On Thu, Nov 29, 2018 at 10:58:40AM -0800, Linus Torvalds wrote: > On Thu, Nov 29, 2018 at 10:47 AM Steven Rostedt <rost...@goodmis.org> wrote: > > > > Note, we do have a bit of control at what is getting called. The patch > > set requires that the callers are wrapped in macros. We should not > > allow just any random callers (like from asm). > > Actually, I'd argue that asm is often more controlled than C code. > > Right now you can do odd things if you really want to, and have the > compiler generate indirect calls to those wrapper functions. > > For example, I can easily imagine a pre-retpoline compiler turning > > if (cond) > fn1(a,b) > else > fn2(a,b); > > into a function pointer conditional > > (cond ? fn1 : fn2)(a,b); > > and honestly, the way "static_call()" works now, can you guarantee > that the call-site doesn't end up doing that, and calling the > trampoline function for two different static calls from one indirect > call? > > See what I'm talking about? Saying "callers are wrapped in macros" > doesn't actually protect you from the compiler doing things like that. > > In contrast, if the call was wrapped in an inline asm, we'd *know* the > compiler couldn't turn a "call wrapper(%rip)" into anything else.
I think objtool could warn about many such issues, including function pointer references to trampolines and short tail call jumps. -- Josh
