Hi Steve,
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.
I've tried to implement ustring and u-offsets, but I got some issues.
- access_ok() warns if it is called in IRQ context (kprobes is.) - copy_from_user uses access_ok(), so it is not designed for irq handler.
Moreover, if we have different kernel/user address spaces, we have to assign target user-pages to kernel vma. Can we do that (doesn't it involve mutex locks)?
If not, I think what we can do "in kprobes" is only probe_kernel_read() and strncpy_from_unsafe(). This means, on the architechture whoes kernel address space doesn't map user space, we can not support user-space data fetching in kprobe evevnts.
Thank you,