Re: [PATCH v12 2/3] ptrace,syscall_user_dispatch: checkpoint/restore support for SUD

[Dmitry Safonov]

On 2/28/23 16:52, Oleg Nesterov wrote:
> On 02/27, Dmitry Safonov wrote:
>>
>>> +int syscall_user_dispatch_set_config(struct task_struct *task, unsigned long size,
>>> +                                    void __user *data)
>>> +{
>>> +       int rc;
>>> +       struct ptrace_sud_config cfg;
>>> +
>>> +       if (size != sizeof(struct ptrace_sud_config))
>>> +               return -EINVAL;
>>> +
>>> +       if (copy_from_user(&cfg, data, sizeof(struct ptrace_sud_config)))
>>> +               return -EFAULT;
>>
>> It seems that the tool you want here would be copy_struct_from_user(),
>> which is designed for extendable syscalls.
> 
> Hmm. Why?
> 
> In this case ksize == usize, so why do we need copy_struct_from_user ?

In case the structure extends in future, that will let newer userspace
run on an older kernel (as long as it doesn't use [set] any new fields).
With regular sizeof(struct ptrace_sud_config) instead of adding
size-related defines.

It was Christian's idea how-to add/design new syscalls in an
"extensible" manner. Here are his LPC slides:
https://lpc.events/event/7/contributions/657/attachments/639/1159/extensible_syscalls.pdf
[7/18 slide on checks]
And an LWN article:
https://lwn.net/Articles/830666/

Thanks,
          Dmitry