[PATCH perf/core 09/22] uprobes/x86: Add uprobe syscall to speed up uprobe

Jiri Olsa
2025-04-21T21:46:21Z

Adding new uprobe syscall that calls uprobe handlers for given
'breakpoint' address.

The idea is that the 'breakpoint' address calls the user space
trampoline which executes the uprobe syscall.

The syscall handler reads the return address of the initial call
to retrieve the original 'breakpoint' address. With this address
we find the related uprobe object and call its consumers.

Adding the arch_uprobe_trampoline_mapping function that provides
uprobe trampoline mapping. This mapping is backed with one global
page initialized at __init time and shared by the all the mapping
instances.

We do not allow to execute uprobe syscall if the caller is not
from uprobe trampoline mapping.

The uprobe syscall ensures the consumer (bpf program) sees registers
values in the state before the trampoline was called.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/x86/kernel/uprobes.c | 122 +++++++++++++++++++++++++
include/linux/syscalls.h | 2 +
include/linux/uprobes.h | 1 +
kernel/events/uprobes.c | 17 ++++
kernel/sys_ni.c | 1 +
6 files changed, 144 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index cfb5ca41e30d..9fd1291e7bdf 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -345,6 +345,7 @@
333 common io_pgetevents sys_io_pgetevents
334 common rseq sys_rseq
335 common uretprobe sys_uretprobe
+336 common uprobe sys_uprobe
# don't use numbers 387 through 423, add new calls after the last
# 'common' entry
424 common pidfd_send_signal sys_pidfd_send_signal
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 023c55d52138..01b3035e01ea 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -739,6 +739,128 @@ void arch_uprobe_clear_state(struct mm_struct *mm)
hlist_for_each_entry_safe(tramp, n, &state->head_tramps, node)
destroy_uprobe_trampoline(tramp);
}
+
+static bool __in_uprobe_trampoline(unsigned long ip)
+{
+ struct vm_area_struct *vma = vma_lookup(current->mm, ip);
+
+ return vma && vma_is_special_mapping(vma, &tramp_mapping);
+}
+
+static bool in_uprobe_trampoline(unsigned long ip)
+{
+ struct mm_struct *mm = current->mm;
+ bool found, retry = true;
+ unsigned int seq;
+
+ rcu_read_lock();
+ if (mmap_lock_speculate_try_begin(mm, &seq)) {
+ found = __in_uprobe_trampoline(ip);
+ retry = mmap_lock_speculate_retry(mm, seq);
+ }
+ rcu_read_unlock();
+
+ if (retry) {
+ mmap_read_lock(mm);
+ found = __in_uprobe_trampoline(ip);
+ mmap_read_unlock(mm);
+ }
+ return found;
+}
+
+SYSCALL_DEFINE0(uprobe)
+{
+ struct pt_regs *regs = task_pt_regs(current);
+ unsigned long ip, sp, ax_r11_cx_ip[4];
+ int err;
+
+ /* Allow execution only from uprobe trampolines. */
+ if (!in_uprobe_trampoline(regs->ip))
+ goto sigill;
+
+ err = copy_from_user(ax_r11_cx_ip, (void __user *)regs->sp, sizeof(ax_r11_cx_ip));
+ if (err)
+ goto sigill;
+
+ ip = regs->ip;
+
+ /*
+ * expose the "right" values of ax/r11/cx/ip/sp to uprobe_consumer/s, plus:
+ * - adjust ip to the probe address, call saved next instruction address
+ * - adjust sp to the probe's stack frame (check trampoline code)
+ */
+ regs->ax = ax_r11_cx_ip[0];
+ regs->r11 = ax_r11_cx_ip[1];
+ regs->cx = ax_r11_cx_ip[2];
+ regs->ip = ax_r11_cx_ip[3] - 5;
+ regs->sp += sizeof(ax_r11_cx_ip);
+ regs->orig_ax = -1;
+
+ sp = regs->sp;
+
+ handle_syscall_uprobe(regs, regs->ip);
+
+ /*
+ * Some of the uprobe consumers has changed sp, we can do nothing,
+ * just return via iret.
+ */
+ if (regs->sp != sp)
+ return regs->ax;
+
+ regs->sp -= sizeof(ax_r11_cx_ip);
+
+ /* for the case uprobe_consumer has changed ax/r11/cx */
+ ax_r11_cx_ip[0] = regs->ax;
+ ax_r11_cx_ip[1] = regs->r11;
+ ax_r11_cx_ip[2] = regs->cx;
+
+ /* keep return address unless we are instructed otherwise */
+ if (ax_r11_cx_ip[3] - 5 != regs->ip)
+ ax_r11_cx_ip[3] = regs->ip;
+
+ regs->ip = ip;
+
+ err = copy_to_user((void __user *)regs->sp, ax_r11_cx_ip, sizeof(ax_r11_cx_ip));
+ if (err)
+ goto sigill;
+
+ /* ensure sysret, see do_syscall_64() */
+ regs->r11 = regs->flags;
+ regs->cx = regs->ip;
+ return 0;
+
+sigill:
+ force_sig(SIGILL);
+ return -1;
+}
+
+asm (
+ ".pushsection .rodata\n"
+ ".balign " __stringify(PAGE_SIZE) "\n"
+ "uprobe_trampoline_entry:\n"
+ "push %rcx\n"
+ "push %r11\n"
+ "push %rax\n"
+ "movq $" __stringify(__NR_uprobe) ", %rax\n"
+ "syscall\n"
+ "pop %rax\n"
+ "pop %r11\n"
+ "pop %rcx\n"
+ "ret\n"
+ ".balign " __stringify(PAGE_SIZE) "\n"
+ ".popsection\n"
+);
+
+extern u8 uprobe_trampoline_entry[];
+
+static int __init arch_uprobes_init(void)
+{
+ tramp_mapping_pages[0] = virt_to_page(uprobe_trampoline_entry);
+ return 0;
+}
+
+late_initcall(arch_uprobes_init);
+
#else /* 32-bit: */
/*
* No RIP-relative addressing on 32-bit
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index e5603cc91963..b0cc60f1c458 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -998,6 +998,8 @@ asmlinkage long sys_ioperm(unsigned long from, unsigned long num, int on);

asmlinkage long sys_uretprobe(void);

+asmlinkage long sys_uprobe(void);
+
/* pciconfig: alpha, arm, arm64, ia64, sparc */
asmlinkage long sys_pciconfig_read(unsigned long bus, unsigned long dfn,
unsigned long off, unsigned long len,
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index bc532d086813..bbe218ff16cc 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -239,6 +239,7 @@ 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 arch_uprobe_clear_state(struct mm_struct *mm);
extern void arch_uprobe_init_state(struct mm_struct *mm);
+extern void handle_syscall_uprobe(struct pt_regs *regs, unsigned long bp_vaddr);
#else /* !CONFIG_UPROBES */
struct uprobes_state {
};
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index a3107f63f295..97a7b9f0c7ca 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2782,6 +2782,23 @@ static void handle_swbp(struct pt_regs *regs)
rcu_read_unlock_trace();
}

+void handle_syscall_uprobe(struct pt_regs *regs, unsigned long bp_vaddr)
+{
+ struct uprobe *uprobe;
+ int is_swbp;
+
+ guard(rcu_tasks_trace)();
+
+ uprobe = find_active_uprobe_rcu(bp_vaddr, &is_swbp);
+ if (!uprobe)
+ return;
+ if (!get_utask())
+ return;
+ if (arch_uprobe_ignore(&uprobe->arch, regs))
+ return;
+ handler_chain(uprobe, regs);
+}
+
/*
* Perform required fix-ups and disable singlestep.
* Allow pending signals to take effect.
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index c00a86931f8c..bf5d05c635ff 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -392,3 +392,4 @@ COND_SYSCALL(setuid16);
COND_SYSCALL(rseq);

COND_SYSCALL(uretprobe);
+COND_SYSCALL(uprobe);
--
2.49.0