Re: eBPF verifier thoughts (Re: [PATCH v15 net-next 00/11] eBPF syscall, verifier, testsuite)

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Sep 26, 2014 at 3:26 PM, Alexei Starovoitov <ast@plumgrid.com> wrote:
> On Fri, Sep 26, 2014 at 3:07 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Fri, Sep 26, 2014 at 3:03 PM, Alexei Starovoitov <ast@plumgrid.com> wrote:
>>> On Fri, Sep 26, 2014 at 2:47 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>>
>>>> Can't you just disallow the 1-byte write to the stack?
>>>
>>> of course not.
>>> That would be extremely limiting to users.
>>> Can you actually see yourself living with stack that only
>>> allows 8-byte writes/reads?
>>> The stack usage will increase a lot, since all char/short
>>> stack variables will become 8-byte...
>>
>> How about requiring that sub-8-byte stack accesses only be to integer slots?
>
> you mean to reject the sub-8-byte write early if it's going
> into space where pointers were stored?
> That will limit stack reuse.
> gcc/llvm generate code where the same stack location
> is used by different variables during life of the function.
> So if I reject the write early, it will break otherwise valid
> programs.

I think that a sub-8-byte write to an integer slot should leave it as
an integer and a sub-8-byte write to a non-integer slot should turn
that slot into an integer (if conversions to integer are permitted) or
be rejected otherwise.  gcc/llvm could emit an 8-byte write first, as
needed, to make this valid.

Alternatively, an integer stack slot could have a bitmask indicating
which bytes are valid.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC