Re: [PATCH perf/core 08/22] uprobes/x86: Add mapping for optimized uprobe trampolines

Oleg Nesterov
2025-04-27T14:57:00Z

On 04/21, Jiri Olsa wrote:
>
> +static unsigned long find_nearest_page(unsigned long vaddr)
> +{
> + struct vm_area_struct *vma, *prev = NULL;
> + unsigned long prev_vm_end = PAGE_SIZE;
> + VMA_ITERATOR(vmi, current->mm, 0);
> +
> + vma = vma_next(&vmi);
> + while (vma) {
> + if (prev)
> + prev_vm_end = prev->vm_end;
> + if (vma->vm_start - prev_vm_end >= PAGE_SIZE) {
> + if (is_reachable_by_call(prev_vm_end, vaddr))
> + return prev_vm_end;
> + if (is_reachable_by_call(vma->vm_start - PAGE_SIZE, vaddr))
> + return vma->vm_start - PAGE_SIZE;
> + }
> + prev = vma;
> + vma = vma_next(&vmi);
> + }
> +
> + return 0;
> +}

This can be simplified afaics... We don't really need prev, and we can
use for_each_vma(),

static unsigned long find_nearest_page(unsigned long vaddr)
{
struct vm_area_struct *vma;
unsigned long prev_vm_end = PAGE_SIZE;
VMA_ITERATOR(vmi, current->mm, 0);

for_each_vma(vmi, vma) {
if (vma->vm_start - prev_vm_end >= PAGE_SIZE) {
if (is_reachable_by_call(prev_vm_end, vaddr))
return prev_vm_end;
if (is_reachable_by_call(vma->vm_start - PAGE_SIZE, vaddr))
return vma->vm_start - PAGE_SIZE;
}
prev_vm_end = vma->vm_end;
}

return 0;
}

> +static struct uprobe_trampoline *create_uprobe_trampoline(unsigned long vaddr)
> +{
> + struct pt_regs *regs = task_pt_regs(current);
> + struct mm_struct *mm = current->mm;
> + struct uprobe_trampoline *tramp;
> + struct vm_area_struct *vma;
> +
> + if (!user_64bit_mode(regs))
> + return NULL;

Cosmetic, but I think it would be better to move this check into the
caller, uprobe_trampoline_get().

> + vma = _install_special_mapping(mm, tramp->vaddr, PAGE_SIZE,
> + VM_READ|VM_EXEC|VM_MAYEXEC|VM_MAYREAD|VM_DONTCOPY|VM_IO,
> + &tramp_mapping);

Note that xol_add_vma() -> _install_special_mapping() uses VM_SEALED_SYSMAP.
Perhaps create_uprobe_trampoline() should use this flag too for consistency?

Oleg.