From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 20 Aug 2025 16:39:46 +0000 (+0200)
Subject: fork: kill the pointless lower_32_bits() in create_io_thread(), kernel_thread(),... 
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=f7071db2fe3d20991a35043b32012e1b37d32cc0;p=users%2Fhch%2Fmisc.git

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.

Link: https://lkml.kernel.org/r/20250820163946.GA18549@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/kernel/fork.c b/kernel/fork.c
index e06cfaa85a84..a8674ba2b33b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2537,11 +2537,9 @@ 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,
@@ -2653,9 +2651,8 @@ 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,
@@ -2671,9 +2668,8 @@ 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,
 	};