Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

On Thu, Jul 12, 2018 at 08:50:46AM -0700, Linus Torvalds wrote:

But "write()" simply is *NOT* a good "command" interface. If you want
to send a command, use an ioctl or a system call.

Because it's not just about credentials. It's not just about fooling a
suid app into writing an error message to a descriptor you wrote. It's
also about things like "splice()", which can write to your target
using a kernel buffer, and thus trick you into doing a command while
we have the context set to kernel addresses.

Wait a sec - that's only a problem if your command contains pointer-chasing
et.al.  Which is why e.g. /dev/sg is fucked in head.  But for something that
is plain text, what's the problem with splice/write/sendmsg/whatever?

I'm not talking about this particular interface, but "write is bad for
commands" as general policy looks missing the point.  If anything, it's
pointer-chasing crap that should be banned everywhere.  Just look at SG_IO -
it's a ioctl, and it's absolute garbage...