Re: [PATCHv4 bpf-next 2/7] uprobe: Add uretprobe syscall to speed up return probe

[Jiri Olsa]

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:
> > 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