Re: [PATCH bpf-next 05/13] uprobes: Add mapping for optimized uprobe trampolines

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

On Wed, Dec 11, 2024 at 5:35 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding support to add special mapping for for user space trampoline

typo: for for

> with following functions:
>
>   uprobe_trampoline_get - find or add related uprobe_trampoline
>   uprobe_trampoline_put - remove ref or destroy uprobe_trampoline
>
> The user space trampoline is exported as architecture specific user space
> special mapping, which is provided by arch_uprobe_trampoline_mapping
> function.
>
> The uprobe trampoline needs to be callable/reachable from the probe address,
> so while searching for available address we use arch_uprobe_is_callable
> 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.
>
> Locking is provided by callers in following changes.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/linux/uprobes.h |  12 +++++
>  kernel/events/uprobes.c | 114 ++++++++++++++++++++++++++++++++++++++++
>  kernel/fork.c           |   1 +
>  3 files changed, 127 insertions(+)
>

Ran out of time for today, will continue tomorrow for the rest of
patches. Some comments below.

The numbers are really encouraging, though!

> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index 8843b7f99ed0..c4ee755ca2a1 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -16,6 +16,7 @@
>  #include <linux/types.h>
>  #include <linux/wait.h>
>  #include <linux/timer.h>
> +#include <linux/mutex.h>
>
>  struct uprobe;
>  struct vm_area_struct;
> @@ -172,6 +173,13 @@ struct xol_area;
>
>  struct uprobes_state {
>         struct xol_area         *xol_area;
> +       struct hlist_head       tramp_head;
> +};
> +

should we make uprobe_state be linked by a pointer from mm_struct
instead of increasing mm for each added field? right now it's
embedded, I don't think it's problematic to allocate it on demand and
keep it until mm_struct is freed

> +struct uprobe_trampoline {
> +       struct hlist_node       node;
> +       unsigned long           vaddr;
> +       atomic64_t              ref;
>  };
>
>  extern void __init uprobes_init(void);
> @@ -220,6 +228,10 @@ extern int arch_uprobe_verify_opcode(struct arch_uprobe *auprobe, struct page *p
>                                      unsigned long vaddr, uprobe_opcode_t *new_opcode,
>                                      int nbytes);
>  extern bool arch_uprobe_is_register(uprobe_opcode_t *insn, int nbytes);
> +extern struct uprobe_trampoline *uprobe_trampoline_get(unsigned long vaddr);
> +extern void uprobe_trampoline_put(struct uprobe_trampoline *area);
> +extern bool arch_uprobe_is_callable(unsigned long vtramp, unsigned long vaddr);
> +extern const struct vm_special_mapping *arch_uprobe_trampoline_mapping(void);
>  #else /* !CONFIG_UPROBES */
>  struct uprobes_state {
>  };
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 8068f91de9e3..f57918c624da 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -615,6 +615,118 @@ set_orig_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long v
>                         (uprobe_opcode_t *)&auprobe->insn, UPROBE_SWBP_INSN_SIZE);
>  }
>
> +bool __weak arch_uprobe_is_callable(unsigned long vtramp, unsigned long vaddr)

bikeshedding some more, I still find "is_callable" confusing. How
about "is_reachable_by_call"? slightly verbose, but probably more
meaningful?

> +{
> +       return false;
> +}
> +
> +const struct vm_special_mapping * __weak arch_uprobe_trampoline_mapping(void)
> +{
> +       return NULL;
> +}
> +
> +static unsigned long find_nearest_page(unsigned long vaddr)
> +{
> +       struct mm_struct *mm = current->mm;
> +       struct vm_area_struct *vma, *prev;
> +       VMA_ITERATOR(vmi, mm, 0);
> +
> +       prev = vma_next(&vmi);

minor: we are missing an opportunity to add something between
[PAGE_SIZE, <first_vma_start>). Probably fine, but why not?

> +       vma = vma_next(&vmi);
> +       while (vma) {
> +               if (vma->vm_start - prev->vm_end  >= PAGE_SIZE) {
> +                       if (arch_uprobe_is_callable(prev->vm_end, vaddr))
> +                               return prev->vm_end;
> +                       if (arch_uprobe_is_callable(vma->vm_start - PAGE_SIZE, vaddr))
> +                               return vma->vm_start - PAGE_SIZE;
> +               }
> +
> +               prev = vma;
> +               vma = vma_next(&vmi);
> +       }
> +
> +       return 0;
> +}
> +

[...]

> +struct uprobe_trampoline *uprobe_trampoline_get(unsigned long vaddr)
> +{
> +       struct uprobes_state *state = &current->mm->uprobes_state;
> +       struct uprobe_trampoline *tramp = NULL;
> +
> +       hlist_for_each_entry(tramp, &state->tramp_head, node) {
> +               if (arch_uprobe_is_callable(tramp->vaddr, vaddr)) {
> +                       atomic64_inc(&tramp->ref);
> +                       return tramp;
> +               }
> +       }
> +
> +       tramp = create_uprobe_trampoline(vaddr);
> +       if (!tramp)
> +               return NULL;
> +
> +       hlist_add_head(&tramp->node, &state->tramp_head);
> +       return tramp;
> +}
> +
> +static void destroy_uprobe_trampoline(struct uprobe_trampoline *tramp)
> +{
> +       hlist_del(&tramp->node);
> +       kfree(tramp);

hmm... shouldn't this be RCU-delayed (RCU Tasks Trace for uprobes),
otherwise we might have some CPU executing code in that trampoline,
no?

> +}
> +

[...]