Messages in this thread |  | | Date | Mon, 8 Oct 2018 12:39:28 -0400 | From | Steven Rostedt <> | Subject | Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function" |
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On Mon, 8 Oct 2018 09:29:56 -0700 Andy Lutomirski <luto@amacapital.net> wrote:
> > On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <peterz@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. >
FYI, your email client is horrible to read from decent email clients :-p
Anyway,
I'd like to have these "dynamic functions" be "special" where they can't be added to tables or what not. If you need to add one to a table or function pointer, then you need to have a wrapper function that does the call. I think we can come up with some kind of wrapper or linker magic to enforce this too.
-- Steve
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