Subject: Re: [net-next v2] tcp: Explicitly mark reserved field in tcp_zerocopy_receive args.

From: Leon Romanovsky <[email-masked]>
Date: Tue, 9 Feb 2021 21:01:52 +0200

On Tue, Feb 09, 2021 at 08:59:38AM -0800, Jakub Kicinski wrote:
> On Tue, 9 Feb 2021 08:15:11 +0200 Leon Romanovsky wrote:
> > At least in my tree, we have the length check:
> >   4155                 if (len > sizeof(zc)) {
> >   4156                         len = sizeof(zc);
> >   4157                         if (put_user(len, optlen))
> >   4158                                 return -EFAULT;
> >   4159                 }
> >
> >
> > Ad David wrote below, the "if (zc.reserved)" is enough.
> >
> > We have following options:
> > 1. Old kernel that have sizeof(sz) upto .reserved and old userspace
> > -> len <= sizeof(sz) - works correctly.
> > 2. Old kernel that have sizeof(sz) upto .reserved and new userspace that
> > sends larger struct -> "f (len > sizeof(zc))" will return -EFAULT
>
> Based on the code you quoted? I don't see how. Maybe I need a vacation.
> put_user() just copies len back to user space after truncation.

It is me who needs to go to vacation, you are right it doesn't return
for lengths larger than sizeof(zc).

>
> > 3. New kernel that have sizeof(sz) beyond reserved and old userspace
> > -> any new added field to struct sz should be checked and anyway it is the same as item 1.
> > 4. New kernel and new userspace
> > -> standard flow.