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    SubjectRe: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
    On Mon, May 16, 2022 at 2:54 AM Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > 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?

    What's the concern with the endbr? Dropping it would currently break
    the CFI+IBT combination on newer hardware, no?

    > Also, perhaps s/CFI_CLANG/KERNEL_CFI/ or somesuch, so that GCC might
    > also implement this same scheme (in time)?

    I'm fine with renaming the config.

    > > ; 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.

    With the current compiler patch, LLVM sets up function arguments after
    the CFI check. if it's a problem, we can look into changing that.

    Sami

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