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

Andrii Nakryiko
2024-12-13T01:02:04Z

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?

> +}
> + …