• DevSummit
• 201808
• Toolchain

Session Leader: jhb

Notes: https://hackmd.io/BjHsH7MzTLuAcUF_P7JJxA#

Toolchain and Build

• basic build requirements
  • compiler, etc. to bootstrap a new release
  • Guarantee: latest X release can build stable/X+1
  • Willing to guarantee: X-1 and X-2 userland will work on X-CURRENT kernel
• cross-building from a non-FreeBSD host
  • patch to build without depending on tools in $PATH
  • Can we enumerate all build tools?
  • Once it works, add CI
  • Avoid host $PATH
  • Wrapper scripts for non-FreeBSD hosted builds
  • arichardson patch is ~ +1500 -69

    • https://github.com/arichardson/cheribsd/commits/crossbuild-aug2018

• META_MODE
  • How do we finally turn it on? -- src.conf(5)
  • Can you use this without filemon.ko (cross-build)?
    • Would have to check in the dependencies, but wouldn't update them when making changes
  • LLVM sanitizers
    • more architectures (aarch64 and mips64 ASAN in review)
    • amd64 ASAN LLVM tests don't all pass
    • Need a matrix sowing what works
      • asan + threads issue?
    • kernel ubsan? (relatively straightforward)
    • Couple test suite with CI with sanitizers
    • which ones do we want?
      • ASAN
      • UBSAN
      • TSAN
      • profile?
      • xray?
        • msan?
    • Build infrastructure (make goo?) e.g. WITH_ASAN
    • Separate ASAN-built libraries as an option?
• SSP in GENERIC-DEBUG? WITH_SSP=yes?
• LLD
  • Current state
    • bootstrap + system linker on amd64, arm64
    • bootstrap linker on i386, armv7
  • Near future (12.0 if possible, else 13)
    • system linker on i386, armv7
    • armv7 flip on and watch for ports fallout
    • MIPS64? (requires -z notext)
• Userland LTO
  • requires llvm-ar, possibly llvm-nm
  • requires rtld in 11+
  • option for base system? (WITH_LTO)
  • integration/deployment questions
    • -flto produces .o that contain llvm IR (not ELF)
    • llvm-ar emits llvm IR archives
    • new extension for LTO lib vs new libpath vs fat .a
• Kernel LTO
  • barfed on Dtrace (ctfconvert is unhappy)
    • OpenBSD approach might address this
  • debugging and observability suffers
    • Need to be careful with optimizing out "unnecessary" memory zeroing etc.
    • Link some parts with -flto and not others?
    • -flto=thin vs -flto

• C11 Atomics vs <machine/atomic.h>

  • marginally better asm
  • ordering with I/O (bus_space_barrier and bus_dmamap_sync)

  • issue with using C11 memory_order_consume to implement RCU in the Linux kernel (papers at http://www.rdrop.com/users/paulmck/RCU/)

• Enabling compiler support for mitigation features
  • per-arch considerations
  • CFI -- needs someone to actually try and test

  • safe stack -- ^

  • PIE binaries (WITH_PIE)
• Compressed debug sections / .gdb_index
  • Enable on supported toolchains -- needs someone to do and test, probably shouldn't have WITH/WITHOUT, just enable by default
• Switch llvm back to dwarf4 by default
  • emaste should do this tonight during the hacking lounge
• clang support for more architectures?
  • RISC-V (can build test application with clang 8, not link)
  • MIPS? (do we care about mips32)
    • mips32 TLS segfaults
    • TLS model is always dynamic
  • powerpc
• Using newer features (like ifuncs / optimized ifuncs)
  • Ok to start ditching GCC 4.2.x / ld.bfd 2.17.50
  • Requiring vs optionally using features in only one modern toolchain
  • Should we support both toolchains on platforms that support both? Generally yes.
  • Defer question about desupporting modern alternate toolchain.
• Starting to use C++11 in userland on architectures?
• Using external toolchain more often (e.g. in regular amd64 development)
  • Manual override to force existing system toolchain
  • Actual testing / documentation of llvm-xtoolchain port
• LLVM vs elftoolchain
  • ar, nm - for LTO and large .a
    • bug fix needed for elftoolchain, possibly new functionality to be added
  • size, objcopy, readelf, strings - leave as elftc
• External GCC (base/*)
• Killing src/contrib/gcc
• Killing src/contrib/binutils
• Working with compiler-provided headers (std*.h, etc.)