linux-kernel - [PATCH RESEND v11 06/13] x86/um: nommu: process/thread handling

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Message-ID: <5c56554cf8c0215ea378b4c5935664ebc7a27d50.1758181109.git.thehajime@gmail.com>
Date: Thu, 18 Sep 2025 16:39:01 +0900
From: Hajime Tazaki <thehajime@...il.com>
To: linux-um@...ts.infradead.org
Cc: thehajime@...il.com,
	ricarkol@...gle.com,
	Liam.Howlett@...cle.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH RESEND v11 06/13] x86/um: nommu: process/thread handling

Since ptrace facility isn't used under !MMU of UML, there is different
code path to invoke processes/threads; there are no external process
used, and need to properly configure some of registers (fs segment
register for TLS, etc) on every context switch, etc.

Signals aren't delivered in non-ptrace syscall entry/leave so, we also
need to handle pending signal by ourselves.

ptrace related syscalls are not tested yet so, marked
arch_has_single_step() unsupported in !MMU environment.

Signed-off-by: Hajime Tazaki <thehajime@...il.com>
Signed-off-by: Ricardo Koller <ricarkol@...gle.com>
---
 arch/um/include/asm/ptrace-generic.h |  2 +-
 arch/x86/um/Makefile                 |  3 +-
 arch/x86/um/nommu/Makefile           |  2 +-
 arch/x86/um/nommu/entry_64.S         |  2 ++
 arch/x86/um/nommu/syscalls.h         |  2 ++
 arch/x86/um/nommu/syscalls_64.c      | 50 ++++++++++++++++++++++++++++
 6 files changed, 58 insertions(+), 3 deletions(-)
 create mode 100644 arch/x86/um/nommu/syscalls_64.c

diff --git a/arch/um/include/asm/ptrace-generic.h b/arch/um/include/asm/ptrace-generic.h
index 62e9916078ec..5aa38fe6b2fb 100644
--- a/arch/um/include/asm/ptrace-generic.h
+++ b/arch/um/include/asm/ptrace-generic.h
@@ -14,7 +14,7 @@ struct pt_regs {
 	struct uml_pt_regs regs;
 };
 
-#define arch_has_single_step()	(1)
+#define arch_has_single_step()	(IS_ENABLED(CONFIG_MMU))
 
 #define EMPTY_REGS { .regs = EMPTY_UML_PT_REGS }
 
diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
index 227af2a987e2..53c9ebb3c41c 100644
--- a/arch/x86/um/Makefile
+++ b/arch/x86/um/Makefile
@@ -27,7 +27,8 @@ subarch-y += ../kernel/sys_ia32.o
 
 else
 
-obj-y += syscalls_64.o vdso/
+obj-y += vdso/
+obj-$(CONFIG_MMU) += syscalls_64.o
 
 subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o \
 	../lib/memmove_64.o ../lib/memset_64.o
diff --git a/arch/x86/um/nommu/Makefile b/arch/x86/um/nommu/Makefile
index ebe47d4836f4..4018d9e0aba0 100644
--- a/arch/x86/um/nommu/Makefile
+++ b/arch/x86/um/nommu/Makefile
@@ -5,4 +5,4 @@ else
 	BITS := 64
 endif
 
-obj-y = do_syscall_$(BITS).o entry_$(BITS).o os-Linux/
+obj-y = do_syscall_$(BITS).o entry_$(BITS).o syscalls_$(BITS).o os-Linux/
diff --git a/arch/x86/um/nommu/entry_64.S b/arch/x86/um/nommu/entry_64.S
index 485c578aae64..a58922fc81e5 100644
--- a/arch/x86/um/nommu/entry_64.S
+++ b/arch/x86/um/nommu/entry_64.S
@@ -86,6 +86,8 @@ END(__kernel_vsyscall)
  */
 ENTRY(userspace)
 
+	/* set stack and pt_regs to the current task */
+	call	arch_set_stack_to_current
 	/* clear direction flag to meet ABI */
 	cld
 	/* align the stack for x86_64 ABI */
diff --git a/arch/x86/um/nommu/syscalls.h b/arch/x86/um/nommu/syscalls.h
index a2433756b1fc..ce16bf8abd59 100644
--- a/arch/x86/um/nommu/syscalls.h
+++ b/arch/x86/um/nommu/syscalls.h
@@ -13,4 +13,6 @@
 extern long current_top_of_stack;
 extern long current_ptregs;
 
+void arch_set_stack_to_current(void);
+
 #endif
diff --git a/arch/x86/um/nommu/syscalls_64.c b/arch/x86/um/nommu/syscalls_64.c
new file mode 100644
index 000000000000..d56027ebc651
--- /dev/null
+++ b/arch/x86/um/nommu/syscalls_64.c
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2003 - 2007 Jeff Dike (jdike@...dtoit,linux.intel}.com)
+ * Copyright 2003 PathScale, Inc.
+ *
+ * Licensed under the GPL
+ */
+
+#include <linux/sched.h>
+#include <linux/sched/mm.h>
+#include <linux/syscalls.h>
+#include <linux/uaccess.h>
+#include <asm/prctl.h> /* XXX This should get the constants from libc */
+#include <registers.h>
+#include <os.h>
+#include "syscalls.h"
+
+void arch_set_stack_to_current(void)
+{
+	current_top_of_stack = task_top_of_stack(current);
+	current_ptregs = (long)task_pt_regs(current);
+}
+
+void arch_switch_to(struct task_struct *to)
+{
+	/*
+	 * In !CONFIG_MMU, it doesn't ptrace thus,
+	 * The FS_BASE registers are saved here.
+	 */
+	current_top_of_stack = task_top_of_stack(to);
+	current_ptregs = (long)task_pt_regs(to);
+
+	if ((to->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)] == 0) ||
+	    (to->mm == NULL))
+		return;
+
+	/* this changes the FS on every context switch */
+	arch_prctl(to, ARCH_SET_FS,
+		   (void __user *) to->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)]);
+}
+
+SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
+		unsigned long, prot, unsigned long, flags,
+		unsigned long, fd, unsigned long, off)
+{
+	if (off & ~PAGE_MASK)
+		return -EINVAL;
+
+	return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
+}
-- 
2.43.0