Fix building the ia32 vDSO with --no-undefined-version

[val_packett.cool]

Summary

With an external toolchain that's all LLVM 16 including the binutils,
the build fails because --no-undefined-version became the default in lld:

ld.lld: error: version script assignment of 'FBSD_1.7' to symbol '__vdso_ia32_osigcode' failed: symbol not defined
ld.lld: error: version script assignment of 'FBSD_1.7' to symbol '__vdso_lcall_tramp' failed: symbol not defined

We can preprocess the linker script to fix this.

Sponsored by: https://www.patreon.com/valpackett

Test Plan

My setup for using an all-llvm toolchain is this:

/etc/src.conf:

CROSS_TOOLCHAIN=llvmroot
LLVM_ROOT=/usr/local/llvm16

/usr/local/share/toolchains/llvmroot.mk (I wonder why this file doesn't already exist somewhere… well maybe in the future the llvm16.mk etc. files would include the binutils):

X_COMPILER_TYPE=clang
XCC=${LLVM_ROOT}/bin/clang
XCXX=${LLVM_ROOT}/bin/clang++
XCPP=${LLVM_ROOT}/bin/clang-cpp
XLLVM_LINK=${LLVM_ROOT}/bin/llvm-link
XLD=${LLVM_ROOT}/bin/ld.lld
XAS=${LLVM_ROOT}/bin/llvm-as
XAR=${LLVM_ROOT}/bin/llvm-ar
XNM=${LLVM_ROOT}/bin/llvm-nm
XOBJCOPY=${LLVM_ROOT}/bin/llvm-objcopy
XRANLIB=${LLVM_ROOT}/bin/llvm-ranlib
XSTRINGS=${LLVM_ROOT}/bin/llvm-strings
XSIZE=${LLVM_ROOT}/bin/llvm-size
XSTRIPBIN=${LLVM_ROOT}/bin/llvm-strip

Confirming that the ifdefs happened by seeing the freebsd4 symbol:

$ readelf -a /usr/obj/usr/home/val/base/amd64.amd64/sys/GENERIC/elf-vdso32.so.1
…
Symbol table '.dynsym' contains 3 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000230     0 NOTYPE  GLOBAL DEFAULT    9 __vdso_ia32_sigcode@@FBSD_1.7 (2)
     2: 0000000000000244     0 NOTYPE  GLOBAL DEFAULT    9 __vdso_freebsd4_ia32_sigcode@@FBSD_1.7 (2)

[kib]

Did the build failed, or only issued a warning?

I do not think we should preprocess linker script for this issue, PROVIDE-ing the symbols should be enough. At least for me D38392 made the build silent with ld.lld from llvm16.

[val_packett.cool]

Yes, it has failed. I wouldn't be that bothered with a little warning :)

Oh, that's an interesting trick! Doesn't seem to be working for me though.

I went with preprocessing because that's what the build system does for everything else (processing the SYMBOL_MAPS in bsd.symver.mk).

[val_packett.cool]

btw, what toggles the error is --(no-)undefined-version, no became the default. It seems difficult to make *everything* work with --no-undefined-version, but this was the only thing in the kernel.

[kib]

In D38389#872944, @val_packett.cool wrote:

Yes, it has failed. I wouldn't be that bothered with a little warning :)

Oh, that's an interesting trick! Doesn't seem to be working for me though.

I went with preprocessing because that's what the build system does for everything else (processing the SYMBOL_MAPS in bsd.symver.mk).

Yes. and it was unfortunate decision at that time (same as our policy for using symver for userspace).
I updated my review to always define the symbols in .S file instead.

[val_packett.cool]

ok, let's go with D38392