linux-kernel - Re: [PATCH v2 6/7] ptrace: introduce PTRACE_SET_SYSCALL

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Message-ID: <20250116152137.GE21801@redhat.com>
Date: Thu, 16 Jan 2025 16:21:38 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Dmitry V. Levin" <ldv@...ace.io>
Cc: Eugene Syromyatnikov <evgsyr@...il.com>,
	Mike Frysinger <vapier@...too.org>,
	Renzo Davoli <renzo@...unibo.it>,
	Davide Berardi <berardi.dav@...il.com>,
	strace-devel@...ts.strace.io, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v2 6/7] ptrace: introduce PTRACE_SET_SYSCALL_INFO request

On 01/13, Dmitry V. Levin wrote:
>
> +static int
> +ptrace_set_syscall_info(struct task_struct *child, unsigned long user_size,
> +			void __user *datavp)
> +{
> +	struct pt_regs *regs = task_pt_regs(child);
> +	struct ptrace_syscall_info info;
> +	int error;
> +
> +	BUILD_BUG_ON(sizeof(struct ptrace_syscall_info) < PTRACE_SYSCALL_INFO_SIZE_VER0);
> +
> +	if (user_size < PTRACE_SYSCALL_INFO_SIZE_VER0 || user_size > PAGE_SIZE)
> +		return -EINVAL;
> +
> +	error = copy_struct_from_user(&info, sizeof(info), datavp, user_size);

OK, I certainly can't understand why copy_struct_from_user/check_zeroed_user
is useful, at least in this case. In particular, this won't allow to run the
new code (which uses the "extended" ptrace_syscall_info) on the older kernels?

Can't we just use user_size as a version number?

We can also turn info->reserved into info->version filled by
ptrace_get_syscall_info().

ptrace_set_syscall_info() can check that info->version matches user_size.

Oleg.