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fork: kill the pointless lower_32_bits() in create_io_thread(), kernel_thread(), and user_mode_thread()

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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>
This commit is contained in:
Oleg Nesterov
2025-08-20 18:39:46 +02:00
committed by Andrew Morton
parent b32730e68d
commit f7071db2fe
1 changed files with 6 additions and 10 deletions
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16
kernel/fork.c
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@@ -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,
};