Re: [PATCH 1/1] LoongArch: Add static PIE support

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

On 22/09/22 06:19, Xi Ruoyao via Libc-alpha wrote:
> If the compiler is new enough, enable static PIE support.  In the static
> PIE version of _start (in rcrt1.o), use la.pcrel instead of la.got
> because in a static PIE we cannot use GOT entries until the dynamic
> relocations for GOT are resolved.
> ---
>  sysdeps/loongarch/configure    | 37 ++++++++++++++++++++++++++++++++++
>  sysdeps/loongarch/configure.ac | 14 +++++++++++++
>  sysdeps/loongarch/start.S      | 14 ++++++++++---
>  3 files changed, 62 insertions(+), 3 deletions(-)
> 
> diff --git a/sysdeps/loongarch/configure b/sysdeps/loongarch/configure
> index 43b54d4965..7e671241a8 100644
> --- a/sysdeps/loongarch/configure
> +++ b/sysdeps/loongarch/configure
> @@ -3,3 +3,40 @@
>  
>  $as_echo "#define HIDDEN_VAR_NEEDS_DYNAMIC_RELOC 1" >>confdefs.h
>  
> +
> +
> +
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if $CC is sufficient to build static PIE on LoongArch" >&5
> +$as_echo_n "checking if $CC is sufficient to build static PIE on LoongArch... " >&6; }
> +if ${libc_cv_static_pie_on_loongarch+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +
> +cat confdefs.h - <<_ACEOF >conftest.$ac_ext
> +/* end confdefs.h.  */
> +
> +int
> +main ()
> +{
> +
> +#if !defined __GNUC__ || __GNUC__ < 13
> +#error insufficient compiler for static PIE on LoongArch
> +#endif
> +
> +  ;
> +  return 0;
> +}
> +_ACEOF
> +if ac_fn_c_try_cpp "$LINENO"; then :
> +  libc_cv_static_pie_on_loongarch=yes
> +else
> +  libc_cv_static_pie_on_loongarch=no
> +fi
> +rm -f conftest.err conftest.i conftest.$ac_ext
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_static_pie_on_loongarch" >&5
> +$as_echo "$libc_cv_static_pie_on_loongarch" >&6; }
> +if test "$libc_cv_static_pie_on_loongarch" = yes; then
> +  $as_echo "#define SUPPORT_STATIC_PIE 1" >>confdefs.h
> +
> +fi
> diff --git a/sysdeps/loongarch/configure.ac b/sysdeps/loongarch/configure.ac
> index f744367bf3..624897bfb3 100644
> --- a/sysdeps/loongarch/configure.ac
> +++ b/sysdeps/loongarch/configure.ac
> @@ -4,3 +4,17 @@ GLIBC_PROVIDES dnl See aclocal.m4 in the top level source directory.
>  dnl It is always possible to access static and hidden symbols in an
>  dnl position independent way.
>  AC_DEFINE(HIDDEN_VAR_NEEDS_DYNAMIC_RELOC)
> +
> +dnl test if GCC is new enough for static PIE.
> +AC_CACHE_CHECK([if $CC is sufficient to build static PIE on LoongArch],
> +libc_cv_static_pie_on_loongarch, [
> +AC_PREPROC_IFELSE([AC_LANG_PROGRAM([[]], [[
> +#if !defined __GNUC__ || __GNUC__ < 13
> +#error insufficient compiler for static PIE on LoongArch
> +#endif
> +]])],
> +	[libc_cv_static_pie_on_loongarch=yes],
> +	[libc_cv_static_pie_on_loongarch=no])])
> +if test "$libc_cv_static_pie_on_loongarch" = yes; then
> +  AC_DEFINE(SUPPORT_STATIC_PIE)
> +fi

I am not sure if checking compiler version is the best option, since this can
backported without breaking ABI.  Maybe a best check would be to see if
-fpie/-fPIE and check if the required relocation if any is generated, as done
by s390x with 728894dba4a19578bd803906de184a8dd51ed13c.


> diff --git a/sysdeps/loongarch/start.S b/sysdeps/loongarch/start.S
> index e66af16d57..05cabd9b96 100644
> --- a/sysdeps/loongarch/start.S
> +++ b/sysdeps/loongarch/start.S
> @@ -60,9 +60,17 @@ ENTRY (ENTRY_POINT)
>  	cfi_undefined (1)
>  	or		a5, a0, zero /* rtld_fini */
>  
> +#if defined(PIC) && !defined(SHARED)
> +/* For static PIE, the GOT cannot be used in _start because the GOT entries are
> +   offsets instead of real addresses before __libc_start_main.  */
> +# define LA la.pcrel
> +#else
>  /* We must get symbol main through GOT table, since main may not be local.
>     For instance: googletest defines main in dynamic library.  */
> -	la.got		a0, t0, main
> +# define LA la.got
> +#endif
> +
> +	LA		a0, t0, main
>  	REG_L		a1, sp, 0
>  	ADDI		a2, sp, SZREG
>  
> @@ -73,9 +81,9 @@ ENTRY (ENTRY_POINT)
>  	move		a4, zero /* used to be fini */
>  	or		a6, sp, zero /* stack_end */
>  
> -	la.got		ra, t0, __libc_start_main
> +	LA		ra, t0, __libc_start_main
>  	jirl		ra, ra, 0
>  
> -	la.got		ra, t0, abort
> +	LA		ra, t0, abort
>  	jirl		ra, ra, 0
>  END (ENTRY_POINT)