Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] static_call,x86: Robustify trampoline patching | Date | Wed, 3 Nov 2021 10:01:40 +0000 |
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From: Peter Zijlstra > Sent: 03 November 2021 08:36 > > On Tue, Nov 02, 2021 at 05:20:05PM -0700, Andy Lutomirski wrote: > > I think that's a big mistake -- any sane ENDBR-using scheme would > > really prefer that ENDBR to be right next to the actual function body, > > and really any scheme would benefit due to better cache locality. > > Agreed, IBT/BTI want the landing pad in front of the actual function. > > > But, more importantly, IMO any sane ENDBR-using scheme wants to > > generate the indirect stub as part of code gen for the actual > > function. > > Sorta, I really want to be able to not have a landing pad for functions > whose address is never taken. At that point it doesn't matter if it gets > generated along with the function and then stripped/poisoned later, or > generated later. > > As such, the landing pad should not be part of the function proper, > direct calls should never observe it. > > Less landing pads is more better.
One problem is when a direct call is 'too far' for a call instruction. IIRC this can happen in arm64 with modules (all 64bit except x86?). So an indirect call has to be used instead - which needs the landing pad. Although it may actually be better to put a trampoline (landing pad + near jump) elsewhere and have the module loader do the correct fixup. (Is the loader already generating a trampoline in the module code?) The function body can then be cache-line aligned - with its benefits.
Can't anything that can write instructions always use a retpoline to implement a jump indirect to an arbitrary address? (Not to mention just generating the required code rather than a call.)
AFAICT CFI is all about detecting invalid values in function pointer tables. It doesn't really protect in any way from JIT code doing incorrect things.
David
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