linux-kernel - [PATCH RFC v2 04/28] ARM: process: Rewrite ret_from

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Message-Id: <20241029-arm-generic-entry-v2-4-573519abef38@linaro.org>
Date: Tue, 29 Oct 2024 11:52:44 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Oleg Nesterov <oleg@...hat.com>, Russell King <linux@...linux.org.uk>, 
 Kees Cook <kees@...nel.org>, Andy Lutomirski <luto@...capital.net>, 
 Will Drewry <wad@...omium.org>, Frederic Weisbecker <frederic@...nel.org>, 
 "Paul E. McKenney" <paulmck@...nel.org>, 
 Jinjie Ruan <ruanjinjie@...wei.com>, Arnd Bergmann <arnd@...db.de>, 
 Ard Biesheuvel <ardb@...nel.org>, Al Viro <viro@...iv.linux.org.uk>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH RFC v2 04/28] ARM: process: Rewrite ret_from_fork i C

Move the direct assembly call to schedule_tail() and the
call starting a kernel thread into a C version of ret_from_fork()
in process.c and rename the old ret_from_fork to ret_from_fork_asm
following the pattern in x86.

Leave a comment on what we will do when we get to switching to
generic entry with this as a base.

Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
 arch/arm/include/asm/switch_to.h |  4 ++++
 arch/arm/kernel/entry-common.S   | 22 +++++++++++++++-------
 arch/arm/kernel/process.c        | 24 ++++++++++++++++++++++--
 3 files changed, 41 insertions(+), 9 deletions(-)

diff --git a/arch/arm/include/asm/switch_to.h b/arch/arm/include/asm/switch_to.h
index 9372348516ce..18d084aff5a4 100644
--- a/arch/arm/include/asm/switch_to.h
+++ b/arch/arm/include/asm/switch_to.h
@@ -17,6 +17,10 @@
 #define __complete_pending_tlbi()
 #endif
 
+asmlinkage void ret_from_fork_asm(void);
+__visible void ret_from_fork(struct task_struct *prev, struct pt_regs *regs,
+			     int (*fn)(void *), void *fn_arg);
+
 /*
  * switch_to(prev, next) should switch from task `prev' to `next'
  * `prev' will never be the same as `next'.  schedule() itself
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 2f93c2f6d5b8..df6961a1006b 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -5,6 +5,7 @@
  *  Copyright (C) 2000 Russell King
  */
 
+#include <linux/cfi_types.h>
 #include <asm/assembler.h>
 #include <asm/unistd.h>
 #include <asm/ftrace.h>
@@ -128,15 +129,22 @@ ENDPROC(ret_to_user)
 /*
  * This is how we return from a fork.
  */
-ENTRY(ret_from_fork)
-	bl	schedule_tail
-	cmp	r5, #0
-	movne	r0, r4
-	badrne	lr, 1f
-	retne	r5
+SYM_TYPED_FUNC_START(ret_from_fork_asm)
+	/*
+	 * ret_from_fork() expects:
+	 * r0: prev
+	 * r1: regs
+	 * r2: fn() for kernel process or NULL
+	 * r3: fn_arg for kernel process or N/A
+	 */
+	mov	r1, sp
+	mov	r2, r5
+	mov	r3, r4
+	bl	ret_from_fork
+	mov	r0, #0
 1:	get_thread_info tsk
 	b	ret_to_user
-ENDPROC(ret_from_fork)
+SYM_FUNC_END(ret_from_fork_asm)
 
 /*=============================================================================
  * SWI handler
diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index e16ed102960c..81c1c3f98834 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -29,6 +29,7 @@
 #include <asm/processor.h>
 #include <asm/thread_notify.h>
 #include <asm/stacktrace.h>
+#include <asm/switch_to.h>
 #include <asm/system_misc.h>
 #include <asm/mach/time.h>
 #include <asm/tls.h>
@@ -230,7 +231,26 @@ void flush_thread(void)
 	thread_notify(THREAD_NOTIFY_FLUSH, thread);
 }
 
-asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");
+/* The assembly stub sets up the parameters and calls ret_from_fork() */
+asmlinkage void ret_from_fork_asm(void) __asm__("ret_from_fork_asm");
+
+__visible void ret_from_fork(struct task_struct *prev, struct pt_regs *regs,
+				     int (*fn)(void *), void *fn_arg)
+{
+	schedule_tail(prev);
+
+	/* Is this a kernel thread? */
+	if (unlikely(fn)) {
+		fn(fn_arg);
+		/*
+		 * A kernel thread is allowed to return here after successfully
+		 * calling kernel_execve().  Exit to userspace to complete the
+		 * execve() syscall.
+		 */
+	}
+
+	/* syscall_exit_to_user_mode(regs); here once we switch to generic entry */
+}
 
 int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 {
@@ -263,7 +283,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		thread->cpu_context.r5 = (unsigned long)args->fn;
 		childregs->ARM_cpsr = SVC_MODE;
 	}
-	thread->cpu_context.pc = (unsigned long)ret_from_fork;
+	thread->cpu_context.pc = (unsigned long)ret_from_fork_asm;
 	thread->cpu_context.sp = (unsigned long)childregs;
 
 	clear_ptrace_hw_breakpoint(p);

-- 
2.46.2