Re: bpf pointer alignment validation

[Daniel Borkmann]

On 05/10/2017 05:57 PM, David Miller wrote:

From: Daniel Borkmann <dan...@iogearbox.net>
Date: Wed, 10 May 2017 17:51:50 +0200

Would probably be good nevertheless to have this as a flag for
program loads, which gets then passed through to the verifier to
explicitly enable strict alignment checks.

Might certainly aide developing & testing programs on archs with
efficient unaligned access and later actually running them on archs
that don't have it. (And at minimum, it also helps for checking
the test suite against the verifier.)

Ok, I can implement this flag.

The only question is where to put it?  An unused bit in the program
type? :-)

See for example 7f677633379b ("bpf: introduce BPF_F_ALLOW_OVERRIDE
flag"). We can add a flags field to the prog loading part of union
bpf_attr; we would need to make sure to update BPF_PROG_LOAD_LAST_FIELD
to the new member, and to reject unknown flags, of course. Then the
syscall will handle compat with older binaries just fine by design,
the main bpf syscall code and CHECK_ATTR() macros will ensure this
(backward compat, and to a limited degree also forward compat).