Re: [PATCH v16 18/18] arm64: vdso: Expose sigreturn address on vdso to the kernel

[Thomas Weißschuh]

On Mon, Jun 29, 2026 at 09:06:16PM +0800, Jinjie Ruan wrote:
> 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 <kevin.brodsky@xxxxxxx>
> Suggested-by: kemal <kmal@xxxxxxx>
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> ---
>  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?

>  /* 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().


Thomas