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

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Message-ID: <20250116160403.GA3554@strace.io>
Date: Thu, 16 Jan 2025 18:04:03 +0200
From: "Dmitry V. Levin" <ldv@...ace.io>
To: Oleg Nesterov <oleg@...hat.com>
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 Thu, Jan 16, 2025 at 04:21:38PM +0100, Oleg Nesterov wrote:
> 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.

The idea is to use "op" to specify the operation, and "flags" to specify
future extensions to the operation.  For example, we could later add
PTRACE_SYSCALL_INFO_SECCOMP_SKIP operation to specify an exit-like
data for seccomp stops, or some flag to set instruction_pointer or
stack_pointer.  I don't think any of these would require a version field,
though.

That is, the zero check implied by copy_struct_from_user() is not really
needed here since the compatibility is tracked by "op" and "flags":
if "op" and "flags" do not instruct the kernel to use these unknown
extra bits, the kernel is not obliged to check them either.
For the same reason I don't think the kernel is obliged to read more
than sizeof(info) from userspace.

What would you recommend using instead of copy_struct_from_user in this
case?

-- 
ldv