Re: [PATCH] aarch64: Skip traditional GD/LD TLS which are unsupported by Clang and LLD [BZ #28205]

[Szabolcs Nagy via Libc-alpha <libc-alpha@sourceware.org>]

The 08/06/2021 15:41, Fangrui Song via Libc-alpha wrote:
> TLSDESC is the default on aarch64.  Clang doesn't support
> -mtls-dialect=trad.  Its integrated assembler doesn't support the
> marker.  LLD's doesn't support R_AARCH64_TLSGD_*/R_AARCH64_TLSLD_*
> relocations.  Just skip the tests.
> 
> With https://sourceware.org/pipermail/libc-alpha/2021-August/129904.html
> ("aarch64: Make elf_machine_{load_address, dynamic} robust [BZ #28203]"),
> if we allow LLD in configure.ac,
> `make check` test results of LLD are on par with GNU ld.
> ---
>  config.h.in       |  3 +++
>  configure         | 38 ++++++++++++++++++++++++++++++++++++++
>  configure.ac      | 24 ++++++++++++++++++++++++
>  elf/tst-tls1.c    |  7 +++++--
>  elf/tst-tls2.c    |  6 ++++--
>  elf/tst-tls3.c    |  8 ++++----
>  elf/tst-tlsmod1.c |  6 ++++--
>  elf/tst-tlsmod2.c |  4 +++-
>  elf/tst-tlsmod3.c |  5 ++++-
>  elf/tst-tlsmod4.c |  4 +++-
>  10 files changed, 92 insertions(+), 13 deletions(-)
> 
> diff --git a/config.h.in b/config.h.in
> index 8b45a3a61d..f65605d4bd 100644
> --- a/config.h.in
> +++ b/config.h.in
> @@ -198,6 +198,9 @@
>  /* Define if CC supports attribute retain.  */
>  #undef HAVE_GNU_RETAIN
>  
> +/* Define if CC and LD support -mtls-dialect=trad.  */
> +#undef HAVE_MTLS_DIALECT_TRAD
> +
>  /* Define if the linker defines __ehdr_start.  */
>  #undef HAVE_EHDR_START
>  
> diff --git a/configure.ac b/configure.ac
> index 5632277f9c..c73e271cc8 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1501,6 +1501,30 @@ rm -f conftest*])
>  AC_SUBST(libc_cv_mtls_dialect_gnu2)
>  LIBC_CONFIG_VAR([have-mtls-dialect-gnu2], [$libc_cv_mtls_dialect_gnu2])
>  
> +AC_CACHE_CHECK([for -mtls-dialect=trad], libc_cv_mtls_dialect_trad,
> +[dnl
> +cat > conftest.c <<EOF
> +extern __thread int i;
> +void foo (void)
> +{
> +  i = 10;
> +}
> +EOF
> +if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS
> +			-fPIC -mtls-dialect=trad -shared -o conftest.so conftest.c
> +			1>&AS_MESSAGE_LOG_FD])
> +then
> +  libc_cv_mtls_dialect_trad=yes
> +else
> +  libc_cv_mtls_dialect_trad=no
> +fi
> +rm -f conftest*])
> +if test $libc_cv_mtls_dialect_trad = yes; then
> +  AC_DEFINE(HAVE_MTLS_DIALECT_TRAD)
> +fi
> +AC_SUBST(libc_cv_mtls_dialect_trad)
> +LIBC_CONFIG_VAR([have-mtls-dialect-trad], [$libc_cv_mtls_dialect_trad])

this is a possible way, but i think it's nicer to have

#define HAVE_TRAD_TLS 1

in config.h.in, and then in sysdeps/aarch64/configure.ac

AC_DEFINE(HAVE_TRAD_TLS, 0, [Only TLSDESC is supported])

(haven't tested, but i think it would work)

(-mtls-dialect=trad is aarch64 specific flag, so it's
not nice in the generic configure and the tests don't
actually care about this flag but whether the assembler
and linker would work)