Re: [PATCH v3 10/13] aarch64: configure check for pac-ret code generation

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 15/05/2020 11:40, Szabolcs Nagy wrote:
> Return address signing requires unwinder support, which is
> present in libgcc since >=gcc-7, however due to bugs the
> support may be broken in <gcc-10 (and similarly there may
> be issues in custom unwinders), so pac-ret is not always
> safe to use. So in assembly code glibc should only use
> pac-ret if the compiler uses it too. Unfortunately there
> is no predefined feature macro for it set by the compiler
> so pac-ret is inferred from the code generation.

OK with this fixes below.

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

> ---
>  config.h.in                  |  3 +++
>  sysdeps/aarch64/configure    | 39 ++++++++++++++++++++++++++++++++++++
>  sysdeps/aarch64/configure.ac | 21 +++++++++++++++++++
>  3 files changed, 63 insertions(+)
> 
> diff --git a/config.h.in b/config.h.in
> index 506b0c416c..f441470385 100644
> --- a/config.h.in
> +++ b/config.h.in
> @@ -112,6 +112,9 @@
>  /* AArch64 BTI support enabled.  */
>  #undef HAVE_AARCH64_BTI
>  
> +/* AArch64 PAC-RET code generation is enabled.  */
> +#undef HAVE_AARCH64_PAC_RET
> +
>  /* C-SKY ABI version.  */
>  #undef CSKYABI
>  

Use

  #define HAVE_AARCH64_PAC_RET 0

instead so we can check the define value instead of the its existence.

> diff --git a/sysdeps/aarch64/configure b/sysdeps/aarch64/configure
> index 70477a7fa5..5f1cdf5d9b 100644
> --- a/sysdeps/aarch64/configure
> +++ b/sysdeps/aarch64/configure
> @@ -214,3 +214,42 @@ if test $libc_cv_aarch64_bti = yes; then
>    $as_echo "#define HAVE_AARCH64_BTI 1" >>confdefs.h
>  
>  fi
> +
> +# Check if glibc is built with return address signing, i.e.
> +# if -mbranch-protection=pac-ret is on. We need this because
> +# pac-ret relies on unwinder support so it's not safe to use
> +# it in assembly code unconditionally, but there is no
> +# feature test macro for it in gcc.
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if pac-ret is enabled" >&5
> +$as_echo_n "checking if pac-ret is enabled... " >&6; }
> +if ${libc_cv_aarch64_pac_ret+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +    cat > conftest.c <<EOF
> +int bar (void);
> +int foo (void) { return bar () + 1; }
> +EOF
> +  libc_cv_aarch64_pac_ret=no
> +  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -S -o conftest.s conftest.c'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; } \
> +     && { ac_try='grep -q -E '\''(hint( |	)+25|paciasp)'\'' conftest.s'
> +  { { eval echo "\"\$as_me\":${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_pac_ret=yes
> +  fi
> +  rm -rf conftest.*
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_aarch64_pac_ret" >&5
> +$as_echo "$libc_cv_aarch64_pac_ret" >&6; }
> +if test $libc_cv_aarch64_pac_ret = yes; then
> +  $as_echo "#define HAVE_AARCH64_PAC_RET 1" >>confdefs.h
> +
> +fi
> diff --git a/sysdeps/aarch64/configure.ac b/sysdeps/aarch64/configure.ac
> index 798f494740..8248ecf2ed 100644
> --- a/sysdeps/aarch64/configure.ac
> +++ b/sysdeps/aarch64/configure.ac
> @@ -39,3 +39,24 @@ EOF
>  if test $libc_cv_aarch64_bti = yes; then
>    AC_DEFINE(HAVE_AARCH64_BTI)
>  fi
> +
> +# Check if glibc is built with return address signing, i.e.
> +# if -mbranch-protection=pac-ret is on. We need this because

Two spaces after period.

> +# pac-ret relies on unwinder support so it's not safe to use
> +# it in assembly code unconditionally, but there is no
> +# feature test macro for it in gcc.
> +AC_CACHE_CHECK([if pac-ret is enabled], [libc_cv_aarch64_pac_ret], [dnl
> +  cat > conftest.c <<EOF
> +int bar (void);
> +int foo (void) { return bar () + 1; }
> +EOF
> +  libc_cv_aarch64_pac_ret=no
> +  if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -S -o conftest.s conftest.c]) \
> +     && AC_TRY_COMMAND([grep -q -E '\''(hint( |	)+25|paciasp)'\'' conftest.s])

I am not sure this expression is doing what it is intended to do,
I think you are matching the gcc output because it also add an
extra comment with 'paciasp'.

I think what you need here is "(hint[[:space:]]+25|paciasp)".

> +  then
> +    libc_cv_aarch64_pac_ret=yes
> +  fi
> +  rm -rf conftest.*])
> +if test $libc_cv_aarch64_pac_ret = yes; then
> +  AC_DEFINE(HAVE_AARCH64_PAC_RET)
> +fi
>