linux-kernel - [PATCH] fork: kill the pointless lower_32_bits() in create_io_thread(), kernel_thread(), and user_mode

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Message-ID: <20250820163946.GA18549@redhat.com>
Date: Wed, 20 Aug 2025 18:39:46 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christian Brauner <brauner@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] fork: kill the pointless lower_32_bits() in
 create_io_thread(), kernel_thread(), and user_mode_thread()

Unlike sys_clone(), these helpers have only in kernel users which should
pass the correct "flags" argument. lower_32_bits(flags) just adds the
unnecessary confusion and doesn't allow to use the CLONE_ flags which
don't fit into 32 bits.

create_io_thread() looks especially confusing because:

	- "flags" is a compile-time constant, so lower_32_bits() simply
	  has no effect

	- .exit_signal = (lower_32_bits(flags) & CSIGNAL) is harmless but
	  doesn't look right, copy_process(CLONE_THREAD) will ignore this
	  argument anyway.

None of these helpers actually need CLONE_UNTRACED or "& ~CSIGNAL", but
their presence does not add any confusion and improves code clarity.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/fork.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index af673856499d..0e8805d88ca6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2539,11 +2539,9 @@ struct task_struct * __init fork_idle(int cpu)
 struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
 {
 	unsigned long flags = CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|
-				CLONE_IO;
+			      CLONE_IO|CLONE_VM|CLONE_UNTRACED;
 	struct kernel_clone_args args = {
-		.flags		= ((lower_32_bits(flags) | CLONE_VM |
-				    CLONE_UNTRACED) & ~CSIGNAL),
-		.exit_signal	= (lower_32_bits(flags) & CSIGNAL),
+		.flags		= flags,
 		.fn		= fn,
 		.fn_arg		= arg,
 		.io_thread	= 1,
@@ -2655,9 +2653,8 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, const char *name,
 		    unsigned long flags)
 {
 	struct kernel_clone_args args = {
-		.flags		= ((lower_32_bits(flags) | CLONE_VM |
-				    CLONE_UNTRACED) & ~CSIGNAL),
-		.exit_signal	= (lower_32_bits(flags) & CSIGNAL),
+		.flags		= ((flags | CLONE_VM | CLONE_UNTRACED) & ~CSIGNAL),
+		.exit_signal	= (flags & CSIGNAL),
 		.fn		= fn,
 		.fn_arg		= arg,
 		.name		= name,
@@ -2673,9 +2670,8 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, const char *name,
 pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags)
 {
 	struct kernel_clone_args args = {
-		.flags		= ((lower_32_bits(flags) | CLONE_VM |
-				    CLONE_UNTRACED) & ~CSIGNAL),
-		.exit_signal	= (lower_32_bits(flags) & CSIGNAL),
+		.flags		= ((flags | CLONE_VM | CLONE_UNTRACED) & ~CSIGNAL),
+		.exit_signal	= (flags & CSIGNAL),
 		.fn		= fn,
 		.fn_arg		= arg,
 	};
-- 
2.25.1.362.g51ebf55