From: Fangrui Song <maskray@google.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: GNU C Library <libc-alpha@sourceware.org>,
Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH v2] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]
Date: Sun, 21 Mar 2021 21:40:31 -0700 [thread overview]
Message-ID: <20210322044031.4goih7otuijx4xwg@google.com> (raw)
In-Reply-To: <CAMe9rOqkAMmfxGAAL-_VSJ=d6SnaGUiczRWPkCbir52BsH0r1w@mail.gmail.com>
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
# 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.
next prev parent reply other threads:[~2021-03-22 4:40 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 [this message]
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
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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