[tip: x86/fpu] x86/fpu: Always use memcpy_and_pad() in arch_dup_task_struct()

[tip-bot2 for Oleg Nesterov]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     2d299e3d773d519ee93e5aaa3ffddd4a6276b005
Gitweb:        https://git.kernel.org/tip/2d299e3d773d519ee93e5aaa3ffddd4a6276b005
Author:        Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate:    Sat, 03 May 2025 16:38:50 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Sun, 04 May 2025 10:29:25 +02:00

x86/fpu: Always use memcpy_and_pad() in arch_dup_task_struct()

It makes no sense to copy the bytes after sizeof(struct task_struct),
FPU state will be initialized in fpu_clone().

A plain memcpy(dst, src, sizeof(struct task_struct)) should work too,
but "_and_pad" looks safer.

[ mingo: Simplify it a bit more. ]

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Chang S . Bae <chang.seok.bae@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250503143850.GA8997@xxxxxxxxxx
---
 arch/x86/kernel/process.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 7a1bfb6..9e61807 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -93,11 +93,8 @@ EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid);
  */
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
-	/* init_task is not dynamically sized (incomplete FPU state) */
-	if (unlikely(src == &init_task))
-		memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(init_task), 0);
-	else
-		memcpy(dst, src, arch_task_struct_size);
+	/* fpu_clone() will initialize the "dst_fpu" memory */
+	memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(*dst), 0);
 
 #ifdef CONFIG_VM86
 	dst->thread.vm86 = NULL;