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Date: Wed, 4 May 2022 09:34:18 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Peter Collingbourne <pcc@...gle.com> Cc: Sami Tolvanen <samitolvanen@...gle.com>, Kees Cook <keescook@...omium.org>, Mark Rutland <mark.rutland@....com>, Josh Poimboeuf <jpoimboe@...hat.com>, Will Deacon <will@...nel.org>, Catalin Marinas <catalin.marinas@....com>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Joao Moreira <joao@...rdrivepizza.com>, Sedat Dilek <sedat.dilek@...il.com>, Steven Rostedt <rostedt@...dmis.org>, LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>, linux-hardening@...r.kernel.org, linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, llvm@...ts.linux.dev Subject: Re: [RFC PATCH 00/21] KCFI support On Tue, May 03, 2022 at 03:35:34PM -0700, Peter Collingbourne wrote: > On Mon, May 2, 2022 at 1:02 PM Peter Zijlstra <peterz@...radead.org> wrote: > > > > On Mon, May 02, 2022 at 08:22:57AM -0700, Sami Tolvanen wrote: > > > > > > Anyway, I think I hate that __builtin, I'd *much* rather see a variable > > > > attribute or qualifier for this, such that one can mark a function > > > > pointer as not doing CFI. > > > > > > > > I simply doesn't make sense to have a builtin that operates on an > > > > expression. The whole thing is about indirect calls, IOW function > > > > pointers. > > > > > > I also thought an attribute would be more convenient, but the compiler > > > folks prefer a built-in: > > > > > > https://reviews.llvm.org/D122673 > > > > That seems to mostly worry about C++ things (overload sets, template > > specialization, name mangling) we kernel folks don't seem to much care > > about. > > > > I'll stick with saying type system makes more sense to me though. > > I'd say it's not only the C++ issues but more the "action at a > distance" that's implied by having this be part of the type system. > With this being in the function type it's hard to tell whether any > particular call will have CFI disabled, without needing to go and look > at how the function pointer is defined. Look at how we use volatile: *(volatile int *)(&foo) we don't use volatile on actual variable definitions (much), but instead cast it in at the usage site. Same can be done with this if so desired. > On the other hand, if we > explicitly mark up the calls with CFI disabled, the code becomes > easier to audit (think Rust "unsafe" blocks). I don't know any Rust. To me Rust still looks like line noise. > Does it seem any better to you to have this be marked up via the > function expression, rather than the call? The idea is that this would > always compile to a check-free function call, no matter what "func" > is: > > __builtin_kcfi_call_unchecked(func)(args) > > We already have this, to some degree, with KCFI as implemented: CFI > checks are disabled if the function expression refers to a declared > function. The builtin would allow overriding the decision to also > disable CFI checks for function expressions that use the builtin. It > also wouldn't preclude a type based system later on (the builtin would > become effectively a cast to the "unchecked" type). That's still a bit naf; you've effectively made that builtin a type-cast.
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