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

On Fri, Sep 26, 2014 at 3:26 PM, Alexei Starovoitov [off-list ref] wrote:

On Fri, Sep 26, 2014 at 3:07 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Fri, Sep 26, 2014 at 3:03 PM, Alexei Starovoitov [off-list ref] wrote:

quoted

On Fri, Sep 26, 2014 at 2:47 PM, Andy Lutomirski [off-list ref] wrote:

quoted

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