[PATCH v4] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]
H.J. Lu
hjl.tools@gmail.com
Thu Apr 1 01:00:10 GMT 2021
On Wed, Mar 31, 2021 at 5:02 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2021-03-31, H.J. Lu wrote:
> >On Wed, Mar 31, 2021 at 12:39 PM Fangrui Song <maskray@google.com> 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).
> >> ---
> >> Changes in v1 -> v2:
> >> * Define attribute_used_retain_section
> >> Changes in v2 -> v3:
> >> * Use attribute_used_retain instead attribute_used_retain_section
> >> Changes in v3 -> v4:
> >> * Add LIBC_LINKER_FEATURE detection for -z start-stop-gc and add tst-cleanup* tests
> >> ---
> >> config.h.in | 3 +++
> >> configure | 58 ++++++++++++++++++++++++++++++++++++++++++
> >> configure.ac | 20 +++++++++++++++
> >> include/libc-symbols.h | 14 +++++++---
> >> libio/Makefile | 32 +++++++++++++++++++++++
> >> libio/tst-cleanup.c | 33 ++++++++++++++++++++++++
> >> libio/tst-cleanup.exp | 1 +
> >> 7 files changed, 157 insertions(+), 4 deletions(-)
> >> create mode 100644 libio/tst-cleanup.c
> >> create mode 100644 libio/tst-cleanup.exp
> >>
> >> diff --git a/config.h.in b/config.h.in
> >> index ca1547ae67..95af1ef229 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 fcf43bf7de..fa86348342 100755
> >> --- a/configure
> >> +++ b/configure
> >> @@ -626,6 +626,7 @@ fno_unit_at_a_time
> >> libc_cv_has_glob_dat
> >> libc_cv_hashstyle
> >> libc_cv_fpie
> >> +libc_cv_z_start_stop_gc
> >> libc_cv_z_execstack
> >> libc_cv_z_combreloc
> >> ASFLAGS_config
> >> @@ -4105,6 +4106,29 @@ fi
> >> $as_echo "$libc_cv_textrel_ifunc" >&6; }
> >>
> >>
> >> +# Check if gcc supports attribute retain as it is used in attribute_used_retain 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; }
> >> @@ -5871,6 +5895,40 @@ fi
> >> $as_echo "$libc_linker_feature" >&6; }
> >>
> >>
> >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports -z start-stop-gc" >&5
> >> +$as_echo_n "checking for linker that supports -z start-stop-gc... " >&6; }
> >> +libc_linker_feature=no
> >> +if test x"$gnu_ld" = x"yes"; then
> >> + libc_linker_check=`$LD -v --help 2>/dev/null | grep "\-z start-stop-gc"`
> >> + if test -n "$libc_linker_check"; then
> >> + cat > conftest.c <<EOF
> >> +int _start (void) { return 42; }
> >> +EOF
> >> + if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
> >> + -Wl,-z,start-stop-gc -nostdlib -nostartfiles
> >> + -fPIC -shared -o conftest.so conftest.c
> >> + 1>&5'
> >> + { { eval echo "\"\$as_me\":${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_linker_feature=yes
> >> + fi
> >> + rm -f conftest*
> >> + fi
> >> +fi
> >> +if test $libc_linker_feature = yes; then
> >> + libc_cv_z_start_stop_gc=yes
> >> +else
> >> + libc_cv_z_start_stop_gc=no
> >> +fi
> >> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_linker_feature" >&5
> >> +$as_echo "$libc_linker_feature" >&6; }
> >> +config_vars="$config_vars
> >> +have-z-start-stop-gc = $libc_cv_z_start_stop_gc"
> >> +
> >> { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5
> >> $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
> >> libc_linker_feature=no
> >> diff --git a/configure.ac b/configure.ac
> >> index fce967f2c2..4e2907da92 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 retain as it is used in attribute_used_retain 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 $libc_cv_gcc_retain = yes; then
> >> + AC_DEFINE(HAVE_GCC_RETAIN)
> >> +fi
> >> +
> >> # Check if gcc warns about alias for function with incompatible types.
> >> AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
> >> libc_cv_gcc_incompatible_alias, [dnl
> >> @@ -1317,6 +1333,10 @@ LIBC_LINKER_FEATURE([-z execstack], [-Wl,-z,execstack],
> >> [libc_cv_z_execstack=yes], [libc_cv_z_execstack=no])
> >> AC_SUBST(libc_cv_z_execstack)
> >>
> >> +LIBC_LINKER_FEATURE([-z start-stop-gc], [-Wl,-z,start-stop-gc],
> >> + [libc_cv_z_start_stop_gc=yes], [libc_cv_z_start_stop_gc=no])
> >> +LIBC_CONFIG_VAR([have-z-start-stop-gc], [$libc_cv_z_start_stop_gc])
> >> +
> >> LIBC_LINKER_FEATURE([--no-dynamic-linker],
> >> [-Wl,--no-dynamic-linker],
> >> [libc_cv_no_dynamic_linker=yes],
> >> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> >> index 546fc26a7b..b311c90f0c 100644
> >> --- a/include/libc-symbols.h
> >> +++ b/include/libc-symbols.h
> >> @@ -352,6 +352,12 @@ for linking")
> >>
> >> */
> >>
> >> +#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. */
> >> @@ -367,12 +373,12 @@ 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
> >>
> >> /* Define SET as a symbol set. This may be required (it is in a.out) to
> >> diff --git a/libio/Makefile b/libio/Makefile
> >> index 12ce41038f..439d22e057 100644
> >> --- a/libio/Makefile
> >> +++ b/libio/Makefile
> >> @@ -195,6 +195,20 @@ ifeq (yes,$(build-shared))
> >> tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
> >> $(objpfx)tst-bz24228-mem.out
> >> endif
> >> +
> >> +tests += tst-cleanup-default
> >> +tests-static += tst-cleanup-default
> >> +tests-special += $(objpfx)tst-cleanup-default-cmp.out
> >> +LDFLAGS-tst-cleanup-default = -Wl,--gc-sections
> >> +
> >> +ifeq ($(have-z-start-stop-gc),yes)
> >> +tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> >> +tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> >> +tests-special += $(objpfx)tst-cleanup-start-stop-gc-cmp.out \
> >> + $(objpfx)tst-cleanup-nostart-stop-gc-cmp.out
> >> +LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
> >
> >Won't this test fail if linker supports -z start-stop-gc and GCC doesn't support
> >retain attribute?
>
> Yes, but that doesn't happen in practice. If ld supports -z start-stop-gc,
> the GNU as should be new and the GCC should support 'retain'.
I am using GCC 10 and binutils 2.37.
> But perhaps it is good to prevent the issue, I can change
>
> +ifeq ($(have-z-start-stop-gc),yes)
>
> to
>
> +ifeq ($(have-gnu-retain)$(have-z-start-stop-gc),yesyes)
>
> and add a LIBC_CONFIG_VAR in configure.ac.
>
Yes, please.
Thanks.
--
H.J.
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