobe_events
echo 'objtrace:0x28(arg1):u32:1 if comm == "cat"' > ./events/kprobes/p_bio_add_page_0/trigger
cat  /test.txt > /dev/null
cat ./trace

最后输出的数据:

可以动态抓到bio->bi_iter.bi_size 值的变化。

# tracer: nop
#
# entries-in-buffer/entries-written: 50/50   #P:1
#
#                                _-----=> irqs-off
#                               / _----=> need-resched
#                              | / _---=> hardirq/softirq
#                              || / _--=> preempt-depth
#                              ||| / _-=> migrate-disable
#                              |||| /     delay
#           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
#              | |         |   |||||     |         |
             cat-95      [000] .....     1.412065: _raw_spin_unlock_irqrestore <-event_triggers_call object:0xffff888108af6328 value:0x0
             cat-95      [000] .....     1.412066: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x0
             cat-95      [000] .....     1.412066: __bio_add_page <-bio_add_page object:0xffff888108af6328 value:0x0
             cat-95      [000] .....     1.412066: rcu_read_unlock_strict <-xa_load object:0xffff888108af6328 value:0x1000
             cat-95      [000] .....     1.412066: bio_add_page <-ext4_mpage_readpages object:0xffff888108af6328 value:0x1000
             cat-95      [000] .....     1.412066: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x1000
             cat-95      [000] .....     1.412067: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x1000
             cat-95      [000] .....     1.412067: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x1000
             cat-95      [000] .....     1.412067: __bio_add_page <-bio_add_page object:0xffff888108af6328 value:0x1000
             cat-95      [000] .....     1.412067: rcu_read_unlock_strict <-xa_load object:0xffff888108af6328 value:0x2000
             cat-95      [000] .....     1.412067: bio_add_page <-ext4_mpage_readpages object:0xffff888108af6328 value:0x2000
             cat-95      [000] .....     1.412067: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x2000
             cat-95      [000] .....     1.412067: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x2000
             cat-95      [000] .....     1.412067: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x2000
             cat-95      [000] .....     1.412068: submit_bio <-ext4_mpage_readpages object:0xffff888108af6328 value:0x3000
             cat-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

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