Message ID | 20220626184025.553459-1-maskray@google.com |
---|---|
State | New |
Headers | show |
Series | Makerules: Remove no-op -Wl,-d when linking libc_pic.os | expand |
* Fangrui Song via Libc-alpha: > In GNU ld, -d assigns space to common symbols for -r (i.e. change common > symbols to STB_GLOBAL definitions). This option was added in commit > da2d1bc5adf49352232ad0514e79fbd5dcae08e8 (1998) likely because ld at > that time had a bug that common symbols did not override shared object > definitions. -d has been long unneeded and more so since -fno-common > was added to +cflags. > --- > Makerules | 8 ++------ > 1 file changed, 2 insertions(+), 6 deletions(-) > > diff --git a/Makerules b/Makerules > index dfe89e9e39..d1e139d03c 100644 > --- a/Makerules > +++ b/Makerules > @@ -633,14 +633,10 @@ LDFLAGS-c.so = -nostdlib -nostartfiles > LDLIBS-c.so += $(libc.so-gnulib) > # Give libc.so an entry point and make it directly runnable itself. > LDFLAGS-c.so += -e __libc_main > -# Pre-link the objects of libc_pic.a so that we can locally resolve > -# COMMON symbols before we link against ld.so. This is because ld.so > -# contains some of libc_pic.a already, which will prevent the COMMONs > -# from being allocated in libc.so, which introduces evil dependencies > -# between libc.so and ld.so, which can make it impossible to upgrade. > +# Pre-link the objects of libc_pic.a for .gnu.glibc-stub.* processing. > $(common-objpfx)libc_pic.os: $(common-objpfx)libc_pic.a > $(LINK.o) -nostdlib -nostartfiles -r -o $@ \ > - $(LDFLAGS-c_pic.os) -Wl,-d $(whole-archive) $^ -o $@ > + $(LDFLAGS-c_pic.os) $(whole-archive) $^ -o $@ > > ifeq (,$(strip $(shlib-lds-flags))) > # Generate a list of -R options to excise .gnu.glibc-stub.* sections. This looks okay to me, thanks. Do you know what the .gnu.glibc-stub.* processing processing refers to?
On 2022-06-26, Florian Weimer wrote: >* Fangrui Song via Libc-alpha: > >> In GNU ld, -d assigns space to common symbols for -r (i.e. change common >> symbols to STB_GLOBAL definitions). This option was added in commit >> da2d1bc5adf49352232ad0514e79fbd5dcae08e8 (1998) likely because ld at >> that time had a bug that common symbols did not override shared object >> definitions. -d has been long unneeded and more so since -fno-common >> was added to +cflags. >> --- >> Makerules | 8 ++------ >> 1 file changed, 2 insertions(+), 6 deletions(-) >> >> diff --git a/Makerules b/Makerules >> index dfe89e9e39..d1e139d03c 100644 >> --- a/Makerules >> +++ b/Makerules >> @@ -633,14 +633,10 @@ LDFLAGS-c.so = -nostdlib -nostartfiles >> LDLIBS-c.so += $(libc.so-gnulib) >> # Give libc.so an entry point and make it directly runnable itself. >> LDFLAGS-c.so += -e __libc_main >> -# Pre-link the objects of libc_pic.a so that we can locally resolve >> -# COMMON symbols before we link against ld.so. This is because ld.so >> -# contains some of libc_pic.a already, which will prevent the COMMONs >> -# from being allocated in libc.so, which introduces evil dependencies >> -# between libc.so and ld.so, which can make it impossible to upgrade. >> +# Pre-link the objects of libc_pic.a for .gnu.glibc-stub.* processing. >> $(common-objpfx)libc_pic.os: $(common-objpfx)libc_pic.a >> $(LINK.o) -nostdlib -nostartfiles -r -o $@ \ >> - $(LDFLAGS-c_pic.os) -Wl,-d $(whole-archive) $^ -o $@ >> + $(LDFLAGS-c_pic.os) $(whole-archive) $^ -o $@ >> >> ifeq (,$(strip $(shlib-lds-flags))) >> # Generate a list of -R options to excise .gnu.glibc-stub.* sections. > >This looks okay to me, thanks. > >Do you know what the .gnu.glibc-stub.* processing processing refers >to? [a] Just few lines below, there is # Generate a list of -R options to excise .gnu.glibc-stub.* sections. The idea is to create libc_pic.os from libc_pic.a, strip .gnu.glibc-stub.*, then link libc_pic.os.clean into libc.so. Technically the relocatable link can be omitted. We can change `build-shlib` to link -Wl,--whole-archive libc_pic.a -Wl,--no-whole-archive instead. objcopy -R works on an archive for a long time. I am unsure whether the 1990+ objcopy supports archives. But at a first glance I do not find a good place to insert -Wl,--whole-archive into build-shlib... [b] Around $(objpfx)stubs: $(objs-for-stubs), we can see that .gnu.glibc-stub.* is to generate */stubs files and finally testroot.root/$prefix/include/gnu/stubs-64.h % cat testroot.root/tmp/glibc/lld/include/gnu/stubs-64.h /* This file is automatically generated. It defines a symbol `__stub_FUNCTION' for each function in the C library which is a stub, meaning it will fail every time called, usually setting errno to ENOSYS. */ #ifdef _LIBC #error Applications may not define the macro _LIBC #endif #define __stub___compat_bdflush #define __stub_chflags #define __stub_fchflags #define __stub_gtty #define __stub_revoke #define __stub_setlogin #define __stub_sigreturn #define __stub_stty
* Fangrui Song: >>Do you know what the .gnu.glibc-stub.* processing processing refers >>to? > > [a] Just few lines below, there is > > # Generate a list of -R options to excise .gnu.glibc-stub.* sections. > > The idea is to create libc_pic.os from libc_pic.a, strip .gnu.glibc-stub.*, then link libc_pic.os.clean into libc.so. > > Technically the relocatable link can be omitted. We can change `build-shlib` > to link -Wl,--whole-archive libc_pic.a -Wl,--no-whole-archive instead. objcopy -R works on an archive for > a long time. I am unsure whether the 1990+ objcopy supports archives. > But at a first glance I do not find a good place to insert -Wl,--whole-archive into build-shlib... For linking libc.so, we can list the objects directly, then we don't need -Wl,--whole-archive. For linking ld.so, we do not use -Wl,--whole-archive because we actually want to link a subset. I was under the impression that we rebuild objects for inclusion into ld.so, so I'm rather puzzled that we need this. > [b] Around $(objpfx)stubs: $(objs-for-stubs), we can see that > .gnu.glibc-stub.* is to generate */stubs files and finally testroot.root/$prefix/include/gnu/stubs-64.h > > % cat testroot.root/tmp/glibc/lld/include/gnu/stubs-64.h > /* This file is automatically generated. > It defines a symbol `__stub_FUNCTION' for each function > in the C library which is a stub, meaning it will fail > every time called, usually setting errno to ENOSYS. */ > > #ifdef _LIBC > #error Applications may not define the macro _LIBC > #endif > > #define __stub___compat_bdflush > #define __stub_chflags > #define __stub_fchflags > #define __stub_gtty > #define __stub_revoke > #define __stub_setlogin > #define __stub_sigreturn > #define __stub_stty Those warning sections must propagate into libc.so, so I think we could parse that and the generate header from it.
On 2022-06-27, Florian Weimer wrote: >* Fangrui Song: > >>>Do you know what the .gnu.glibc-stub.* processing processing refers >>>to? >> >> [a] Just few lines below, there is >> >> # Generate a list of -R options to excise .gnu.glibc-stub.* sections. >> >> The idea is to create libc_pic.os from libc_pic.a, strip .gnu.glibc-stub.*, then link libc_pic.os.clean into libc.so. >> >> Technically the relocatable link can be omitted. We can change `build-shlib` >> to link -Wl,--whole-archive libc_pic.a -Wl,--no-whole-archive instead. objcopy -R works on an archive for >> a long time. I am unsure whether the 1990+ objcopy supports archives. >> But at a first glance I do not find a good place to insert -Wl,--whole-archive into build-shlib... > >For linking libc.so, we can list the objects directly, then we don't >need -Wl,--whole-archive. > >For linking ld.so, we do not use -Wl,--whole-archive because we >actually want to link a subset. I was under the impression that we >rebuild objects for inclusion into ld.so, so I'm rather puzzled that >we need this. This plan looks good to me. Removing the relocatable link for libc_pic.os will make incremental build slightly faster. >> [b] Around $(objpfx)stubs: $(objs-for-stubs), we can see that >> .gnu.glibc-stub.* is to generate */stubs files and finally testroot.root/$prefix/include/gnu/stubs-64.h >> >> % cat testroot.root/tmp/glibc/lld/include/gnu/stubs-64.h >> /* This file is automatically generated. >> It defines a symbol `__stub_FUNCTION' for each function >> in the C library which is a stub, meaning it will fail >> every time called, usually setting errno to ENOSYS. */ >> >> #ifdef _LIBC >> #error Applications may not define the macro _LIBC >> #endif >> >> #define __stub___compat_bdflush >> #define __stub_chflags >> #define __stub_fchflags >> #define __stub_gtty >> #define __stub_revoke >> #define __stub_setlogin >> #define __stub_sigreturn >> #define __stub_stty > >Those warning sections must propagate into libc.so, so I think we >could parse that and the generate header from it. The final libc.so does not want .gnu.glibc-stub.* sections. The current build steps look like: 1. ar cruv libc_pic.a `cat csu/stamp.os iconv/stamp.os ...` 2. build libc_pic.os libc_pic.a 3. build libc_pic.os.clean from libc_pic.os 4. build libc.so from libc_pic.os.clean csu/abi-note.o elf/interp.os elf/ld.so ... I think ideally 2 and 3 can be omitted. It seems like you want to omit libc_pic.a as well? That looks good to me. stubs files can be either generated from .os files (in step 1) or libc.so (then a separate strip command on libc.so is needed).
diff --git a/Makerules b/Makerules index dfe89e9e39..d1e139d03c 100644 --- a/Makerules +++ b/Makerules @@ -633,14 +633,10 @@ LDFLAGS-c.so = -nostdlib -nostartfiles LDLIBS-c.so += $(libc.so-gnulib) # Give libc.so an entry point and make it directly runnable itself. LDFLAGS-c.so += -e __libc_main -# Pre-link the objects of libc_pic.a so that we can locally resolve -# COMMON symbols before we link against ld.so. This is because ld.so -# contains some of libc_pic.a already, which will prevent the COMMONs -# from being allocated in libc.so, which introduces evil dependencies -# between libc.so and ld.so, which can make it impossible to upgrade. +# Pre-link the objects of libc_pic.a for .gnu.glibc-stub.* processing. $(common-objpfx)libc_pic.os: $(common-objpfx)libc_pic.a $(LINK.o) -nostdlib -nostartfiles -r -o $@ \ - $(LDFLAGS-c_pic.os) -Wl,-d $(whole-archive) $^ -o $@ + $(LDFLAGS-c_pic.os) $(whole-archive) $^ -o $@ ifeq (,$(strip $(shlib-lds-flags))) # Generate a list of -R options to excise .gnu.glibc-stub.* sections.