On Wed, 20 Feb 2019 09:49:26 -0500 Steven Rostedt rostedt@goodmis.org wrote:
On Wed, 20 Feb 2019 17:10:19 +0900 Masami Hiramatsu mhiramat@kernel.org wrote:
Let me ensure what you want. So you want to access a "string" in user-space, not a data structure? In that case, it is very easy to me. It is enough to add a "ustring" type to kprobe events. For example, do_sys_opsn's path variable is one example. That will be +0(+0(%si)):ustring, and fetcher finally copy the string using strncpy_from_user() instead of strncpy_from_unsafe(). (*)
ustring would be good.
OK.
But if you consider to access a field in a data-structure in user space, it might need some more work (E.g. ioctl's parameter), becase if the __user pointer to the data structure is on the memory, we have to dereference the address inside kernel using probe_kernel_read(), but after getting the data strucutre address, we have to dereference the address with copy_from_user(). At this moment, we have no such strong syntax...
To solve that, maybe we need to introduce something like "back reference" of arguments in the event, e.g.
p somewhere user_data=+0(%si) field_val=+8(\user_data):u32:user
or
p somewhere +0(%si) field_val=+8(\1):u32:user
This ":user" additional suffix tells kprobe events to change fetching method to fetch the data by copy_from_user().
What about just adding 'u' to the end of the offset? Say you have a data structure in kernel space that has a field in user space you want to reference?
field_val=+8u(+0(%si))
Ah, that looks good :~) thank you for this idea!
Although, I would say having ways to access current parameters may also be a nice touch ;-)
Hehe, OK, let's implement both.
Thanks!
-- Steve