linux-kernel - Re: [PATCH v2] signal: add procfd

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Date:   Sun, 2 Dec 2018 09:52:29 +0100
From:   Christian Brauner <christian@...uner.io>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, cyphar@...har.com,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Daniel Colascione <dancol@...gle.com>,
        Tim Murray <timmurray@...gle.com>, linux-man@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall

On Sat, Dec 01, 2018 at 09:28:47AM -0600, Eric W. Biederman wrote:
> 
> It just occurs to me that the simple way to implement
> procfd_sigqueueinfo info is like:
> 
> int copy_siginfo_from_user_any(kernel_siginfo_t *info, siginfo_t *uinfo)
> {
> #ifdef CONFIG_COMPAT
> 	if (in_compat_syscall)
> 		return copy_siginfo_from_user32(info, uinfo);

Right, though that would require a cast afaict.

static int __copy_siginfo_from_user_generic(int signo, kernel_siginfo_t *kinfo,
                                           siginfo_t *info)
{
#ifdef CONFIG_COMPAT
       if (in_compat_syscall())
               return __copy_siginfo_from_user32(
                       signo, kinfo, (struct compat_siginfo __user *)info);
#endif
       return __copy_siginfo_from_user(signo, kinfo, info);
}

It seems that a cast to (compat_siginfo __user *) should be safe in this
context? I've at least seen similar things done for __sys_sendmsg().

> #endif
> 	return copy_siginfo_from_user(info, uinfo);                        
> }
> 
> long procfd_sigqueueinfo(int fd, siginfo_t *uinfo)

**bikeshedding**
Not a fan of that name. I'm going to go with procfd_send_signal().
sigqueue gives non-native speakers a lot of room for spelling errors and
it always seemed opaque to me what this function is doing without
consulting the manpage. :)

> {
> 	kernel_siginfo info;
> 
>         if (copy_siginfo_from_user_any(&info, uinfo))
>         	return -EFAULT;
> 	...;                
> }
> 
> It looks like there is already a place in ptrace.c that already
> hand rolls copy_siginfo_from_user_any.
> 
> So while I would love to figure out the subset of siginfo_t tha we can
> just pass through, as I think that would make a better more forward
> compatible copy_siginfo_from_user32.  I think for this use case we just
> add the in_compat_syscall test and then we just need to ensure this new
> system call is placed in the proper places in the syscall table.
> 
> Because we will need 3 call sights: x86_64, x32 and ia32.  As the layout
> changes between those three subarchitecuters.
> 
> Eric
>