Re: [PATCH v2] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

[Fangrui Song <maskray@google.com>]

On 2021-03-22, H.J. Lu wrote:
>On Sun, Mar 21, 2021 at 9:40 PM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2021-03-21, H.J. Lu wrote:
>> >On Sun, Mar 21, 2021 at 9:01 PM Fangrui Song via Libc-alpha
>> ><libc-alpha@sourceware.org> wrote:
>> >>
>> >> So that text_set_element/data_set_element/bss_set_element defined
>> >> variables will be retained by the linker.
>> >>
>> >> Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
>> >> will not be optimized out; 'retain' prevents section garbage collection
>> >> if the linker support SHF_GNU_RETAIN.
>> >>
>> >> GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
>> >> identifier name sections to be GCed even if there are live
>> >> __start_/__stop_ references.
>> >>
>> >> Without the change, there are some static linking problems, e.g.
>> >> _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
>> >> stdout is not flushed on exit.
>> >>
>> >> Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
>> >> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
>> >> ---
>> >>  config.h.in            |  3 +++
>> >>  configure              | 23 +++++++++++++++++++++++
>> >>  configure.ac           | 16 ++++++++++++++++
>> >>  include/libc-symbols.h | 30 +++++++++++++++++++++---------
>> >>  4 files changed, 63 insertions(+), 9 deletions(-)
>> >>
>> >> diff --git a/config.h.in b/config.h.in
>> >> index f21bf04e47..cc875e90f9 100644
>> >> --- a/config.h.in
>> >> +++ b/config.h.in
>> >> @@ -187,6 +187,9 @@
>> >>  /* Define if gcc supports attribute ifunc.  */
>> >>  #undef HAVE_GCC_IFUNC
>> >>
>> >> +/* Define if gcc supports attribute retain.  */
>> >> +#undef HAVE_GCC_RETAIN
>> >> +
>> >>  /* Define if the linker defines __ehdr_start.  */
>> >>  #undef HAVE_EHDR_START
>> >>
>> >> diff --git a/configure b/configure
>> >> index 37cef37413..28494de748 100755
>> >> --- a/configure
>> >> +++ b/configure
>> >> @@ -4105,6 +4105,29 @@ fi
>> >>  $as_echo "$libc_cv_textrel_ifunc" >&6; }
>> >>
>> >>
>> >> +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
>> >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
>> >> +$as_echo_n "checking for gcc attribute retain support... " >&6; }
>> >> +if ${libc_cv_gcc_retain+:} false; then :
>> >> +  $as_echo_n "(cached) " >&6
>> >> +else
>> >> +  cat > conftest.c <<EOF
>> >> +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
>> >> +EOF
>> >> +libc_cv_gcc_retain=no
>> >> +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
>> >> +   2>&5 ; then
>> >> +  libc_cv_gcc_retain=yes
>> >> +fi
>> >> +rm -f conftest*
>> >> +fi
>> >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
>> >> +$as_echo "$libc_cv_gcc_retain" >&6; }
>> >> +if test $libc_cv_gcc_retain = yes; then
>> >> +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
>> >> +
>> >> +fi
>> >> +
>> >>  # Check if gcc warns about alias for function with incompatible types.
>> >>  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
>> >>  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
>> >> diff --git a/configure.ac b/configure.ac
>> >> index 16b15b6f90..94ad713820 100644
>> >> --- a/configure.ac
>> >> +++ b/configure.ac
>> >> @@ -707,6 +707,22 @@ fi
>> >>  rm -f conftest*])
>> >>  AC_SUBST(libc_cv_textrel_ifunc)
>> >>
>> >> +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
>> >> +AC_CACHE_CHECK([for gcc attribute retain support],
>> >> +              libc_cv_gcc_retain, [dnl
>> >> +cat > conftest.c <<EOF
>> >> +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
>> >> +EOF
>> >> +libc_cv_gcc_retain=no
>> >> +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
>> >> +   2>&AS_MESSAGE_LOG_FD ; then
>> >> +  libc_cv_gcc_retain=yes
>> >> +fi
>> >> +rm -f conftest*])
>> >> +if test $libc_cv_gcc_retain = yes; then
>> >> +  AC_DEFINE(HAVE_GCC_RETAIN)
>> >> +fi
>> >> +
>> >>  # Check if gcc warns about alias for function with incompatible types.
>> >>  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
>> >>                libc_cv_gcc_incompatible_alias, [dnl
>> >> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
>> >> index c83e550b03..f8eafcf1f3 100644
>> >> --- a/include/libc-symbols.h
>> >> +++ b/include/libc-symbols.h
>> >> @@ -373,17 +373,29 @@ for linking")
>> >>
>> >>  /* These are all done the same way in ELF.
>> >>     There is a new section created for each set.  */
>> >> -#ifdef SHARED
>> >> +#ifdef HAVE_GCC_RETAIN
>> >> +# ifdef SHARED
>> >>  /* When building a shared library, make the set section writable,
>> >> -   because it will need to be relocated at run time anyway.  */
>> >> -# define _elf_set_element(set, symbol) \
>> >> -  static const void *__elf_set_##set##_element_##symbol##__ \
>> >> -    __attribute__ ((used, section (#set))) = &(symbol)
>> >
>> >You should define __attribute_used_and_retain__ instead.
>> >
>> >> +  because it will need to be relocated at run time anyway.  */
>> >> +#  define _elf_set_element(set, symbol) \
>> >> +     static const void *__elf_set_##set##_element_##symbol##__ \
>> >> +       __attribute__ ((used, retain, section (#set))) = &(symbol)
>> >> +# else
>> >> +#  define _elf_set_element(set, symbol) \
>> >> +     static const void *const __elf_set_##set##_element_##symbol##__ \
>> >> +       __attribute__ ((used, retain, section (#set))) = &(symbol)
>> >> +# endif
>> >>  #else
>> >> -# define _elf_set_element(set, symbol) \
>> >> -  static const void *const __elf_set_##set##_element_##symbol##__ \
>> >> -    __attribute__ ((used, section (#set))) = &(symbol)
>> >> -#endif
>> >> +# ifdef SHARED
>> >> +#  define _elf_set_element(set, symbol) \
>> >> +     static const void *__elf_set_##set##_element_##symbol##__ \
>> >> +       __attribute__ ((used, section (#set))) = &(symbol)
>> >> +# else
>> >> +#  define _elf_set_element(set, symbol) \
>> >> +     static const void *const __elf_set_##set##_element_##symbol##__ \
>> >> +       __attribute__ ((used, section (#set))) = &(symbol)
>> >> +# endif
>> >> +#endif /* HAVE_GCC_RETAIN  */
>> >>
>> >>  /* Define SET as a symbol set.  This may be required (it is in a.out) to
>> >>     be able to use the set's contents.  */
>> >> --
>> >> 2.31.0.rc2.261.g7f71774620-goog
>> >>
>> >
>> >libc.a is an installed file.  Does this work with older linkers?
>> >
>> >--
>> >H.J.
>>
>> How about
>>
>>    /* These are all done the same way in ELF.
>>       There is a new section created for each set.  */
>>    #ifdef HAVE_GCC_RETAIN
>>    # define attribute_used_retain_section(sec) \
>>        __attribute__ ((used, retain, section (sec)))
>>    #else
>
>__attribute_used_and_retain__ is more flex and can be used
>without section.
>
>>    # define attribute_used_retain_section(sec) \
>>        __attribute__ ((used, section (sec)))
>>    #endif
>>    #ifdef SHARED
>>    /* When building a shared library, make the set section writable,
>>     because it will need to be relocated at run time anyway.  */
>>    # define _elf_set_element(set, symbol) \
>>        static const void *__elf_set_##set##_element_##symbol##__ \
>>          attribute_used_retain_section(#set) = &(symbol)
>>    #else
>>    # define _elf_set_element(set, symbol) \
>>        static const void *const __elf_set_##set##_element_##symbol##__ \
>>          attribute_used_retain_section(#set) = &(symbol)
>>    #endif
>> ? Yes, older linkers just ignore unknown section flags. That is the ELF spirit.
>
>
>
>-- 
>H.J.

+#ifdef HAVE_GCC_RETAIN
+# define attribute_used_retain __attribute__ ((__used__, __retain__))
+#else
+# define attribute_used_retain __attribute__ ((__used__))
+#endif
+
  /* Symbol set support macros.  */
  
  /* Make SYMBOL, which is in the text segment, an element of SET.  */
@@ -377,12 +383,12 @@ for linking")
  /* When building a shared library, make the set section writable,
     because it will need to be relocated at run time anyway.  */
  # define _elf_set_element(set, symbol) \
-  static const void *__elf_set_##set##_element_##symbol##__ \
-    __attribute__ ((used, section (#set))) = &(symbol)
+    static const void *__elf_set_##set##_element_##symbol##__ \
+      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
  #else
  # define _elf_set_element(set, symbol) \
-  static const void *const __elf_set_##set##_element_##symbol##__ \
-    __attribute__ ((used, section (#set))) = &(symbol)
+    static const void *const __elf_set_##set##_element_##symbol##__ \
+      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
  #endif