From: "H.J. Lu" <hjl.tools@gmail.com>
To: "Fāng-ruì Sòng" <maskray@google.com>
Cc: GNU C Library <libc-alpha@sourceware.org>,
Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH v3] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]
Date: Tue, 30 Mar 2021 12:30:32 -0700 [thread overview]
Message-ID: <CAMe9rOq6fUtzVP=evOJtYaHh0R-am0S8U73_S8mq5XRECnxfKA@mail.gmail.com> (raw)
In-Reply-To: <CAFP8O3LJJPMnhfaC4HXeJpcW7-XBPejcTyZ7VbMTb_z+1oUNfQ@mail.gmail.com>
On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>
> On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Mon, Mar 29, 2021 at 4:24 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
> > > ---
> > > config.h.in | 3 +++
> > > configure | 23 +++++++++++++++++++++++
> > > configure.ac | 16 ++++++++++++++++
> > > include/libc-symbols.h | 14 ++++++++++----
> > > 4 files changed, 52 insertions(+), 4 deletions(-)
> > >
> > > 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..6a6b8ac264 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 fce967f2c2..031b8c3fb3 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 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
> > > --
> > > 2.31.0.291.g576ba9dcdaf-goog
> > >
> >
> > Need gc-sections tests for linkers with and without -z start-stop-gc.
> >
> > --
> > H.J.
>
> I don't know how to add such a test... It needs to check flush at ext
> time but I cannot find a template in libio/tst-* ...
You can add a special rule to check for linker output with
--print-gc-sections.
--
H.J.
next prev parent reply other threads:[~2021-03-30 19:31 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 1:10 [PATCH] " Fangrui Song
2021-03-15 7:28 ` Florian Weimer
2021-03-15 7:45 ` Fāng-ruì Sòng
2021-03-15 8:13 ` Florian Weimer
2021-03-22 4:00 ` [PATCH v2] " Fangrui Song
2021-03-22 4:24 ` H.J. Lu
2021-03-22 4:40 ` Fangrui Song
2021-03-22 20:28 ` H.J. Lu
2021-03-23 4:14 ` Fangrui Song
2021-03-29 23:11 ` H.J. Lu
2021-03-29 23:24 ` [PATCH v3] " Fangrui Song
2021-03-30 18:17 ` H.J. Lu
2021-03-30 19:17 ` Fāng-ruì Sòng
2021-03-30 19:30 ` H.J. Lu [this message]
2021-03-31 4:34 ` Fāng-ruì Sòng
2021-03-31 16:17 ` H.J. Lu
2021-03-31 18:09 ` Fāng-ruì Sòng
2021-03-31 18:57 ` H.J. Lu
2021-03-31 19:39 ` [PATCH v4] " Fangrui Song
2021-03-31 23:38 ` H.J. Lu
2021-04-01 0:02 ` Fangrui Song
2021-04-01 1:00 ` H.J. Lu
2021-04-01 1:06 ` [PATCH v5] " Fangrui Song
2021-04-01 12:52 ` H.J. Lu
2021-04-02 3:23 ` Fangrui Song
2021-04-02 14:14 ` H.J. Lu
2021-04-02 17:09 ` Fangrui Song
2021-04-02 17:33 ` H.J. Lu
2021-04-03 18:02 ` Fangrui Song
2021-04-03 20:47 ` H.J. Lu
2021-04-03 21:57 ` Fangrui Song
2021-04-05 1:55 ` H.J. Lu
2021-04-05 18:17 ` Fangrui Song
2021-04-05 20:35 ` H.J. Lu
2021-04-05 21:03 ` Fangrui Song
2021-04-05 21:58 ` H.J. Lu
2021-04-06 21:46 ` Fangrui Song
2021-04-09 22:36 ` Fāng-ruì Sòng
2021-04-16 3:51 ` Fāng-ruì Sòng
2021-04-16 13:00 ` H.J. Lu
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