linux-kernel - Re: [PATCH 3/3] x86: avoid copying dynamic FP state from init

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Message-ID: <Z78SVdv5YKie-Mcp@gmail.com>
Date: Wed, 26 Feb 2025 14:08:37 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Benjamin Berg <benjamin@...solutions.net>
Cc: linux-arch@...r.kernel.org, linux-um@...ts.infradead.org,
	x86@...nel.org, briannorris@...omium.org,
	linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
	Benjamin Berg <benjamin.berg@...el.com>
Subject: Re: [PATCH 3/3] x86: avoid copying dynamic FP state from init_task


* Benjamin Berg <benjamin@...solutions.net> wrote:

> From: Benjamin Berg <benjamin.berg@...el.com>
> 
> The init_task instance of struct task_struct is statically allocated and
> may not contain the full FP state for userspace. As such, limit the copy
> to the valid area of init_task and fill the rest with zero.
> 
> Note that the FP state is only needed for userspace, and as such it is
> entirely reasonable for init_task to not contain parts of it.
> 
> Signed-off-by: Benjamin Berg <benjamin.berg@...el.com>
> Fixes: 5aaeb5c01c5b ("x86/fpu, sched: Introduce CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT and use it on x86")
> ---
>  arch/x86/kernel/process.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index f63f8fd00a91..1be45fe70cad 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -92,7 +92,15 @@ EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid);
>   */
>  int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>  {
> -	memcpy(dst, src, arch_task_struct_size);
> +	/* init_task is not dynamically sized (incomplete FPU state) */
> +	if (unlikely(src == &init_task)) {
> +		memcpy(dst, src, sizeof(init_task));
> +		memset((void *)dst + sizeof(init_task), 0,
> +		       arch_task_struct_size - sizeof(init_task));
> +	} else {
> +		memcpy(dst, src, arch_task_struct_size);

Note that this patch, while it still applies cleanly, crashes/hangs the 
x86-64 defconfig kernel bootup in the early boot phase in a KVM guest 
bootup.

Thanks,

	Ingo