On Wed, 20 Feb 2019 17:10:19 +0900 Masami Hiramatsu <mhiramat@xxxxxxxxxx> 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. > > 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)) Although, I would say having ways to access current parameters may also be a nice touch ;-) -- Steve
