Re: [RFC] systemtap: begin the process of using proper kernel APIs(part1: use kprobe symbol_name/offset instead of address)

From: Masami Hiramatsu
Date: Thu Jul 17 2008 - 17:38:35 EST

James Bottomley wrote:
> On Thu, 2008-07-17 at 09:18 -0500, James Bottomley wrote:
>> OK, thought about it. There seem to be two possible solutions
>> 1. Get systemtap always to offset from non-static functions. This
>> will use the standard linker to ensure uniqueness (a module
>> qualifier will still need to be added to the struct kprobe for
>> lookup, since modules can duplicate unexported kernel symbols).
>> 2. Add the filename as a discriminator for duplicate symbols in the
>> kallsyms program (would still need module qualifier). This is
>> appealing because the path name would be printed in the kernel
>> trace to help with oops tracking
>> This is where negotiations come in. To me 2. looks to be better because
>> it will help us with oops tracking. On the other hand, it's usually
>> pretty obvious from the stack trace context which files the duplicate
>> symbols are actually in; what do other people think?
> Just by way of illustration, this is systemtap fixed up according to
> suggestion number 1. You can see now using your test case that we get:
> # probes
> kernel.function("do_open@fs/block_dev.c:929") /* pc=<lookup_bdev+0x90> */ /* <- kernel.function("do_open") */
> kernel.function("do_open@fs/nfsctl.c:24") /* pc=<sys_nfsservctl+0x6a> */ /* <- kernel.function("do_open") */
> kernel.function("do_open@ipc/mqueue.c:642") /* pc=<sys_mq_unlink+0x130> */ /* <- kernel.function("do_open") */

Hi James,

Thank you for updating the patch.
Unfortunately, I found another scenario; if someone make a module which
has EXPORT_SYMBOL(do_open), it's a non-static function. but there are
other static version do_open in kallsyms.
Here, I tested it and got below;

$ stap --kelf -e 'probe module("test").function("do_open"){}' -p2
# probes
module("test").function("do_open@?") /* pc=<do_open+0x0> */ /* <- module("test").function("do_open") */

And I think similar issue will occur even if it is embedded in vmlinux.

By the way, can this patch solve the issue of -ffunction-sections?

Anyway, I think we still need solution no.2.

Thank you,

Masami Hiramatsu

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

e-mail: mhiramat@xxxxxxxxxx

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