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

From: Alexei Starovoitov
Date: Fri Sep 26 2014 - 18:26:49 EST

On Fri, Sep 26, 2014 at 3:07 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Fri, Sep 26, 2014 at 3:03 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>> On Fri, Sep 26, 2014 at 2:47 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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
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