Re: [PATCH v4 10/13] x86/um: nommu: signal handling

[Hajime Tazaki <thehajime@gmail.com>]

On Sun, 08 Dec 2024 19:15:37 +0900,
Hajime Tazaki wrote:
> 
> This commit updates the behavior of signal handling under !MMU
> environment. 1) the stack preparation for the signal handlers and
> 2) restoration of stack after rt_sigreturn(2) syscall.  Those are needed
> as the stack usage on vfork(2) syscall is different.
> 
> It also adds the follow up routine for SIGSEGV as a signal delivery runs
> in the same stack frame while we have to avoid endless SIGSEGV.
(snip)
> diff --git a/arch/x86/um/nommu/signal.c b/arch/x86/um/nommu/signal.c
> new file mode 100644
> index 000000000000..a94e9b86273a
> --- /dev/null
> +++ b/arch/x86/um/nommu/signal.c
> @@ -0,0 +1,43 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/syscalls.h>
> +#include <linux/kernel.h>
> +#include <asm/sigframe.h>
> +
> +#include <sysdep/signal.h>
> +
> +int arch_setup_signal_stack_si(struct rt_sigframe __user **frame,
> +			       struct ksignal *ksig)
> +{
> +	int err = 0;
> +
> +	/*
> +	 * we need to push handler address at top of stack, as
> +	 * __kernel_vsyscall, called after this returns with ret with
> +	 * stack contents, thus push the handler here.
> +	 */
> +	*frame = (struct rt_sigframe __user *) ((unsigned long) *frame -
> +					       sizeof(unsigned long));
> +	err |= __put_user((unsigned long)ksig->ka.sa.sa_handler,
> +			  (unsigned long *)*frame);
> +
> +	return err;

this has a serious issue to handle signals.
I also found that manipulation of stack pointer makes the code
cryptic..

I will clean up those parts in the next version.

-- Hajime