Messages in this thread | | | Date | Mon, 16 May 2022 11:54:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG |
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On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote: > With CONFIG_CFI_CLANG, the compiler injects a type preamble > immediately before each function and a check to validate the target > function type before indirect calls: > > ; type preamble > __cfi_function: > int3 > int3 > mov <id>, %eax > int3 > int3 > function: > ...
When I enable CFI_CLANG and X86_KERNEL_IBT I get:
0000000000000c80 <__cfi_io_schedule_timeout>: c80: cc int3 c81: cc int3 c82: b8 b5 b1 39 b3 mov $0xb339b1b5,%eax c87: cc int3 c88: cc int3
0000000000000c89 <io_schedule_timeout>: c89: f3 0f 1e fa endbr64
That seems unfortunate. Would it be possible to get an additional compiler option to suppress the endbr for all symbols that get a __cfi_ preaamble?
Also, perhaps s/CFI_CLANG/KERNEL_CFI/ or somesuch, so that GCC might also implement this same scheme (in time)?
> ; indirect call check > cmpl <id>, -6(%r11) > je .Ltmp1 > ud2 > .Ltmp1: > call __x86_indirect_thunk_r11
The first one I try and find looks like:
26: 41 81 7b fa a6 96 9e 38 cmpl $0x389e96a6,-0x6(%r11) 2e: 74 02 je 32 <__traceiter_sched_kthread_stop+0x29> 30: 0f 0b ud2 32: 4c 89 f6 mov %r14,%rsi 35: e8 00 00 00 00 call 3a <__traceiter_sched_kthread_stop+0x31> 36: R_X86_64_PLT32 __x86_indirect_thunk_r11-0x4
This must not be. If I'm to rewrite that lot to:
movl $\hash, %r10d sub $9, %r11 call *%r11 .nop 4
Then there must not be spurious instruction in between the ud2 and the indirect call/retpoline thing.
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