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

On Thu, May 15, 2025 at 5:13 AM Jiri Olsa [off-list ref] 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);
+}

[...]