Re: [PATCHv2 perf/core 08/22] uprobes/x86: Add mapping for optimized uprobe trampolines

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, May 15, 2025 at 5:13 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding support to add special mapping for user space trampoline with
> following functions:
>
>   uprobe_trampoline_get - find or add uprobe_trampoline
>   uprobe_trampoline_put - remove or destroy uprobe_trampoline
>
> The user space trampoline is exported as arch specific user space special
> mapping through tramp_mapping, which is initialized in following changes
> with new uprobe syscall.
>
> The uprobe trampoline needs to be callable/reachable from the probed address,
> so while searching for available address we use is_reachable_by_call function
> to decide if the uprobe trampoline is callable from the probe address.
>
> All uprobe_trampoline objects are stored in uprobes_state object and are
> cleaned up when the process mm_struct goes down. Adding new arch hooks
> for that, because this change is x86_64 specific.
>
> Locking is provided by callers in following changes.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  arch/x86/kernel/uprobes.c | 115 ++++++++++++++++++++++++++++++++++++++
>  include/linux/uprobes.h   |   6 ++
>  kernel/events/uprobes.c   |  10 ++++
>  kernel/fork.c             |   1 +
>  4 files changed, 132 insertions(+)
>

[...]

> +static unsigned long find_nearest_page(unsigned long vaddr)
> +{
> +       struct vm_unmapped_area_info info = {
> +               .length     = PAGE_SIZE,
> +               .align_mask = ~PAGE_MASK,
> +               .flags      = VM_UNMAPPED_AREA_TOPDOWN,
> +               .low_limit  = 0,

would this, technically, allow to allocate memory at NULL (0x0000)
address? should this start at PAGE_SIZE?

> +               .high_limit = ULONG_MAX,
> +       };
> +       unsigned long limit, call_end = vaddr + 5;
> +
> +       if (!check_add_overflow(call_end, INT_MIN, &limit))
> +               info.low_limit = limit;
> +       if (!check_add_overflow(call_end, INT_MAX, &limit))
> +               info.high_limit = limit;
> +       return vm_unmapped_area(&info);
> +}

[...]