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    SubjectRe: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
    On Fri, Feb 11, 2022 at 5:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
    > I think we'll end up with something related to KCFI, but with distinct
    > differences:
    >
    > - 32bit immediates for smaller code

    Sure, I don't see issues with that. Based on a quick test with
    defconfig, this reduces vmlinux size by 0.30%.

    > - __kcfi_check_fail() is out for smaller code

    I'm fine with adding a trap mode that's used by default, but having
    more helpful diagnostics when something fails is useful even in
    production systems in my experience. This change results in a vmlinux
    that's another 0.92% smaller.

    > Which then yields:
    >
    > caller:
    > cmpl $0xdeadbeef, -0x4(%rax) # 7 bytes
    > je 1f # 2 bytes
    > ud2 # 2 bytes
    > 1: call __x86_indirect_thunk_rax # 5 bytes

    Note that the compiler might not emit this *exact* sequence of
    instructions. For example, Clang generates this for events_sysfs_show
    with the modified KCFI patch:

    2274: cmpl $0x4d7bed9e,-0x4(%r11)
    227c: jne 22c0 <events_sysfs_show+0x6c>
    227e: call 2283 <events_sysfs_show+0x2f>
    227f: R_X86_64_PLT32 __x86_indirect_thunk_r11-0x4
    ...
    22c0: ud2

    In this case the function has two indirect calls and Clang seems to
    prefer to emit just one ud2.

    > .align 16
    > .byte 0xef, 0xbe, 0xad, 0xde # 4 bytes
    > func:
    > endbr # 4 bytes

    Here func is no longer aligned to 16 bytes, in case that's important.

    > Further, Andrew put in the request for __attribute__((cfi_seed(blah)))
    > to allow distinguishing indirect functions with otherwise identical
    > signature; eg. cookie = hash32(blah##signature).

    Sounds reasonable.

    > Did I miss anything? Got anything wrong?

    How would you like to deal with the 4-byte hashes in objtool? We
    either need to annotate all function symbols in the kernel, or we need
    a way to distinguish the hashes from random instructions, so we can
    also have functions that don't have a type hash.

    Sami

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