[PATCH] ld: Add -Bsymbolic-non-weak-functions
Fangrui Song
i@maskray.me
Fri Sep 1 01:10:43 GMT 2023
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Fri Sep 1 01:10:43 GMT 2023
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On Sun, Dec 5, 2021 at 8:53 PM Fangrui Song <i@maskray.me> wrote: > > On Mon, Aug 23, 2021 at 3:14 AM Fangrui Song <i@maskray.me> wrote: > > > > FWIW Android ndk plans to use -Bsymbolic-non-weak-functions. > > The behavior will be quite similar to Mach-O, so pretty safe. > > -Bsymbolic-non-weak-functions or -Bsymbolic-non-weak is still worth considering. > (The behavior will be similar to Mach-O's model (strong definitions > are non-interposable by default; weak definitions are coalesced).) Recently I got interest from Meta (https://reviews.llvm.org/D158322) and SerenityOS[1] . I hope that this patch will be considered:) [1]: "We (SerenityOS) use __attribute__((weak)) to provide stubs for expensive features (Unicode and Locale data) in libraries, which are interposed by the real implementation if the application requests it" > > On Sat, May 22, 2021 at 11:10 PM Fangrui Song via Binutils > > <binutils@sourceware.org> wrote: > > > > > > This option is a subset of -Bsymbolic-functions: only STB_GLOBAL are > > > considered. Vague linkage functions are STB_WEAK. A vague linkage > > > function may have different addresses in a -Bsymbolic-functions linked > > > shared object and outside the shared object. > > > -Bsymbolic-non-weak-functions can keep pointer equality while providing > > > most benefits: (a) fewer JUMP_SLOT (symbol lookups) (b) avoid PLT > > > entries for default visibility defined functions. > > > > > > PR 27871 > > > include/ > > > * bfdlink.h (struct bfd_link_info): Add dynamic_weak_functions. > > > ld/ > > > * ldlex.h (enum option_values): Add OPTION_SYMBOLIC_NON_WEAK_FUNCTIONS. > > > * lexsup.c (struct ld_options): Add -Bsymbolic-non-weak-functions. > > > (enum symbolic_enum): Add symbolic_non_weak_functions. > > > (parse_args): Handle -Bsymbolic-non-weak-functions. > > > * ld.texi: Document -Bsymbolic-non-weak-functions. > > > * NEWS: Mention -Bsymbolic-non-weak-functions. > > > * testsuite/ld-elf/shared.exp: Add tests. > > > * testsuite/ld-elf/symbolic-non-weak-func.s: New file. > > > * testsuite/ld-elf/symbolic-non-weak-func-a.rd: Likewise. > > > * testsuite/ld-elf/symbolic-non-weak-func-b.rd: Likewise. > > > --- > > > bfd/elflink.c | 12 +++++----- > > > include/bfdlink.h | 3 +++ > > > ld/NEWS | 2 ++ > > > ld/ld.texi | 15 ++++++++++--- > > > ld/ldlex.h | 1 + > > > ld/lexsup.c | 17 +++++++++++--- > > > ld/testsuite/ld-elf/shared.exp | 22 +++++++++++++++++++ > > > .../ld-elf/symbolic-non-weak-func-a.rd | 4 ++++ > > > .../ld-elf/symbolic-non-weak-func-b.rd | 4 ++++ > > > ld/testsuite/ld-elf/symbolic-non-weak-func.s | 18 +++++++++++++++ > > > 10 files changed, 85 insertions(+), 13 deletions(-) > > > create mode 100644 ld/testsuite/ld-elf/symbolic-non-weak-func-a.rd > > > create mode 100644 ld/testsuite/ld-elf/symbolic-non-weak-func-b.rd > > > create mode 100644 ld/testsuite/ld-elf/symbolic-non-weak-func.s > > > > > > diff --git a/bfd/elflink.c b/bfd/elflink.c > > > index 0e1871aaac9..cbdac95f18a 100644 > > > --- a/bfd/elflink.c > > > +++ b/bfd/elflink.c > > > @@ -597,14 +597,12 @@ bfd_elf_link_mark_dynamic_symbol (struct bfd_link_info *info, > > > if(h->dynamic || bfd_link_relocatable (info)) > > > return; > > > > > > + int type = sym != NULL ? ELF_ST_TYPE (sym->st_info) : STT_NOTYPE; > > > if ((info->dynamic_data > > > - && (h->type == STT_OBJECT > > > - || h->type == STT_COMMON > > > - || (sym != NULL > > > - && (ELF_ST_TYPE (sym->st_info) == STT_OBJECT > > > - || ELF_ST_TYPE (sym->st_info) == STT_COMMON)))) > > > - || (d != NULL > > > - && h->non_elf > > > + && (type == STT_OBJECT || type == STT_COMMON)) > > > + || (info->dynamic_weak_functions && type == STT_FUNC > > > + && ELF_ST_BIND (sym->st_info) == STB_WEAK) > > > + || (d != NULL && h->non_elf > > > && (*d->match) (&d->head, NULL, h->root.root.string))) > > > { > > > h->dynamic = 1; > > > diff --git a/include/bfdlink.h b/include/bfdlink.h > > > index 7f1b12dbf37..e1576405007 100644 > > > --- a/include/bfdlink.h > > > +++ b/include/bfdlink.h > > > @@ -369,6 +369,9 @@ struct bfd_link_info > > > /* TRUE if all data symbols should be dynamic. */ > > > unsigned int dynamic_data: 1; > > > > > > + /* TRUE if all weak function symbols should be dynamic. */ > > > + unsigned int dynamic_weak_functions: 1; > > > + > > > /* TRUE if section groups should be resolved. */ > > > unsigned int resolve_section_groups: 1; > > > > > > diff --git a/ld/NEWS b/ld/NEWS > > > index a5ed9058c04..86bf9a8f9ab 100644 > > > --- a/ld/NEWS > > > +++ b/ld/NEWS > > > @@ -10,6 +10,8 @@ > > > > > > * Add -Bno-symbolic to cancel -Bsymbolic and -Bsymbolic-functions. > > > > > > +* Add -Bsymbolic-non-weak-functions as a safe subset of -Bsymbolic-functions. > > > + > > > Changes in 2.36: > > > > > > * Add libdep plugin, for linking dependencies of static libraries that > > > diff --git a/ld/ld.texi b/ld/ld.texi > > > index 29874688a73..3807ab98f57 100644 > > > --- a/ld/ld.texi > > > +++ b/ld/ld.texi > > > @@ -1620,7 +1620,7 @@ libraries. > > > > > > @kindex -Bsymbolic > > > @item -Bsymbolic > > > -When creating a shared library, bind references to global symbols to the > > > +When creating a shared library, bind references to non-local symbols to the > > > definition within the shared library, if any. Normally, it is possible > > > for a program linked against a shared library to override the definition > > > within the shared library. This option is only meaningful on ELF > > > @@ -1628,11 +1628,20 @@ platforms which support shared libraries. > > > > > > @kindex -Bsymbolic-functions > > > @item -Bsymbolic-functions > > > -When creating a shared library, bind references to global function > > > -symbols to the definition within the shared library, if any. > > > +When creating a shared library, bind references to non-local function > > > +symbols to the definition within the shared library, if any. A vague linkage > > > +function definition is weak. It may have different addresses in the linked > > > +shared library and outside the shared library. > > > This option is only meaningful on ELF platforms which support shared > > > libraries. > > > > > > +@kindex -Bsymbolic-non-weak-functions > > > +@item -Bsymbolic-non-weak-functions > > > +When creating a shared library, bind references to @code{STB_GLOBAL} function > > > +symbols to the definition within the shared library, if any. Noticeably this > > > +option skips C++ vague linkage functions and is thus safe. > > > +This option is only meaningful on ELF platforms which support shared libraries. > > > + > > > @kindex -Bno-symbolic > > > @item -Bno-symbolic > > > This option can cancel previously specified @samp{-Bsymbolic} and > > > diff --git a/ld/ldlex.h b/ld/ldlex.h > > > index e0f0241d9cc..188160c6b9d 100644 > > > --- a/ld/ldlex.h > > > +++ b/ld/ldlex.h > > > @@ -64,6 +64,7 @@ enum option_values > > > OPTION_SORT_SECTION, > > > OPTION_STATS, > > > OPTION_SYMBOLIC, > > > + OPTION_SYMBOLIC_NON_WEAK_FUNCTIONS, > > > OPTION_SYMBOLIC_FUNCTIONS, > > > OPTION_TASK_LINK, > > > OPTION_TBSS, > > > diff --git a/ld/lexsup.c b/ld/lexsup.c > > > index dcb2d9d9ab3..64db6a89460 100644 > > > --- a/ld/lexsup.c > > > +++ b/ld/lexsup.c > > > @@ -305,9 +305,11 @@ static const struct ld_option ld_options[] = > > > { {"Bno-symbolic", no_argument, NULL, OPTION_NO_SYMBOLIC}, > > > '\0', NULL, N_("Don't bind global references locally"), ONE_DASH }, > > > { {"Bsymbolic", no_argument, NULL, OPTION_SYMBOLIC}, > > > - '\0', NULL, N_("Bind global references locally"), ONE_DASH }, > > > + '\0', NULL, N_("Bind default visibility defined symbols locally for -shared"), ONE_DASH }, > > > + { {"Bsymbolic-non-weak-functions", no_argument, NULL, OPTION_SYMBOLIC_NON_WEAK_FUNCTIONS}, > > > + '\0', NULL, N_("Bind default visibility defined STB_GLOBAL function symbols locally for -shared"), ONE_DASH }, > > > { {"Bsymbolic-functions", no_argument, NULL, OPTION_SYMBOLIC_FUNCTIONS}, > > > - '\0', NULL, N_("Bind global function references locally"), ONE_DASH }, > > > + '\0', NULL, N_("Bind default visibility defined function symbols locally for -shared"), ONE_DASH }, > > > { {"check-sections", no_argument, NULL, OPTION_CHECK_SECTIONS}, > > > '\0', NULL, N_("Check section addresses for overlaps (default)"), > > > TWO_DASHES }, > > > @@ -611,8 +613,9 @@ parse_args (unsigned argc, char **argv) > > > enum symbolic_enum > > > { > > > symbolic_unset = 0, > > > - symbolic, > > > + symbolic_non_weak_functions, > > > symbolic_functions, > > > + symbolic, > > > } opt_symbolic = symbolic_unset; > > > enum dynamic_list_enum > > > { > > > @@ -1309,6 +1312,9 @@ parse_args (unsigned argc, char **argv) > > > case OPTION_SYMBOLIC: > > > opt_symbolic = symbolic; > > > break; > > > + case OPTION_SYMBOLIC_NON_WEAK_FUNCTIONS: > > > + opt_symbolic = symbolic_non_weak_functions; > > > + break; > > > case OPTION_SYMBOLIC_FUNCTIONS: > > > opt_symbolic = symbolic_functions; > > > break; > > > @@ -1888,6 +1894,11 @@ parse_args (unsigned argc, char **argv) > > > link_info.dynamic = true; > > > link_info.dynamic_data = true; > > > break; > > > + case symbolic_non_weak_functions: > > > + link_info.dynamic = true; > > > + link_info.dynamic_data = true; > > > + link_info.dynamic_weak_functions = true; > > > + break; > > > } > > > > > > if (!bfd_link_dll (&link_info)) > > > diff --git a/ld/testsuite/ld-elf/shared.exp b/ld/testsuite/ld-elf/shared.exp > > > index d00358e47ef..0ddc4b1cfcd 100644 > > > --- a/ld/testsuite/ld-elf/shared.exp > > > +++ b/ld/testsuite/ld-elf/shared.exp > > > @@ -457,6 +457,28 @@ run_ld_link_tests [list \ > > > "symbolic-func.so"] \ > > > ] > > > > > > +if {[istarget "aarch64*-*-*"] || [istarget "powerpc*-*-*"] || > > > + [istarget "i?86-*-*"] || [istarget "x86_64-*-*"]} { > > > + run_ld_link_tests [list \ > > > + [list "-Bsymbolic-non-weak-functions -Bsymbolic" \ > > > + "-shared -Bsymbolic-non-weak-functions -Bsymbolic" "" "$AFLAGS_PIC" \ > > > + {symbolic-non-weak-func.s} {{readelf {-r --wide} symbolic-non-weak-func-a.rd}} \ > > > + "symbolic-non-weak-func-a.so"] \ > > > + ] > > > + run_ld_link_tests [list \ > > > + [list "-Bsymbolic-non-weak-functions" \ > > > + "-shared -Bsymbolic-non-weak-functions" "" "$AFLAGS_PIC" \ > > > + {symbolic-non-weak-func.s} {{readelf {-r --wide} symbolic-non-weak-func-b.rd}} \ > > > + "symbolic-non-weak-func-b.so"] \ > > > + ] > > > + run_ld_link_tests [list \ > > > + [list "-Bsymbolic-functions -Bsymbolic-non-weak-functions" \ > > > + "-shared -Bsymbolic-functions -Bsymbolic-non-weak-functions" "" "$AFLAGS_PIC" \ > > > + {symbolic-non-weak-func.s} {{readelf {-r --wide} symbolic-non-weak-func-b.rd}} \ > > > + "symbolic-non-weak-func-b.so"] \ > > > + ] > > > +} > > > + > > > run_ld_link_tests [list \ > > > [list "Build pr20995.so" \ > > > "-shared" "" "$AFLAGS_PIC" \ > > > diff --git a/ld/testsuite/ld-elf/symbolic-non-weak-func-a.rd b/ld/testsuite/ld-elf/symbolic-non-weak-func-a.rd > > > new file mode 100644 > > > index 00000000000..ef591840f5c > > > --- /dev/null > > > +++ b/ld/testsuite/ld-elf/symbolic-non-weak-func-a.rd > > > @@ -0,0 +1,4 @@ > > > +#... > > > +[0-9a-f]+ +[0-9a-f]+ +R_.*_RELATIVE .* > > > +[0-9a-f]+ +[0-9a-f]+ +R_.*_RELATIVE .* > > > +[0-9a-f]+ +[0-9a-f]+ +R_.*_RELATIVE .* > > > diff --git a/ld/testsuite/ld-elf/symbolic-non-weak-func-b.rd b/ld/testsuite/ld-elf/symbolic-non-weak-func-b.rd > > > new file mode 100644 > > > index 00000000000..34228b0627b > > > --- /dev/null > > > +++ b/ld/testsuite/ld-elf/symbolic-non-weak-func-b.rd > > > @@ -0,0 +1,4 @@ > > > +#... > > > +[0-9a-f]+ +[0-9a-f]+ +R_.*_RELATIVE .* > > > +[0-9a-f]+ +[0-9a-f]+ +R_.*_RELATIVE .* > > > +[0-9a-f]+ +[0-9a-f]+ +R_.* weak_fun.* > > > diff --git a/ld/testsuite/ld-elf/symbolic-non-weak-func.s b/ld/testsuite/ld-elf/symbolic-non-weak-func.s > > > new file mode 100644 > > > index 00000000000..e259f12bfc1 > > > --- /dev/null > > > +++ b/ld/testsuite/ld-elf/symbolic-non-weak-func.s > > > @@ -0,0 +1,18 @@ > > > + .text > > > + .global global_fun > > > + .type global_fun, %function > > > +global_fun: > > > + .space 4 > > > + .weak weak_fun > > > + .type weak_fun, %function > > > +weak_fun: > > > + .space 4 > > > + > > > + .section .data,"aw",%progbits > > > + .p2align 3 > > > + .dc.a global_data > > > + .dc.a global_fun > > > + .dc.a weak_fun > > > + > > > + .global global_data > > > +global_data: > > > -- > > > 2.31.1.818.g46aad6cb9e-goog > > >
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