Re: [PATCH bpf-next 06/13] uprobes/x86: Add uprobe syscall to speed up uprobe

Thomas Weißschuh
2024-12-13T15:12:48Z

On 2024-12-13 15:51:44+0100, Jiri Olsa wrote:
> On Fri, Dec 13, 2024 at 02:48:00PM +0100, Thomas Weißschuh wrote:
>
> SNIP
>
> > > +static int tramp_mremap(const struct vm_special_mapping *sm, struct vm_area_struct *new_vma)
> > > +{
> > > + return -EPERM;
> > > +}
> > > +
> > > +static struct vm_special_mapping tramp_mapping = {
> > > + .name = "[uprobes-trampoline]",
> > > + .mremap = tramp_mremap,
> > > +};
> > > +
> > > +SYSCALL_DEFINE0(uprobe)
> > > +{
> > > + struct pt_regs *regs = task_pt_regs(current);
> > > + struct vm_area_struct *vma;
> > > + unsigned long bp_vaddr;
> > > + int err;
> > > +
> > > + err = copy_from_user(&bp_vaddr, (void __user *)regs->sp + 3*8, sizeof(bp_vaddr));
> >
> > A #define for the magic values would be nice.
>
> the 3*8 is to skip 3 values pushed on stack and get the return ip value,
> I'd prefer to keep 3*8 but it's definitely missing explaining comment
> above, wdyt?

A comment sounds good.

> > > + if (err) {
> > > + force_sig(SIGILL);
> > > + return -1;
> > > + }
> > > +
> > > + /* Allow execution only from uprobe trampolines. */
> > > + vma = vma_lookup(current->mm, regs->ip);
> > > + if (!vma || vma->vm_private_data != (void *) &tramp_mapping) {
> >
> > vma_is_special_mapping()
>
> did not know about this function, thanks
>
> >
> > > + force_sig(SIGILL);
> > > + return -1;
> > > + }
> > > +
> > > + handle_syscall_uprobe(regs, bp_vaddr - 5);
> > > + return 0;
> > > +}
> > > +
> > > +asm (
> > > + ".pushsection .rodata\n"
> > > + ".global uprobe_trampoline_entry\n"
> > > + "uprobe_trampoline_entry:\n"
> > > + "endbr64\n"
> > > + "push %rcx\n"
> > > + "push %r11\n"
> > > + "push %rax\n"
> > > + "movq $" __stringify(__NR_uprobe) ", %rax\n"
> > > + "syscall\n"
> > > + "pop %rax\n"
> > > + "pop %r11\n"
> > > + "pop %rcx\n"
> > > + "ret\n"
> > > + ".global uprobe_trampoline_end\n"
> > > + "uprobe_trampoline_end:\n"
> > > + ".popsection\n"
> > > +);
> > > +
> > > +extern __visible u8 uprobe_trampoline_entry[];
> > > +extern __visible u8 uprobe_trampoline_end[];
> > > +
> > > +const struct vm_special_mapping *arch_uprobe_trampoline_mapping(void)
> > > +{
> > > + struct pt_regs *regs = task_pt_regs(current);
> > > +
> > > + return user_64bit_mode(regs) ? &tramp_mapping : NULL;
> > > +}
> > > +
> > > +static int __init arch_uprobes_init(void)
> > > +{
> > > + unsigned long size = uprobe_trampoline_end - uprobe_trampoline_entry;
> > > + static struct page *pages[2];
> > > + struct page *page;
> > > +
> > > + page = alloc_page(GFP_HIGHUSER);
> >
> > That page could be in static memory, removing the need for the explicit
> > allocation. It could also be __ro_after_init.
> > Then tramp_mapping itself can be const.
>
> hum, how would that look like? I think that to get proper page object
> you have to call alloc_page or some other page alloc family function..
> what do I miss?

static u8 trampoline_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE);
static struct page *tramp_mapping_pages[2] __ro_after_init;

static const struct vm_special_mapping tramp_mapping = {
.name = "[uprobes-trampoline]",
.pages = tramp_mapping_pages,
.mremap = tramp_mremap,
};

static int __init arch_uprobes_init(void)
{
...
trampoline_pages[0] = virt_to_page(trampoline_page);
...
}

Untested, but it's similar to the stuff the vDSO implementations are
doing which I am working with at the moment.

> >
> > Also this seems to waste the page on 32bit kernels.
>
> it's inside CONFIG_X86_64 ifdef
>
> >
> > > + if (!page)
> > > + return -ENOMEM;
> > > + pages[0] = page;
> > > + tramp_mapping.pages = (struct page **) &pages;
> >
> > tramp_mapping.pages = pages; ?
>
> I think the compiler will cry about *pages[2] vs **pages types mismatch,
> but I'll double check that

It compiles for me.

> thanks,
> jirka
>
> >
> > > + arch_uprobe_copy_ixol(page, 0, uprobe_trampoline_entry, size);
> > > + return 0;
> > > +}
> > > +
> > > +late_initcall(arch_uprobes_init);
> > > +
> > > /*
> > > * If arch_uprobe->insn doesn't use rip-relative addressing, return
> > > * immediately. Otherwise, rewrite the instruction so that it accesses
> >
> > [..]