Date:   Sun, 15 May 2022 18:39:06 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Airlie <airlied@...ux.ie>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>,
        Yanteng Si <siyanteng@...ngson.cn>,
        Huacai Chen <chenhuacai@...il.com>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Eric Biederman <ebiederm@...ssion.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH V10 14/22] LoongArch: Add signal handling support

Hi,

On 5/14/22 16:03, Huacai Chen wrote:
> Add ucontext/sigcontext definition and signal handling support for
> LoongArch.
>
> Cc: Eric Biederman <ebiederm@...ssion.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
>   arch/loongarch/include/uapi/asm/sigcontext.h |  63 ++
>   arch/loongarch/include/uapi/asm/signal.h     |  13 +
>   arch/loongarch/include/uapi/asm/ucontext.h   |  35 ++
>   arch/loongarch/kernel/signal.c               | 568 +++++++++++++++++++
>   4 files changed, 679 insertions(+)
>   create mode 100644 arch/loongarch/include/uapi/asm/sigcontext.h
>   create mode 100644 arch/loongarch/include/uapi/asm/signal.h
>   create mode 100644 arch/loongarch/include/uapi/asm/ucontext.h
>   create mode 100644 arch/loongarch/kernel/signal.c
>
> diff --git a/arch/loongarch/include/uapi/asm/sigcontext.h b/arch/loongarch/include/uapi/asm/sigcontext.h
> new file mode 100644
> index 000000000000..efeb8b3f8236
> --- /dev/null
> +++ b/arch/loongarch/include/uapi/asm/sigcontext.h
> @@ -0,0 +1,63 @@
> +/* SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note */
> +/*
> + * Author: Hanlu Li <lihanlu@...ngson.cn>
> + *         Huacai Chen <chenhuacai@...ngson.cn>
> + *
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + */
> +#ifndef _UAPI_ASM_SIGCONTEXT_H
> +#define _UAPI_ASM_SIGCONTEXT_H
> +
> +#include <linux/types.h>
> +#include <linux/posix_types.h>
> +
> +/* FP context was used */
> +#define USED_FP			(1 << 0)
> +/* Load/Store access flags for address error */
> +#define ADRERR_RD		(1 << 30)
> +#define ADRERR_WR		(1 << 31)
I've searched GitHub globally, and my local glibc checkout, for usages 
of these 3 constants, and there seems to be none; please consider 
removing these if doable. We don't want cruft in uapi right from the 
beginning.
> +
> +struct sigcontext {
> +	__u64	sc_pc;
> +	__u64	sc_regs[32];
> +	__u32	sc_flags;
> +	__u64	sc_extcontext[0] __attribute__((__aligned__(16)));
> +};
> +
> +#define CONTEXT_INFO_ALIGN	16
> +struct _ctxinfo {
> +	__u32	magic;
> +	__u32	size;
> +	__u64	padding;	/* padding to 16 bytes */
> +};
> +
> +/* FPU context */
> +#define FPU_CTX_MAGIC		0x46505501
> +#define FPU_CTX_ALIGN		8
> +struct fpu_context {
> +	__u64	regs[32];
> +	__u64	fcc;
> +	__u32	fcsr;
> +};
The 3 structs above should already see usage downstream (glibc and other 
low-level friends), so they probably shouldn't be touched by now. At 
least I can't see problems.
> +
> +/* LSX context */
> +#define LSX_CTX_MAGIC		0x53580001
> +#define LSX_CTX_ALIGN		16
> +struct lsx_context {
> +	__u64	regs[2*32];
> +	__u64	fcc;
> +	__u32	fcsr;
> +	__u32	vcsr;
> +};
> +
> +/* LASX context */
> +#define LASX_CTX_MAGIC		0x41535801
> +#define LASX_CTX_ALIGN		32
> +struct lasx_context {
> +	__u64	regs[4*32];
> +	__u64	fcc;
> +	__u32	fcsr;
> +	__u32	vcsr;
> +};
Do we want to freeze the LSX/LASX layout this early, before any detail 
of said extension are published? We'll need to update kernel later 
anyway, so I'd recommend leaving them out for the initial bring-up.
> +
> +#endif /* _UAPI_ASM_SIGCONTEXT_H */
> diff --git a/arch/loongarch/include/uapi/asm/signal.h b/arch/loongarch/include/uapi/asm/signal.h
> new file mode 100644
> index 000000000000..992d965aa13f
> --- /dev/null
> +++ b/arch/loongarch/include/uapi/asm/signal.h
> @@ -0,0 +1,13 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +/*
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + */
> +#ifndef _UAPI_ASM_SIGNAL_H
> +#define _UAPI_ASM_SIGNAL_H
> +
> +#define MINSIGSTKSZ 4096
> +#define SIGSTKSZ    16384

I've checked this set of values is also the choice for arch/alpha and 
arch/sparc, and that the Loongson devs are aware of the LWN article [1] 
about x86's story around insufficient signal stack size. So I'd approve 
this.

[1]: https://lwn.net/Articles/874846/

> +
> +#include <asm-generic/signal.h>
> +
> +#endif
> diff --git a/arch/loongarch/include/uapi/asm/ucontext.h b/arch/loongarch/include/uapi/asm/ucontext.h
> new file mode 100644
> index 000000000000..12577e22b1c7
> --- /dev/null
> +++ b/arch/loongarch/include/uapi/asm/ucontext.h
> @@ -0,0 +1,35 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef __LOONGARCH_UAPI_ASM_UCONTEXT_H
> +#define __LOONGARCH_UAPI_ASM_UCONTEXT_H
> +
> +/**
> + * struct ucontext - user context structure
> + * @uc_flags:
> + * @uc_link:
> + * @uc_stack:
> + * @uc_mcontext:	holds basic processor state
> + * @uc_sigmask:
> + * @uc_extcontext:	holds extended processor state
Document the other fields? ;-)
> + */
> +struct ucontext {
> +	unsigned long		uc_flags;
> +	struct ucontext		*uc_link;
> +	stack_t			uc_stack;
> +	sigset_t		uc_sigmask;
> +	/* There's some padding here to allow sigset_t to be expanded in the
> +	 * future.  Though this is unlikely, other architectures put uc_sigmask
> +	 * at the end of this structure and explicitly state it can be
> +	 * expanded, so we didn't want to box ourselves in here. */
> +	__u8		  __unused[1024 / 8 - sizeof(sigset_t)];
> +	/* We can't put uc_sigmask at the end of this structure because we need
> +	 * to be able to expand sigcontext in the future.  For example, the
> +	 * vector ISA extension will almost certainly add ISA state.  We want
> +	 * to ensure all user-visible ISA state can be saved and restored via a
> +	 * ucontext, so we're putting this at the end in order to allow for
> +	 * infinite extensibility.  Since we know this will be extended and we
> +	 * assume sigset_t won't be extended an extreme amount, we're
> +	 * prioritizing this. */
> +	struct sigcontext	uc_mcontext;
> +};
This is in turn copied from arch/riscv (who got it from arch/arm64, 
thanks to both arches!), so we shouldn't be too bad future-proof-wise. 
But do we want an attribution to arch/riscv too, like they did for 
arch/arm64? (Stating that you referred to arch/riscv for inspiration.)
> +
> +#endif /* __LOONGARCH_UAPI_ASM_UCONTEXT_H */
> diff --git a/arch/loongarch/kernel/signal.c b/arch/loongarch/kernel/signal.c
> new file mode 100644
> index 000000000000..489e169e1d13
> --- /dev/null
> +++ b/arch/loongarch/kernel/signal.c
> @@ -0,0 +1,568 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Author: Hanlu Li <lihanlu@...ngson.cn>
> + *         Huacai Chen <chenhuacai@...ngson.cn>
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + *
> + * Derived from MIPS:
> + * Copyright (C) 1991, 1992  Linus Torvalds
> + * Copyright (C) 1994 - 2000  Ralf Baechle
> + * Copyright (C) 1999, 2000 Silicon Graphics, Inc.
> + * Copyright (C) 2014, Imagination Technologies Ltd.
> + */
> +#include <linux/audit.h>
> +#include <linux/cache.h>
> +#include <linux/context_tracking.h>
> +#include <linux/irqflags.h>
> +#include <linux/sched.h>
> +#include <linux/mm.h>
> +#include <linux/personality.h>
> +#include <linux/smp.h>
> +#include <linux/kernel.h>
> +#include <linux/signal.h>
> +#include <linux/errno.h>
> +#include <linux/wait.h>
> +#include <linux/ptrace.h>
> +#include <linux/unistd.h>
> +#include <linux/compiler.h>
> +#include <linux/syscalls.h>
> +#include <linux/uaccess.h>
> +
> +#include <asm/asm.h>
> +#include <asm/cacheflush.h>
> +#include <asm/cpu-features.h>
> +#include <asm/fpu.h>
> +#include <asm/ucontext.h>
> +#include <asm/vdso.h>
> +
> +#ifdef DEBUG_SIG
> +#  define DEBUGP(fmt, args...) printk("%s: " fmt, __func__, ##args)
> +#else
> +#  define DEBUGP(fmt, args...)
> +#endif
> +
> +/* Make sure we will not lose FPU ownership */
> +#define lock_fpu_owner()	({ preempt_disable(); pagefault_disable(); })
> +#define unlock_fpu_owner()	({ pagefault_enable(); preempt_enable(); })
> +
> +/* Assembly functions to move context to/from the FPU */
> +extern asmlinkage int
> +_save_fp_context(void __user *fpregs, void __user *fcc, void __user *csr);
> +extern asmlinkage int
> +_restore_fp_context(void __user *fpregs, void __user *fcc, void __user *csr);
> +
> +struct rt_sigframe {
> +	struct siginfo rs_info;
> +	struct ucontext rs_uctx;
> +};
> +
> +struct _ctx_layout {
> +	struct _ctxinfo *addr;
> +	unsigned int size;
> +};
> +
> +struct extctx_layout {
> +	unsigned long size;
> +	unsigned int flags;
> +	struct _ctx_layout fpu;
> +	struct _ctx_layout lsx;
> +	struct _ctx_layout lasx;
> +	struct _ctx_layout end;
> +};
> +
> +static void __user *get_ctx_through_ctxinfo(struct _ctxinfo *info)
> +{
> +	return (void __user *)((char *)info + sizeof(struct _ctxinfo));
> +}
> +
> +/*
> + * Thread saved context copy to/from a signal context presumed to be on the
> + * user stack, and therefore accessed with appropriate macros from uaccess.h.
> + */
> +static int copy_fpu_to_sigcontext(struct fpu_context __user *ctx)
> +{
> +	int i;
> +	int err = 0;
> +	uint64_t __user *regs	= (uint64_t *)&ctx->regs;
> +	uint64_t __user *fcc	= &ctx->fcc;
> +	uint32_t __user *fcsr	= &ctx->fcsr;
> +
> +	for (i = 0; i < NUM_FPU_REGS; i++) {
> +		err |=
> +		    __put_user(get_fpr64(&current->thread.fpu.fpr[i], 0),
> +			       &regs[i]);
> +	}
> +	err |= __put_user(current->thread.fpu.fcc, fcc);
> +	err |= __put_user(current->thread.fpu.fcsr, fcsr);
> +
> +	return err;
> +}
> +
> +static int copy_fpu_from_sigcontext(struct fpu_context __user *ctx)
> +{
> +	int i;
> +	int err = 0;
> +	u64 fpr_val;
> +	uint64_t __user *regs	= (uint64_t *)&ctx->regs;
> +	uint64_t __user *fcc	= &ctx->fcc;
> +	uint32_t __user *fcsr	= &ctx->fcsr;
> +
> +	for (i = 0; i < NUM_FPU_REGS; i++) {
> +		err |= __get_user(fpr_val, &regs[i]);
> +		set_fpr64(&current->thread.fpu.fpr[i], 0, fpr_val);
> +	}
> +	err |= __get_user(current->thread.fpu.fcc, fcc);
> +	err |= __get_user(current->thread.fpu.fcsr, fcsr);
> +
> +	return err;
> +}
> +
> +/*
> + * Wrappers for the assembly _{save,restore}_fp_context functions.
> + */
> +static int save_hw_fpu_context(struct fpu_context __user *ctx)
> +{
> +	uint64_t __user *regs	= (uint64_t *)&ctx->regs;
> +	uint64_t __user *fcc	= &ctx->fcc;
> +	uint32_t __user *fcsr	= &ctx->fcsr;
> +
> +	return _save_fp_context(regs, fcc, fcsr);
> +}
> +
> +static int restore_hw_fpu_context(struct fpu_context __user *ctx)
> +{
> +	uint64_t __user *regs	= (uint64_t *)&ctx->regs;
> +	uint64_t __user *fcc	= &ctx->fcc;
> +	uint32_t __user *fcsr	= &ctx->fcsr;
> +
> +	return _restore_fp_context(regs, fcc, fcsr);
> +}
> +
> +int fpcsr_pending(unsigned int __user *fpcsr)
Is "fpcsr" just "fcsr"?
> +{
> +	int err, sig = 0;
> +	unsigned int csr, enabled;
> +
> +	err = __get_user(csr, fpcsr);
> +	enabled = ((csr & FPU_CSR_ALL_E) << 24);
> +	/*
> +	 * If the signal handler set some FPU exceptions, clear it and
> +	 * send SIGFPE.
> +	 */
> +	if (csr & enabled) {
> +		csr &= ~enabled;
> +		err |= __put_user(csr, fpcsr);
> +		sig = SIGFPE;
> +	}
> +	return err ?: sig;
> +}
> +
> +/*
> + * Helper routines
> + */
> +static int protected_save_fpu_context(struct extctx_layout *extctx)
> +{
> +	int err = 0;
> +	struct _ctxinfo __user *info = extctx->fpu.addr;
> +	struct fpu_context __user *fpu_ctx = (struct fpu_context *)get_ctx_through_ctxinfo(info);
> +	uint64_t __user *regs	= (uint64_t *)&fpu_ctx->regs;
> +	uint64_t __user *fcc	= &fpu_ctx->fcc;
> +	uint32_t __user *fcsr	= &fpu_ctx->fcsr;
> +
> +	while (1) {
> +		lock_fpu_owner();
> +		if (is_fpu_owner())
> +			err = save_hw_fpu_context(fpu_ctx);
> +		else
> +			err = copy_fpu_to_sigcontext(fpu_ctx);
> +		unlock_fpu_owner();
> +
> +		err |= __put_user(FPU_CTX_MAGIC, &info->magic);
> +		err |= __put_user(extctx->fpu.size, &info->size);
> +
> +		if (likely(!err))
> +			break;
> +		/* Touch the FPU context and try again */
> +		err = __put_user(0, &regs[0]) |
> +			__put_user(0, &regs[31]) |
> +			__put_user(0, fcc) |
> +			__put_user(0, fcsr);
> +		if (err)
> +			return err;	/* really bad sigcontext */
> +	}
> +
> +	return err;
> +}
> +
> +static int protected_restore_fpu_context(struct extctx_layout *extctx)
> +{
> +	int err = 0, sig = 0, tmp __maybe_unused;
> +	struct _ctxinfo __user *info = extctx->fpu.addr;
> +	struct fpu_context __user *fpu_ctx = (struct fpu_context *)get_ctx_through_ctxinfo(info);
> +	uint64_t __user *regs	= (uint64_t *)&fpu_ctx->regs;
> +	uint64_t __user *fcc	= &fpu_ctx->fcc;
> +	uint32_t __user *fcsr	= &fpu_ctx->fcsr;
> +
> +	err = sig = fpcsr_pending(fcsr);
> +	if (err < 0)
> +		return err;
> +
> +	while (1) {
> +		lock_fpu_owner();
> +		if (is_fpu_owner())
> +			err = restore_hw_fpu_context(fpu_ctx);
> +		else
> +			err = copy_fpu_from_sigcontext(fpu_ctx);
> +		unlock_fpu_owner();
> +
> +		if (likely(!err))
> +			break;
> +		/* Touch the FPU context and try again */
> +		err = __get_user(tmp, &regs[0]) |
> +			__get_user(tmp, &regs[31]) |
> +			__get_user(tmp, fcc) |
> +			__get_user(tmp, fcsr);
> +		if (err)
> +			break;	/* really bad sigcontext */
> +	}
> +
> +	return err ?: sig;
> +}
> +
> +static int setup_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
> +			    struct extctx_layout *extctx)
> +{
> +	int i, err = 0;
> +	struct _ctxinfo __user *info;
> +
> +	err |= __put_user(regs->csr_era, &sc->sc_pc);
> +	err |= __put_user(extctx->flags, &sc->sc_flags);
> +
> +	err |= __put_user(0, &sc->sc_regs[0]);
> +	for (i = 1; i < 32; i++)
> +		err |= __put_user(regs->regs[i], &sc->sc_regs[i]);
> +
> +	if (extctx->fpu.addr)
> +		err |= protected_save_fpu_context(extctx);
> +
> +	/* Set the "end" magic */
> +	info = (struct _ctxinfo *)extctx->end.addr;
> +	err |= __put_user(0, &info->magic);
> +	err |= __put_user(0, &info->size);
> +
> +	return err;
> +}
> +
> +static int parse_extcontext(struct sigcontext __user *sc, struct extctx_layout *extctx)
> +{
> +	int err = 0;
> +	unsigned int magic, size;
> +	struct _ctxinfo __user *info = (struct _ctxinfo __user *)&sc->sc_extcontext;
> +
> +	while(1) {
> +		err |= __get_user(magic, &info->magic);
> +		err |= __get_user(size, &info->size);
> +		if (err)
> +			return err;
> +
> +		switch (magic) {
> +		case 0: /* END */
> +			goto done;
> +
> +		case FPU_CTX_MAGIC:
> +			if (size < (sizeof(struct _ctxinfo) +
> +				    sizeof(struct fpu_context)))
> +				goto invalid;
> +			extctx->fpu.addr = info;
> +			break;
> +
> +		default:
> +			goto invalid;
> +		}
> +
> +		info = (struct _ctxinfo *)((char *)info + size);
> +	}
> +
> +done:
> +	return 0;
> +
> +invalid:
> +	return -EINVAL;
> +}
> +
> +static int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
> +{
> +	int i, err = 0;
> +	struct extctx_layout extctx;
> +
> +	memset(&extctx, 0, sizeof(struct extctx_layout));
> +
> +	err = __get_user(extctx.flags, &sc->sc_flags);
> +	if (err)
> +		goto bad;
> +
> +	err = parse_extcontext(sc, &extctx);
> +	if (err)
> +		goto bad;
> +
> +	conditional_used_math(extctx.flags & USED_FP);
> +
> +	/*
> +	 * The signal handler may have used FPU; give it up if the program
> +	 * doesn't want it following sigreturn.
> +	 */
> +	if (!(extctx.flags & USED_FP))
> +		lose_fpu(0);
> +
> +	/* Always make any pending restarted system calls return -EINTR */
> +	current->restart_block.fn = do_no_restart_syscall;
> +
> +	err |= __get_user(regs->csr_era, &sc->sc_pc);
> +	for (i = 1; i < 32; i++)
> +		err |= __get_user(regs->regs[i], &sc->sc_regs[i]);
> +
> +	if (extctx.fpu.addr)
> +		err |= protected_restore_fpu_context(&extctx);
> +
> +bad:
> +	return err;
> +}
> +
> +static unsigned int handle_flags(void)
> +{
> +	unsigned int flags = 0;
> +
> +	flags |= used_math() ? USED_FP : 0;
> +
> +	switch (current->thread.error_code) {
> +	case 1:
> +		flags |= ADRERR_RD;
> +		break;
> +	case 2:
> +		flags |= ADRERR_WR;
> +		break;
> +	}
> +
> +	return flags;
> +}
> +
> +static unsigned long extframe_alloc(struct extctx_layout *extctx,
> +				    struct _ctx_layout *layout,
> +				    size_t size, unsigned int align, unsigned long base)
> +{
> +	unsigned long new_base = base - size;
> +
> +	new_base = round_down(new_base, (align < 16 ? 16 : align));
> +	new_base -= sizeof(struct _ctxinfo);
> +
> +	layout->addr = (void *)new_base;
> +	layout->size = (unsigned int)(base - new_base);
> +	extctx->size += layout->size;
> +
> +	return new_base;
> +}
> +
> +static unsigned long setup_extcontext(struct extctx_layout *extctx, unsigned long sp)
> +{
> +	unsigned long new_sp = sp;
> +
> +	memset(extctx, 0, sizeof(struct extctx_layout));
> +
> +	extctx->flags = handle_flags();
> +
> +	/* Grow down, alloc "end" context info first. */
> +	new_sp -= sizeof(struct _ctxinfo);
> +	extctx->end.addr = (void *)new_sp;
> +	extctx->end.size = (unsigned int)sizeof(struct _ctxinfo);
> +	extctx->size += extctx->end.size;
> +
> +	if (extctx->flags & USED_FP) {
> +		if (cpu_has_fpu)
> +			new_sp = extframe_alloc(extctx, &extctx->fpu,
> +			  sizeof(struct fpu_context), FPU_CTX_ALIGN, new_sp);
> +	}
> +
> +	return new_sp;
> +}
> +
> +void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
> +			  struct extctx_layout *extctx)
> +{
> +	unsigned long sp;
> +
> +	/* Default to using normal stack */
> +	sp = regs->regs[3];
> +
> +	/*
> +	 * If we are on the alternate signal stack and would overflow it, don't.
> +	 * Return an always-bogus address instead so we will die with SIGSEGV.
> +	 */
> +	if (on_sig_stack(sp) &&
> +	    !likely(on_sig_stack(sp - sizeof(struct rt_sigframe))))
> +		return (void __user __force *)(-1UL);
> +
> +	sp = sigsp(sp, ksig);
> +	sp = round_down(sp, 16);
> +	sp = setup_extcontext(extctx, sp);
> +	sp -= sizeof(struct rt_sigframe);
> +
> +	if (!IS_ALIGNED(sp, 16))
> +		BUG();
> +
> +	return (void __user *)sp;
> +}
> +
> +/*
> + * Atomically swap in the new signal mask, and wait for a signal.
> + */
> +
> +asmlinkage long sys_rt_sigreturn(void)
> +{
> +	int sig;
> +	sigset_t set;
> +	struct pt_regs *regs;
> +	struct rt_sigframe __user *frame;
> +
> +	regs = current_pt_regs();
> +	frame = (struct rt_sigframe __user *)regs->regs[3];
> +	if (!access_ok(frame, sizeof(*frame)))
> +		goto badframe;
> +	if (__copy_from_user(&set, &frame->rs_uctx.uc_sigmask, sizeof(set)))
> +		goto badframe;
> +
> +	set_current_blocked(&set);
> +
> +	sig = restore_sigcontext(regs, &frame->rs_uctx.uc_mcontext);
> +	if (sig < 0)
> +		goto badframe;
> +	else if (sig)
> +		force_sig(sig);
> +
> +	regs->regs[0] = 0; /* No syscall restarting */
> +	if (restore_altstack(&frame->rs_uctx.uc_stack))
> +		goto badframe;
> +
> +	return regs->regs[4];
> +
> +badframe:
> +	force_sig(SIGSEGV);
> +	return 0;
> +}
> +
> +static int setup_rt_frame(void *sig_return, struct ksignal *ksig,
> +			  struct pt_regs *regs, sigset_t *set)
> +{
> +	int err = 0;
> +	struct extctx_layout extctx;
> +	struct rt_sigframe __user *frame;
> +
> +	frame = get_sigframe(ksig, regs, &extctx);
> +	if (!access_ok(frame, sizeof(*frame) + extctx.size))
> +		return -EFAULT;
> +
> +	/* Create siginfo.  */
> +	err |= copy_siginfo_to_user(&frame->rs_info, &ksig->info);
> +
> +	/* Create the ucontext.	 */
> +	err |= __put_user(0, &frame->rs_uctx.uc_flags);
> +	err |= __put_user(NULL, &frame->rs_uctx.uc_link);
> +	err |= __save_altstack(&frame->rs_uctx.uc_stack, regs->regs[3]);
> +	err |= setup_sigcontext(regs, &frame->rs_uctx.uc_mcontext, &extctx);
> +	err |= __copy_to_user(&frame->rs_uctx.uc_sigmask, set, sizeof(*set));
> +
> +	if (err)
> +		return -EFAULT;
> +
> +	/*
> +	 * Arguments to signal handler:
> +	 *
> +	 *   a0 = signal number
> +	 *   a1 = pointer to siginfo
> +	 *   a2 = pointer to ucontext
> +	 *
> +	 * c0_era point to the signal handler, $r3 (sp) points to
> +	 * the struct rt_sigframe.
> +	 */
> +	regs->regs[4] = ksig->sig;
> +	regs->regs[5] = (unsigned long) &frame->rs_info;
> +	regs->regs[6] = (unsigned long) &frame->rs_uctx;
> +	regs->regs[3] = (unsigned long) frame;
> +	regs->regs[1] = (unsigned long) sig_return;
> +	regs->csr_era = (unsigned long) ksig->ka.sa.sa_handler;
> +
> +	DEBUGP("SIG deliver (%s:%d): sp=0x%p pc=0x%lx ra=0x%lx\n",
> +	       current->comm, current->pid,
> +	       frame, regs->csr_era, regs->regs[1]);
> +
> +	return 0;
> +}
> +
> +static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
> +{
> +	int ret;
> +	sigset_t *oldset = sigmask_to_save();
> +	void *vdso = current->mm->context.vdso;
> +
> +	/* Are we from a system call? */
> +	if (regs->regs[0]) {
> +		switch (regs->regs[4]) {
> +		case -ERESTART_RESTARTBLOCK:
> +		case -ERESTARTNOHAND:
> +			regs->regs[4] = -EINTR;
> +			break;
> +		case -ERESTARTSYS:
> +			if (!(ksig->ka.sa.sa_flags & SA_RESTART)) {
> +				regs->regs[4] = -EINTR;
> +				break;
> +			}
> +			fallthrough;
> +		case -ERESTARTNOINTR:
> +			regs->regs[4] = regs->orig_a0;
> +			regs->csr_era -= 4;
> +		}
> +
> +		regs->regs[0] = 0;	/* Don't deal with this again.	*/
> +	}
> +
> +	rseq_signal_deliver(ksig, regs);
> +
> +	ret = setup_rt_frame(vdso + current->thread.vdso->offset_sigreturn, ksig, regs, oldset);
> +
> +	signal_setup_done(ret, ksig, 0);
> +}
> +
> +void arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal)
> +{
> +	struct ksignal ksig;
> +
> +	if (has_signal && get_signal(&ksig)) {
> +		/* Whee!  Actually deliver the signal.	*/
> +		handle_signal(&ksig, regs);
> +		return;
> +	}
> +
> +	/* Are we from a system call? */
> +	if (regs->regs[0]) {
> +		switch (regs->regs[4]) {
> +		case -ERESTARTNOHAND:
> +		case -ERESTARTSYS:
> +		case -ERESTARTNOINTR:
> +			regs->regs[4] = regs->orig_a0;
> +			regs->csr_era -= 4;
> +			break;
> +
> +		case -ERESTART_RESTARTBLOCK:
> +			regs->regs[4] = regs->orig_a0;
> +			regs->regs[11] = __NR_restart_syscall;
> +			regs->csr_era -= 4;
> +			break;
> +		}
> +		regs->regs[0] = 0;	/* Don't deal with this again.	*/
> +	}
> +
> +	/*
> +	 * If there's no signal to deliver, we just put the saved sigmask
> +	 * back
> +	 */
> +	restore_saved_sigmask();
> +}

This patch looks reasonably good, with the comments addressed (revised 
or replied in case you want to keep as-is):

Reviewed-by: WANG Xuerui <git@...0n.name>

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