Re: [PATCH RFCv2 08/18] uprobes/x86: Add uprobe syscall to speed up uprobe

Andrii Nakryiko
2025-02-25T17:11:04Z

On Tue, Feb 25, 2025 at 5:35 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Mon, Feb 24, 2025 at 11:22:42AM -0800, Alexei Starovoitov wrote:
> > On Mon, Feb 24, 2025 at 6:08 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > >
> > > +SYSCALL_DEFINE0(uprobe)
> > > +{
> > > + struct pt_regs *regs = task_pt_regs(current);
> > > + unsigned long bp_vaddr;
> > > + int err;
> > > +
> > > + err = copy_from_user(&bp_vaddr, (void __user *)regs->sp + 3*8, sizeof(bp_vaddr));
> > > + if (err) {
> > > + force_sig(SIGILL);
> > > + return -1;
> > > + }
> > > +
> > > + /* Allow execution only from uprobe trampolines. */
> > > + if (!in_uprobe_trampoline(regs->ip)) {
> > > + force_sig(SIGILL);
> > > + return -1;
> > > + }
> > > +
> > > + handle_syscall_uprobe(regs, bp_vaddr - 5);
> > > + return 0;
> > > +}
> > > +
> > > +asm (
> > > + ".pushsection .rodata\n"
> > > + ".balign " __stringify(PAGE_SIZE) "\n"
> > > + "uprobe_trampoline_entry:\n"
> > > + "endbr64\n"
> >
> > why endbr is there?
> > The trampoline is called with a direct call.
>
> ok, that's wrong, will remove that
>
> >
> > > + "push %rcx\n"
> > > + "push %r11\n"
> > > + "push %rax\n"
> > > + "movq $" __stringify(__NR_uprobe) ", %rax\n"
> >
> > To avoid introducing a new syscall for a very similar operation
> > can we disambiguate uprobe vs uretprobe via %rdi or
> > some other way?
> > imo not too late to change uretprobe api.
> > Maybe it was discussed already.
>
> yes, I recall discussing that early during uretprobe work with the decision to
> have separate syscalls for each uprobe and uretprobe.. however wrt recent seccomp
> changes, it might be easier just to add argument to uretprobe syscall to handle
> uprobe
>
> too bad it's not the other way around.. uprobe syscall with argument to do uretprobe
> would sound better

It's an "internal" syscall, why can't we rename it, if we want to?

Though I'm not sure I see the problem having both sys_uprobe and
sys_uretprobe, tbh. We just add sys_uprobe to the same list(s) that
sys_uretprobe is in for seccomp.

>
> >
> > > + "syscall\n"
> > > + "pop %rax\n"
> > > + "pop %r11\n"
> > > + "pop %rcx\n"
> > > + "ret\n"
> >
> > In later patches I see nop5 is replaced with a call to
> > uprobe_trampoline_entry, but which part saves
> > rdi and other regs?
> > Compiler doesn't automatically spill/fill around USDT's nop/nop5.
> > Selftest is doing:
> > +__naked noinline void uprobe_test(void)
> > so just lucky ?
>
> if you mean registers that would carry usdt arguments, ebpf programs
> access those based on assembler operand string stored in usdt record:
>
> stapsdt 0x00000048 NT_STAPSDT (SystemTap probe descriptors)
> Provider: test
> Name: usdt3
> Location: 0x0000000000712f2f, Base: 0x0000000002f516b0, Semaphore: 0x0000000003348ec2
> -> Arguments: -4@-1192(%rbp) -8@-1200(%rbp) 8@%rax
>
> it's up to bpf program to know which register(+offset) to access, libbpf have all
> this logic hidden behind usdt_manager_attach_usdt and bpf_usdt_arg bpf call
>
> the trampoline only saves rcx/r11/rax, because they are changed by syscall instruction
>
> but actually I forgot to load these saved values (of rcx/r11/rax) and rsp into regs that
> are passed to ebpf program, (like we do in uretprobe syscall) will fix that in next version
>
> I'll add tests for optimized usdt with more arguments
>
> thanks,
> jirka