On 6/30/2026 11:32 PM, Thomas Weißschuh wrote:

On Mon, Jun 29, 2026 at 09:06:16PM +0800, Jinjie Ruan wrote:

quoted

Syscall User Dispatch (SUD) requires the signal trampoline code to not be
intercepted. This is necessary to support returning with a locked selector
while avoiding infinite recursion back into the signal handler.

Implement arch_syscall_is_vdso_sigreturn() for arm64 to support this
exclusion mechanism. For native 64-bit tasks, it checks whether the current
PC matches the 'svc #0' instruction inside the vDSO sigreturn trampoline.

	SYM_CODE_START(__kernel_rt_sigreturn)
	        mov     x8, #__NR_rt_sigreturn
	        svc     #0
	SYM_CODE_END(__kernel_rt_sigreturn)

For COMPAT tasks, it verifies if the instruction falls within
the architecture's 'sigpage' range, allowing the kernel to safely bypass
dispatching syscalls originating from these areas back to userspace.

Suggested-by: Kevin Brodsky <redacted>
Suggested-by: kemal <redacted>
Signed-off-by: Jinjie Ruan <redacted>
---
 arch/arm64/include/asm/elf.h |  1 +
 arch/arm64/kernel/vdso.c     | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index d2779d604c7b..f43d4dea916a 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h

@@ -185,6 +185,7 @@ do {									\
 struct linux_binprm;
 extern int arch_setup_additional_pages(struct linux_binprm *bprm,
 				       int uses_interp);
+extern bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs);

A header for that?

Sure, it need a cleanup, as currently, there is no syscall_user_dispatch
header file related to the architecture.

quoted

 /* 1GB of VA */
 #ifdef CONFIG_COMPAT

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 592dd8668de4..5a0314a3c26e 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c

@@ -343,3 +343,19 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	return ret;
 }
+
+bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
+{
+	unsigned long sigtramp;
+
+#ifdef CONFIG_COMPAT
+	if (is_compat_task()) {
+		unsigned long sigpage = (unsigned long)current->mm->context.sigpage;
+
+		return regs->pc >= sigpage && regs->pc < (sigpage + PAGE_SIZE);
+	}
+#endif
+	sigtramp = (unsigned long)VDSO_SYMBOL(current->mm->context.vdso, sigtramp);
+
+	return regs->pc == (sigtramp + 8);

Instead of hardcoding 'sigtramp + 8' you could add a new label to the 'svc #0'
instruction and use that with VDSO_SYMBOL().

It seems that the modification of __kernel_rt_sigreturn() is not recommended

Thomas