On Fri, May 03, 2024 at 01:34:53PM +0200, Peter Zijlstra wrote:

On Thu, May 02, 2024 at 02:23:08PM +0200, Jiri Olsa wrote:

quoted

Adding uretprobe syscall instead of trap to speed up return probe.

At the moment the uretprobe setup/path is:

  - install entry uprobe

  - when the uprobe is hit, it overwrites probed function's return address
    on stack with address of the trampoline that contains breakpoint
    instruction

  - the breakpoint trap code handles the uretprobe consumers execution and
    jumps back to original return address

This patch replaces the above trampoline's breakpoint instruction with new
ureprobe syscall call. This syscall does exactly the same job as the trap
with some more extra work:

  - syscall trampoline must save original value for rax/r11/rcx registers
    on stack - rax is set to syscall number and r11/rcx are changed and
    used by syscall instruction

  - the syscall code reads the original values of those registers and
    restore those values in task's pt_regs area

  - only caller from trampoline exposed in '[uprobes]' is allowed,
    the process will receive SIGILL signal otherwise

Did you consider shadow stacks? IIRC we currently have userspace shadow
stack support available, and that will utterly break all of this.

nope.. I guess it's the extra ret instruction in the trampoline that would
make it crash?

It would be really nice if the new scheme would consider shadow stacks.

I seem to have the hw with support for user_shstk, let me test that

thanks,
jirka