[PATCH] Add an option to allow JumpInstrTables to set unnamed_addr and jumptable on all address-taken functions

[Nick Lewycky]

Tom Roeder wrote:
> On Wed, Jun 18, 2014 at 11:50 PM, Nick Lewycky<nicholas at mxc.ca>  wrote:
>> Tom Roeder wrote:
>>>
>>> On Wed, Jun 18, 2014 at 12:00 AM, Nick Lewycky<nicholas at mxc.ca>   wrote:
>>>>
>>>> Tom Roeder wrote:
>>>>>
>>>>>
>>>>> This small patch adds a new command flag -jump-table-all and an
>>>>> associated
>>>>> TargetOptions field JumpTableAll, and it modifies JumpInstrTables to add
>>>>> unnamed_addr and jumptable to all address-taken functions if the flag is
>>>>> set.
>>>>>
>>>>> This is important for applications like my implementation of
>>>>> control-flow
>>>>> integrity, which want to operate over all address-taken functions and
>>>>> not
>>>>> just the ones that happened to have been marked jumptable.
>>>>
>>>>
>>>>
>>>> Why aren't all functions being marked jumptable when building with CFI?
>>>
>>>
>>> The CFI pass has to operate after the jumptable pass so it has the
>>> requisite information (all the transformations performed by
>>> JumpInstrTable). If I want to mark all functions jumptable, then I
>>> have to do it before JumpInstrTable runs, or during. So, there has to
>>> be something separate from the CFI pass itself that makes this happen.
>>> I could add a separate pass, or I could make JumpInstrTables know
>>> about CFI and set everything as jumptable when it sees CFI requested.
>>> I thought instead it made sense to have this as a feature of
>>> JumpInstrTables itself.
>>
>>
>> I guess what I thought would happen is that you pass "-fcfi" to clang (or
>> whatever frontend) and it marks everything jumptable unnamed_addr for you?
>> ANd also adds the CFI pass to the pass manager?
>
> I don't see why this should have anything to do with a front end
> component, though it might make sense eventually to have a high-level
> flag like that in clang that sets a lower-level flag. Currently, I
> just pass flags directly to LTO through clang with
>
> -Wl,--plugin-opt=-jump-table-type=arity

Mostly I want to have a single place to decide whether we're going to do 
the jump table transform or not. The reason it goes in the frontend is 
because it actually changes language semantics, the frontend may want to 
emit different IR, or may want to warn the user that "&func1 == 
my_funcptr" isn't going to work when CFI is on. If you did have it in 
the frontend, I don't see why you would also need a plugin option.

> Would you be satisfied with me adding a flag in CommandFlags that was
> something like -apply-cfi? This would then set all address-taken
> functions to be unnamed_addr+jumptable and cause the CFI pass to run.
>