[PATCH v3 04/13] aarch64: configure test for BTI support

[Adhemerval Zanella]

On 15/05/2020 11:40, Szabolcs Nagy wrote:
> Check BTI support in the compiler and linker.  The check also
> requires READELF that understands the BTI GNU property note.
> It is expected to succeed with gcc >=gcc-9 configured with
> --enable-standard-branch-protection and binutils >=binutils-2.33.

>From the configure check below I understand that it still possible to
build a glibc with BTI support if the required flags are add on 
CC or CFLAGS.  Maybe make it explicit in the commit message?

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  config.h.in                  |  3 +++
>  sysdeps/aarch64/configure    | 42 ++++++++++++++++++++++++++++++++++++
>  sysdeps/aarch64/configure.ac | 19 ++++++++++++++++
>  3 files changed, 64 insertions(+)
> 
> diff --git a/config.h.in b/config.h.in
> index dea43df438..506b0c416c 100644
> --- a/config.h.in
> +++ b/config.h.in
> @@ -109,6 +109,9 @@
>  /* AArch64 big endian ABI */
>  #undef HAVE_AARCH64_BE
>  
> +/* AArch64 BTI support enabled.  */
> +#undef HAVE_AARCH64_BTI
> +
>  /* C-SKY ABI version.  */
>  #undef CSKYABI
>  

Ok.

> diff --git a/sysdeps/aarch64/configure b/sysdeps/aarch64/configure
> index 5bd355a691..70477a7fa5 100644
> --- a/sysdeps/aarch64/configure
> +++ b/sysdeps/aarch64/configure
> @@ -172,3 +172,45 @@ else
>    config_vars="$config_vars
>  default-abi = lp64"
>  fi
> +
> +# Only consider BTI supported if -mbranch-protection=bti is
> +# on by default in the compiler and the linker produces
> +# binaries with GNU property notes in PT_GNU_PROPERTY segment.
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for BTI support" >&5
> +$as_echo_n "checking for BTI support... " >&6; }
> +if ${libc_cv_aarch64_bti+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +    cat > conftest.c <<EOF
> +void foo (void) { }
> +EOF
> +  libc_cv_aarch64_bti=no
> +  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS -nostdlib -nostartfiles $no_ssp -shared -fPIC -o conftest.so conftest.c'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; } \
> +     && { ac_try='$READELF -lW conftest.so | grep -q GNU_PROPERTY'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; } \
> +     && { ac_try='$READELF -nW conftest.so | grep -q "NT_GNU_PROPERTY_TYPE_0.*AArch64 feature:.* BTI"'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; }
> +  then
> +    libc_cv_aarch64_bti=yes
> +  fi
> +  rm -rf conftest.*
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_aarch64_bti" >&5
> +$as_echo "$libc_cv_aarch64_bti" >&6; }
> +if test $libc_cv_aarch64_bti = yes; then
> +  $as_echo "#define HAVE_AARCH64_BTI 1" >>confdefs.h
> +
> +fi
> diff --git a/sysdeps/aarch64/configure.ac b/sysdeps/aarch64/configure.ac
> index 7851dd4dac..798f494740 100644
> --- a/sysdeps/aarch64/configure.ac
> +++ b/sysdeps/aarch64/configure.ac
> @@ -20,3 +20,22 @@ if test $libc_cv_aarch64_be = yes; then
>  else
>    LIBC_CONFIG_VAR([default-abi], [lp64])
>  fi
> +
> +# Only consider BTI supported if -mbranch-protection=bti is
> +# on by default in the compiler and the linker produces
> +# binaries with GNU property notes in PT_GNU_PROPERTY segment.

Maybe add that user might still set it on CC or CFLAGS?

> +AC_CACHE_CHECK([for BTI support], [libc_cv_aarch64_bti], [dnl
> +  cat > conftest.c <<EOF
> +void foo (void) { }
> +EOF
> +  libc_cv_aarch64_bti=no
> +  if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS -nostdlib -nostartfiles $no_ssp -shared -fPIC -o conftest.so conftest.c]) \
> +     && AC_TRY_COMMAND([$READELF -lW conftest.so | grep -q GNU_PROPERTY]) \
> +     && AC_TRY_COMMAND([$READELF -nW conftest.so | grep -q "NT_GNU_PROPERTY_TYPE_0.*AArch64 feature:.* BTI"])
> +  then
> +    libc_cv_aarch64_bti=yes
> +  fi
> +  rm -rf conftest.*])
> +if test $libc_cv_aarch64_bti = yes; then
> +  AC_DEFINE(HAVE_AARCH64_BTI)
> +fi
> 

Ok.