Messages in this thread | | | Date | Fri, 29 Oct 2021 22:03:24 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v5 00/15] x86: Add support for Clang CFI |
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On Wed, Oct 27, 2021 at 08:50:17AM -0700, Sami Tolvanen wrote: > On Wed, Oct 27, 2021 at 7:18 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > /* > > > * Turns a Clang CFI jump-table entry into an actual function pointer. > > > * These jump-table entries are simply jmp.d32 instruction with their > > > * relative offset pointing to the actual function, therefore decode the > > > * instruction to find the real function. > > > */ > > > static __always_inline void *nocfi_ptr(void *func) > > > { > > > union text_poke_insn insn = *(union text_poke_insn *)func; > > > > > > return func + sizeof(insn) + insn.disp; > > > } > > > > > > But really, that wants to be a compiler intrinsic. > > > > Agreed. We could easily do something similar on arm64, but I'd prefer > > to avoid that too. > > I'll see what we can do. Note that the compiler built-in we previously > discussed would have semantics similar to function_nocfi(). It would > return the raw function address from a symbol name, but it wouldn't > decode the address from an arbitrary pointer, so this would require > something different.
So I had a bit of a peek at what clang generates:
3fa4: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 3fa7: R_X86_64_32S __SCK__x86_pmu_handle_irq 3fab: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 3fae: R_X86_64_32S __SCT__x86_pmu_handle_irq.cfi_jt 3fb2: e8 00 00 00 00 call 3fb7 <init_hw_perf_events+0x1dc> 3fb3: R_X86_64_PLT32 __static_call_update-0x4
So this then gives the trampoline jump table entry to __static_call_update(), with the result that it will rewrite the jump-table entry, not the trampoline!
Now it so happens that the trampoline looks *exactly* like the jump-table entry (one jmp.d32 instruction), so in that regards it'll again 'work'.
But this is all really, as in *really*, wrong. And I'm really sad I'm the one to have to discover this, even though I've mentioned static_call()s being tricky in previous reviews.
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