RE: [PATCH 32/33] sctp: add sctp_sock_get_primary_addr

[David Laight]

From: David Laight
> Sent: 14 May 2020 10:51
> From: Marcelo Ricardo Leitner
> > Sent: 13 May 2020 19:03
> >
> > On Wed, May 13, 2020 at 08:26:47AM +0200, Christoph Hellwig wrote:
> > > Add a helper to directly get the SCTP_PRIMARY_ADDR sockopt from kernel
> > > space without going through a fake uaccess.
> >
> > Same comment as on the other dlm/sctp patch.
> 
> Wouldn't it be best to write sctp_[gs]etsockotp() that
> use a kernel buffer and then implement the user-space
> calls using a wrapper that does the copies to an on-stack
> (or malloced if big) buffer.

Actually looking at __sys_setsockopt() it calls
BPF_CGROUP_RUN_PROG_SETSOCKOPT() which (by the look of it)
can copy the user buffer into malloc()ed memory and
cause set_fs(KERNEL_DS) be called.

The only way to get rid of that set_fs() is to always
have the buffer in kernel memory when the underlying
setsockopt() code is called.

The comment above __sys_[sg]etsockopt() about not knowing
the length is just wrong.
It probably applied to getsockopt() in the dim and distant
past before it was made read-update.

	David

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