Sep 20 – 24, 2021
US/Pacific timezone

objtool on arm64

Sep 24, 2021, 7:35 AM
Microconference1/Virtual-Room (LPC Virtual)


LPC Virtual

Toolchains and Kernel MC Toolchains and Kernel MC


Josh Poimboeuf (Red Hat) Mark Rutland (Arm Ltd) Peter Zijlstra (Intel OTC) Will Deacon


objtool is heavily used on x86, but isn't currently support upstream by arm64.

In order to avoid depending on objtool to enable any kernel features for arm64 and also to avoid disabling compiler optimisations along the lines of when objtool cannot reconstruct the control flow, how much of its functionality is actually required on arm64 and how much of that could be directly implemented by the toolchain instead?


some objtool features on x86 are:

  • validate stack frames

  • generate ORC unwind data (optional)

  • validates unreachable instructions; specifically the lack thereof

  • validates retpoline; or specifically the lack of indirect jump/call
    sites (with annotations for those few that are okay). (optional)

  • validates uaccess rules; specifically no call/ret in between
    __user_access_begin() and __user_access_end(). (optional)

  • validates noinstr annotation; HOWEVER we rely on objtool to NOP
    all __sanitizer_cov_* calls in .noinstr/.entry text sections because
    __no_sanitize_cov is 'broken' in all known compilers.

  • generates __mcount_loc section and NOPs the __fentry call sites

  • generates .static_call_sites section for STATIC_CALL_INLINE support

  • rewrites compiler generates call/jump to the retpoline thunk to an
    alternative such that we can patch out the thunk with an indirect
    call/jmp when retpolines are disabled. (arch dependent)

  • rewrites specific jmp.d8 sites (as found through the __jump_table
    section) to nop2, because GAS is unable to determine if a jmp becomes
    a jmp.d8 or jmp.d32 and emit the right sized nop. (optional)

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Primary authors

Josh Poimboeuf (Red Hat) Mark Rutland (Arm Ltd) Peter Zijlstra (Intel OTC) Will Deacon

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