linux-kernel - [PATCH tip/x86/fpu 4/6] x86/fpu: arch_dup_task_struct: always use memcpy_and

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Message-ID: <20250503143850.GA8997@redhat.com>
Date: Sat, 3 May 2025 16:38:50 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave@...1.net>, Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Chang S . Bae" <chang.seok.bae@...el.com>
Subject: [PATCH tip/x86/fpu 4/6] x86/fpu: arch_dup_task_struct: always use
 memcpy_and_pad()

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 more safe.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 arch/x86/kernel/process.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 7a1bfb61d86f..d01adc028274 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -93,11 +93,9 @@ 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(struct task_struct), 0);
 
 #ifdef CONFIG_VM86
 	dst->thread.vm86 = NULL;
-- 
2.25.1.362.g51ebf55