Re: [PATCH bpf-next 08/13] uprobes/x86: Add support to optimize uprobes

Jiri Olsa
2024-12-13T13:06:55Z

On Fri, Dec 13, 2024 at 11:49:07AM +0100, Peter Zijlstra wrote:
> On Wed, Dec 11, 2024 at 02:33:57PM +0100, Jiri Olsa wrote:
> > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> > index cdea97f8cd39..b2420eeee23a 100644
> > --- a/arch/x86/kernel/uprobes.c
> > +++ b/arch/x86/kernel/uprobes.c
>
> > @@ -1306,3 +1339,132 @@ bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
> > else
> > return regs->sp <= ret->stack;
> > }
> > +
> > +int arch_uprobe_verify_opcode(struct arch_uprobe *auprobe, struct page *page,
> > + unsigned long vaddr, uprobe_opcode_t *new_opcode,
> > + int nbytes)
> > +{
> > + uprobe_opcode_t old_opcode[5];
> > + bool is_call, is_swbp, is_nop5;
> > +
> > + if (!test_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags))
> > + return uprobe_verify_opcode(page, vaddr, new_opcode);
> > +
> > + /*
> > + * The ARCH_UPROBE_FLAG_CAN_OPTIMIZE flag guarantees the following
> > + * 5 bytes read won't cross the page boundary.
> > + */
> > + uprobe_copy_from_page(page, vaddr, (uprobe_opcode_t *) &old_opcode, 5);
> > + is_call = is_call_insn((uprobe_opcode_t *) &old_opcode);
> > + is_swbp = is_swbp_insn((uprobe_opcode_t *) &old_opcode);
> > + is_nop5 = is_nop5_insn((uprobe_opcode_t *) &old_opcode);
> > +
> > + /*
> > + * We allow following trasitions for optimized uprobes:
> > + *
> > + * nop5 -> swbp -> call
> > + * || | |
> > + * |'--<---' |
> > + * '---<-----------'
> > + *
> > + * We return 1 to ack the write, 0 to do nothing, -1 to fail write.
> > + *
> > + * If the current opcode (old_opcode) has already desired value,
> > + * we do nothing, because we are racing with another thread doing
> > + * the update.
> > + */
> > + switch (nbytes) {
> > + case 5:
> > + if (is_call_insn(new_opcode)) {
> > + if (is_swbp)
> > + return 1;
> > + if (is_call && !memcmp(new_opcode, &old_opcode, 5))
> > + return 0;
> > + } else {
> > + if (is_call || is_swbp)
> > + return 1;
> > + if (is_nop5)
> > + return 0;
> > + }
> > + break;
> > + case 1:
> > + if (is_swbp_insn(new_opcode)) {
> > + if (is_nop5)
> > + return 1;
> > + if (is_swbp || is_call)
> > + return 0;
> > + } else {
> > + if (is_swbp || is_call)
> > + return 1;
> > + if (is_nop5)
> > + return 0;
> > + }
> > + }
> > + return -1;
> > +}
> > +
> > +bool arch_uprobe_is_register(uprobe_opcode_t *insn, int nbytes)
> > +{
> > + return nbytes == 5 ? is_call_insn(insn) : is_swbp_insn(insn);
> > +}
> > +
> > +static void __arch_uprobe_optimize(struct arch_uprobe *auprobe, struct mm_struct *mm,
> > + unsigned long vaddr)
> > +{
> > + struct uprobe_trampoline *tramp;
> > + char call[5];
> > +
> > + tramp = uprobe_trampoline_get(vaddr);
> > + if (!tramp)
> > + goto fail;
> > +
> > + relative_call(call, (void *) vaddr, (void *) tramp->vaddr);
> > + if (uprobe_write_opcode(auprobe, mm, vaddr, call, 5))
> > + goto fail;
> > +
> > + set_bit(ARCH_UPROBE_FLAG_OPTIMIZED, &auprobe->flags);
> > + return;
> > +
> > +fail:
> > + /* Once we fail we never try again. */
> > + clear_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags);
> > + uprobe_trampoline_put(tramp);
> > +}
> > +
> > +static bool should_optimize(struct arch_uprobe *auprobe)
> > +{
> > + if (!test_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags))
> > + return false;
> > + if (test_bit(ARCH_UPROBE_FLAG_OPTIMIZED, &auprobe->flags))
> > + return false;
> > + return true;
> > +}
> > +
> > +void arch_uprobe_optimize(struct arch_uprobe *auprobe, unsigned long vaddr)
> > +{
> > + struct mm_struct *mm = current->mm;
> > +
> > + if (!should_optimize(auprobe))
> > + return;
> > +
> > + mmap_write_lock(mm);
> > + if (should_optimize(auprobe))
> > + __arch_uprobe_optimize(auprobe, mm, vaddr);
> > + mmap_write_unlock(mm);
> > +}
> > +
> > +int set_orig_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr)
> > +{
> > + uprobe_opcode_t *insn = (uprobe_opcode_t *) auprobe->insn;
> > +
> > + if (test_bit(ARCH_UPROBE_FLAG_OPTIMIZED, &auprobe->flags))
> > + return uprobe_write_opcode(auprobe, mm, vaddr, insn, 5);
> > +
> > + return uprobe_write_opcode(auprobe, mm, vaddr, insn, UPROBE_SWBP_INSN_SIZE);
> > +}
> > +
> > +bool arch_uprobe_is_callable(unsigned long vtramp, unsigned long vaddr)
> > +{
> > + long delta = (long)(vaddr + 5 - vtramp);
> > + return delta >= INT_MIN && delta <= INT_MAX;
> > +}
>
> All this code is useless on 32bit, right?

yes, there's user_64bit_mode check in arch_uprobe_trampoline_mapping
when getting the trampoline, so above won't get executed in practise..

but I think we should make it more obvious and put the check directly
to arch_uprobe_optimize

jirka