> On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <pet...@infradead.org> wrote:
>
> On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote:
>>> Can't we hijack the relocation records for these functions before they
>>> get thrown out in the (final) link pass or something?
>>
>> I could be talking out my arse here, but I thought we could do this,
>> too, then changed my mind. The relocation records give us the
>> location of the call or jump operand, but they don’t give the address
>> of the beginning of the instruction.
>
> But that's like 1 byte before the operand, right? We could even double check
> this by reading back that byte and ensuring it is in fact 0xE8 (CALL).
>
> AFAICT there is only the _1_ CALL encoding, and that is the 5 byte: E8
> <PLT32>,
> so if we have the PLT32 location, we also have the instruction location. Or am
> I missing something?
There’s also JMP and Jcc, any of which can be used for rail calls, but those
are also one byte. I suppose GCC is unlikely to emit a prefixed form of any of
these. So maybe we really can assume they’re all one byte.
But there is a nasty potential special case: anything that takes the function’s
address. This includes jump tables, computed gotos, and plain old function
pointers. And I suspect that any of these could have one of the rather large
number of CALL/JMP/Jcc bytes before the relocation by coincidence.
