[PATCH RFCv3 09/23] uprobes/x86: Add mapping for optimized uprobe trampolines

Jiri Olsa
2025-03-20T11:43:47Z

Adding support to add special mapping for for user space trampoline
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 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. 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 | 123 ++++++++++++++++++++++++++++++++++++++
include/linux/uprobes.h | 6 ++
kernel/events/uprobes.c | 10 ++++
kernel/fork.c | 1 +
4 files changed, 140 insertions(+)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index b11dcd47edaa..5ee2cce4c63e 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -742,6 +742,129 @@ static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
*sr = utask->autask.saved_scratch_register;
}
}
+
+struct uprobe_trampoline {
+ struct hlist_node node;
+ unsigned long vaddr;
+ atomic64_t ref;
+};
+
+static bool is_reachable_by_call(unsigned long vtramp, unsigned long vaddr)
+{
+ long delta = (long)(vaddr + 5 - vtramp);
+
+ return delta >= INT_MIN && delta <= INT_MAX;
+}
+
+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;
+}
+
+static struct uprobe_trampoline *create_uprobe_trampoline(unsigned long vaddr)
+{
+ struct pt_regs *regs = task_pt_regs(current);
+ const struct vm_special_mapping *mapping;
+ struct mm_struct *mm = current->mm;
+ struct vm_area_struct *vma;
+ struct uprobe_trampoline *tramp;
+
+ mapping = user_64bit_mode(regs) ? &tramp_mapping : NULL;
+ if (!mapping)
+ return NULL;
+
+ vaddr = find_nearest_page(vaddr);
+ if (!vaddr)
+ return NULL;
+
+ tramp = kzalloc(sizeof(*tramp), GFP_KERNEL);
+ if (unlikely(!tramp))
+ return NULL;
+
+ atomic64_set(&tramp->ref, 1);
+ tramp->vaddr = vaddr;
+
+ vma = _install_special_mapping(mm, tramp->vaddr, PAGE_SIZE,
+ VM_READ|VM_EXEC|VM_MAYEXEC|VM_MAYREAD|VM_DONTCOPY|VM_IO,
+ mapping);
+ if (IS_ERR(vma))
+ goto free_area;
+ return tramp;
+
+ free_area:
+ kfree(tramp);
+ return NULL;
+}
+
+__maybe_unused
+static 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->head_tramps, node) {
+ if (is_reachable_by_call(tramp->vaddr, vaddr)) {
+ atomic64_inc(&tramp->ref);
+ return tramp;
+ }
+ }
+
+ tramp = create_uprobe_trampoline(vaddr);
+ if (!tramp)
+ return NULL;
+
+ hlist_add_head(&tramp->node, &state->head_tramps);
+ return tramp;
+}
+
+static void destroy_uprobe_trampoline(struct uprobe_trampoline *tramp)
+{
+ hlist_del(&tramp->node);
+ kfree(tramp);
+}
+
+__maybe_unused
+static void uprobe_trampoline_put(struct uprobe_trampoline *tramp)
+{
+ if (tramp == NULL)
+ return;
+
+ if (atomic64_dec_and_test(&tramp->ref))
+ destroy_uprobe_trampoline(tramp);
+}
+
+void arch_uprobe_init_state(struct mm_struct *mm)
+{
+ INIT_HLIST_HEAD(&mm->uprobes_state.head_tramps);
+}
+
+void arch_uprobe_clear_state(struct mm_struct *mm)
+{
+ struct uprobes_state *state = &mm->uprobes_state;
+ struct uprobe_trampoline *tramp;
+ struct hlist_node *n;
+
+ hlist_for_each_entry_safe(tramp, n, &state->head_tramps, node)
+ destroy_uprobe_trampoline(tramp);
+}
#else /* 32-bit: */
/*
* No RIP-relative addressing on 32-bit
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 4a2b950beefd..7bde68871150 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -17,6 +17,7 @@
#include <linux/wait.h>
#include <linux/timer.h>
#include <linux/seqlock.h>
+#include <linux/mutex.h>

struct uprobe;
struct vm_area_struct;
@@ -185,6 +186,9 @@ struct xol_area;

struct uprobes_state {
struct xol_area *xol_area;
+#ifdef CONFIG_X86_64
+ struct hlist_head head_tramps;
+#endif
};

typedef int (*uprobe_write_verify_t)(struct page *page, unsigned long vaddr, uprobe_opcode_t *opcode, int nbytes);
@@ -233,6 +237,8 @@ extern void *arch_uretprobe_trampoline(unsigned long *psize);
extern unsigned long uprobe_get_trampoline_vaddr(void);
extern void uprobe_copy_from_page(struct page *page, unsigned long vaddr, void *dst, int len);
extern void handle_syscall_uprobe(struct pt_regs *regs, unsigned long bp_vaddr);
+extern void arch_uprobe_clear_state(struct mm_struct *mm);
+extern void arch_uprobe_init_state(struct mm_struct *mm);
#else /* !CONFIG_UPROBES */
struct uprobes_state {
};
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 4de04075576c..9370df47ec71 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1793,6 +1793,14 @@ static struct xol_area *get_xol_area(void)
return area;
}

+void __weak arch_uprobe_clear_state(struct mm_struct *mm)
+{
+}
+
+void __weak arch_uprobe_init_state(struct mm_struct *mm)
+{
+}
+
/*
* uprobe_clear_state - Free the area allocated for slots.
*/
@@ -1804,6 +1812,8 @@ void uprobe_clear_state(struct mm_struct *mm)
delayed_uprobe_remove(NULL, mm);
mutex_unlock(&delayed_uprobe_lock);

+ arch_uprobe_clear_state(mm);
+
if (!area)
return;

diff --git a/kernel/fork.c b/kernel/fork.c
index e27fe5d5a15c..36a0f073b913 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1260,6 +1260,7 @@ static void mm_init_uprobes_state(struct mm_struct *mm)
{
#ifdef CONFIG_UPROBES
mm->uprobes_state.xol_area = NULL;
+ arch_uprobe_init_state(mm);
#endif
}

--
2.49.0