[hjl@gnu-4 tmp]$ cat x.c void foo () { } [hjl@gnu-4 tmp]$ gcc -shared -fpic x.c [hjl@gnu-4 tmp]$ readelf --dyn-syms a.out Symbol table '.dynsym' contains 11 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab 2: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__ 3: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable 4: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@GLIBC_2.2.5 (2) 5: 0000000000004018 0 NOTYPE GLOBAL DEFAULT 20 _edata 6: 00000000000010f9 2 FUNC GLOBAL DEFAULT 11 foo 7: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 21 _end 8: 0000000000004018 0 NOTYPE GLOBAL DEFAULT 21 __bss_start 9: 0000000000001000 0 FUNC GLOBAL DEFAULT 8 _init 10: 00000000000010fc 0 FUNC GLOBAL DEFAULT 12 _fini [hjl@gnu-4 tmp]$ __bss_start, _end and _edata shouldn't be in .dynsym.
I agree that exporting these symbols doesn't seem to be useful. I can't see a good reason why anyone would want access to their values in shared libraries external to the given shared library (and to choose a particular _edata say, from among many possible shared libraries requires dlopen and dlsym). Also, given that the standard scripts define these symbols for shared libraries, it isn't simple for a shared library to access their values in the executable. Again, you'd need to use dlopen and dlsym. So it probably isn't that useful to export these symbols from an executable, but I'm less sure about executables than I am about shared libraries.
(In reply to Alan Modra from comment #1) FreeBSD's libc uses _end to initialize curbrk. At a glance, OpenBSD and NetBSD appear to do this as well. For this use case it is crucial that _end be present in the executable's dynsym, since libc will otherwise use its internal definition. I peeked at the glibc sources; it seems that they rely on Linux's brk() system call returning the current break value when given an input of 0. This is used to lazily initialize the curbrk. We could probably add a similar hack to FreeBSD to get around this, but as noted, we're not the only ones relying on this behaviour.
Should we not define __bss_start/_end/_edata in shared libraries at all and always use the definitions in executable? Will this work for FreeBSD?
(In reply to H.J. Lu from comment #3) I suspect that would address the problem I brought up here: https://sourceware.org/ml/binutils/2018-05/msg00300.html I'm happy to test patches. If I can apply the change to the binutils 2.17.50 sources in FreeBSD's base system, we can test a build of the entire ports tree (~20,000 third-party packages).
(In reply to Mark Johnston from comment #4) > (In reply to H.J. Lu from comment #3) > I suspect that would address the problem I brought up here: > https://sourceware.org/ml/binutils/2018-05/msg00300.html > > I'm happy to test patches. If I can apply the change to the binutils 2.17.50 Please try https://sourceware.org/ml/binutils/2018-05/msg00350.html > sources in FreeBSD's base system, we can test a build of the entire ports > tree (~20,000 third-party packages). I don't believe my patches are suitable for such old binutils.
(In reply to H.J. Lu from comment #5) > (In reply to Mark Johnston from comment #4) > > (In reply to H.J. Lu from comment #3) > > I suspect that would address the problem I brought up here: > > https://sourceware.org/ml/binutils/2018-05/msg00300.html > > > > I'm happy to test patches. If I can apply the change to the binutils 2.17.50 > > Please try > > https://sourceware.org/ml/binutils/2018-05/msg00350.html Thanks, that worked for me. > > sources in FreeBSD's base system, we can test a build of the entire ports > > tree (~20,000 third-party packages). > > I don't believe my patches are suitable for such old binutils. As it turns out, the elf.sc patch may be applied without much effort.
(In reply to Mark Johnston from comment #2) > (In reply to Alan Modra from comment #1) > FreeBSD's libc uses _end to initialize curbrk. At a glance, OpenBSD and > NetBSD appear to do this as well. For this use case it is crucial that _end > be present in the executable's dynsym, since libc will otherwise use its > internal definition. In glibc, ld.so has /* These defined magically in the linker script. */ extern char _begin[] attribute_hidden; extern char _etext[] attribute_hidden; extern char _end[] attribute_hidden; it can't reference _end defined in executables.
Comment #2 implie "_end" must be made dynamic in the executable if referenced from a shared library, and the linker script can't make "_end" hidden when building a shared library. Comment #7 implies that we must continue to define "_end" in linker scripts when building shared libraries.
The master branch has been updated by Alan Modra <amodra@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=fd161d860f1df7140153eab4726705cc3e2727b0 commit fd161d860f1df7140153eab4726705cc3e2727b0 Author: Alan Modra <amodra@gmail.com> Date: Fri Jun 8 20:17:34 2018 +0930 Define various symbols conditionally in shared libraries The values of symbols in shared libraries like _end, _edata, and __bss_start are generally not that useful outside of the shared library. This patch defines them conditionally with PROVIDE, since a shared library might need the local value. An example is glibc ld.so local access to "_begin", "_etext" and "_end". (ld.so gains access to the local values by making the references using hidden visibility. That makes the definitions hidden too.) We can't use PROVIDE_HIDDEN in the linker scripts because the shared library might need the value of the symbol in the executable. An example is freebsd libc dynamic access to "_end". PR ld/23161 * emulparams/aarch64cloudabi.sh: PROVIDE __bss_start__, _bss_end__, and __end__ in shared libraries. * emulparams/aarch64fbsd.sh: Likewise. * emulparams/aarch64linux.sh: Likewise. * emulparams/aarch64linux32.sh: Likewise. * emulparams/armelf_fuchsia.sh: Likewise. * emulparams/armelf_linux.sh: Likewise. * emulparams/armelf_phoenix.sh: Likewise. * emulparams/aarch64elf.sh: Likewise, and __data_start * emulparams/aarch64elf32.sh: Likewise. * emulparams/armelf.sh: Likewise. * emulparams/armnto.sh: Likewise. * emulparams/elf32bmip.sh: Remove duplicate ". = ." from OTHER_GOT_SYMBOLS. PROVIDE _ftext, _fdata, and _fbss in shared libs. * emulparams/elf32bmipn32-defs.sh: Likewise. * emulparams/elf32frv.sh: PROVIDE __end and __data_start in shared libs. * emulparams/elf32lriscv-defs.sh: Tidy. * emulparams/elf32mcore.sh: PROVIDE __bss_start and _bss_end in shared libs * emulparams/elf32ppccommon.sh: PROVIDE __end in shared libs. * emulparams/elf32rl78.sh: Tidy. * emulparams/i386nto.sh: PROVIDE _btext in shared libs. * emulparams/shelf_nto.sh: Likewise. * emulparams/shlelf_nto.sh: Likewise. * emulparams/score3_elf.sh: PROVIDE _gp, _bss_start__, _bss_end__, __bss_end__, __end__, _fdata, _sdata_begin, and _bss_start in shared libs. * scripttempl/elf.sc: Don't use EXECUTABLE_SYMBOLS for shared libraries. PROVIDE _edata, edata, __bss_start, and _end in shared libraries. * testsuite/ld-elf/pr23161.map, * testsuite/ld-elf/pr23161a.c, * testsuite/ld-elf/pr23161b.c, * testsuite/ld-elf/pr23161c.c, * testsuite/ld-elf/pr23161a.rd, * testsuite/ld-elf/pr23161b.rd, * testsuite/ld-elf/pr23161c.rd, * testsuite/ld-elf/pr23161d.rd: New tests. * testsuite/ld-elf/shared.exp: Run ld/23161 tests. * testsuite/ld-elf/pr23162.rd, * testsuite/ld-aarch64/ifunc-1-local.d, * testsuite/ld-aarch64/ifunc-1.d, * testsuite/ld-aarch64/ifunc-2-local.d, * testsuite/ld-aarch64/ifunc-2.d, * testsuite/ld-aarch64/ifunc-21.d, * testsuite/ld-aarch64/ifunc-3a.d, * testsuite/ld-alpha/tlsbin.rd, * testsuite/ld-alpha/tlsbin.sd, * testsuite/ld-alpha/tlsbinr.rd, * testsuite/ld-alpha/tlspic.rd, * testsuite/ld-alpha/tlspic.sd, * testsuite/ld-cris/dso-pltdis1.d, * testsuite/ld-cris/dso-pltdis2.d, * testsuite/ld-cris/dso12-pltdis.d, * testsuite/ld-cris/gotplt1.d, * testsuite/ld-cris/gotplt2.d, * testsuite/ld-cris/gotplt3.d, * testsuite/ld-cris/hiddef1.d, * testsuite/ld-cris/libdso-1.d, * testsuite/ld-cris/libdso-10.d, * testsuite/ld-cris/libdso-11.d, * testsuite/ld-cris/libdso-12.d, * testsuite/ld-cris/libdso-12b.d, * testsuite/ld-cris/libdso-12c.d, * testsuite/ld-cris/libdso-13.d, * testsuite/ld-cris/libdso-13b.d, * testsuite/ld-cris/libdso-14.d, * testsuite/ld-cris/libdso-15.d, * testsuite/ld-cris/libdso-15b.d, * testsuite/ld-cris/libdso-1c.d, * testsuite/ld-cris/libdso-1d.d, * testsuite/ld-cris/libdso-2.d, * testsuite/ld-cris/pic-gc-72.d, * testsuite/ld-cris/pic-gc-73.d, * testsuite/ld-cris/pr16044.d, * testsuite/ld-cris/pv32-1.d, * testsuite/ld-cris/tls-dso-dtpoffd2.d, * testsuite/ld-cris/tls-dso-dtpoffd4.d, * testsuite/ld-cris/tls-dso-tpoffgotcomm1.d, * testsuite/ld-cris/tls-gc-71.d, * testsuite/ld-cris/tls-gd-1.d, * testsuite/ld-cris/tls-gd-1h.d, * testsuite/ld-cris/tls-gd-2.d, * testsuite/ld-cris/tls-gd-2h.d, * testsuite/ld-cris/tls-ie-10.d, * testsuite/ld-cris/tls-ie-11.d, * testsuite/ld-cris/tls-ie-78.d, * testsuite/ld-cris/tls-ie-8.d, * testsuite/ld-cris/tls-ie-9.d, * testsuite/ld-cris/tls-js1.d, * testsuite/ld-cris/tls-ld-4.d, * testsuite/ld-cris/tls-ld-5.d, * testsuite/ld-cris/tls-ld-6.d, * testsuite/ld-cris/tls-ld-7.d, * testsuite/ld-cris/tls-ldgd-14.d, * testsuite/ld-cris/tls-ldgd-15.d, * testsuite/ld-cris/tls-ldgdex-14.d, * testsuite/ld-cris/tls-ldgdex-15.d, * testsuite/ld-cris/tls-ldgdx-14.d, * testsuite/ld-cris/tls-ldgdx-15.d, * testsuite/ld-cris/tls-legdx-16.d, * testsuite/ld-cris/tls-legdx-17.d, * testsuite/ld-cris/tls-local-54.d, * testsuite/ld-cris/tls-local-60.d, * testsuite/ld-cris/tls-local-61.d, * testsuite/ld-cris/tls-local-63.d, * testsuite/ld-cris/tls-local-64.d, * testsuite/ld-cris/tls-ok-30.d, * testsuite/ld-cris/tls-ok-32.d, * testsuite/ld-cris/tls-ok-34.d, * testsuite/ld-cris/tls-und-38.d, * testsuite/ld-cris/tls-und-42.d, * testsuite/ld-cris/tls-und-46.d, * testsuite/ld-cris/tls-und-50.d, * testsuite/ld-cris/weakhiddso.d, * testsuite/ld-cris/weakref2.d, * testsuite/ld-frv/fdpic-shared-1.d, * testsuite/ld-frv/fdpic-shared-2.d, * testsuite/ld-frv/fdpic-shared-3.d, * testsuite/ld-frv/fdpic-shared-4.d, * testsuite/ld-frv/fdpic-shared-5.d, * testsuite/ld-frv/fdpic-shared-7.d, * testsuite/ld-frv/fdpic-shared-8.d, * testsuite/ld-frv/tls-dynamic-2.d, * testsuite/ld-i386/ibt-plt-1.d, * testsuite/ld-i386/ibt-plt-2a.d, * testsuite/ld-i386/ibt-plt-2b.d, * testsuite/ld-i386/ibt-plt-2c.d, * testsuite/ld-i386/ibt-plt-2d.d, * testsuite/ld-i386/ibt-plt-3a.d, * testsuite/ld-i386/ibt-plt-3b.d, * testsuite/ld-i386/ibt-plt-3c.d, * testsuite/ld-i386/ibt-plt-3d.d, * testsuite/ld-i386/plt2.dd, * testsuite/ld-i386/pr20830.d, * testsuite/ld-i386/tlsbin-nacl.rd, * testsuite/ld-i386/tlsbin.rd, * testsuite/ld-i386/tlsbin2-nacl.rd, * testsuite/ld-i386/tlsbin2.rd, * testsuite/ld-i386/tlsbindesc-nacl.rd, * testsuite/ld-i386/tlsbindesc.rd, * testsuite/ld-i386/tlsdesc-nacl.rd, * testsuite/ld-i386/tlsdesc.rd, * testsuite/ld-i386/tlsgdesc-nacl.rd, * testsuite/ld-i386/tlsgdesc.rd, * testsuite/ld-i386/tlsnopic-nacl.rd, * testsuite/ld-i386/tlsnopic.dd, * testsuite/ld-i386/tlsnopic.rd, * testsuite/ld-i386/tlsnopic.sd, * testsuite/ld-i386/tlspic-nacl.rd, * testsuite/ld-i386/tlspic.rd, * testsuite/ld-i386/tlspic2-nacl.rd, * testsuite/ld-i386/tlspic2.rd, * testsuite/ld-ia64/merge1.d, * testsuite/ld-ia64/merge2.d, * testsuite/ld-ia64/merge3.d, * testsuite/ld-ia64/merge4.d, * testsuite/ld-ia64/merge5.d, * testsuite/ld-ia64/tlsbin.rd, * testsuite/ld-ia64/tlspic.rd, * testsuite/ld-ifunc/ifunc-2-i386-now.d, * testsuite/ld-ifunc/ifunc-2-local-i386-now.d, * testsuite/ld-ifunc/ifunc-2-local-x86-64-now.d, * testsuite/ld-ifunc/ifunc-2-local-x86-64.d, * testsuite/ld-ifunc/ifunc-2-x86-64-now.d, * testsuite/ld-ifunc/ifunc-2-x86-64.d, * testsuite/ld-ifunc/pr17154-i386-now.d, * testsuite/ld-ifunc/pr17154-i386.d, * testsuite/ld-ifunc/pr17154-x86-64-now.d, * testsuite/ld-ifunc/pr17154-x86-64.d, * testsuite/ld-m68k/tls-def-1.d, * testsuite/ld-m68k/tls-gd-1.d2, * testsuite/ld-metag/shared.d, * testsuite/ld-metag/stub_pic_app.d, * testsuite/ld-mips-elf/rel32-n32.d, * testsuite/ld-mips-elf/rel32-o32.d, * testsuite/ld-mips-elf/rel64.d, * testsuite/ld-powerpc/ambiguousv1.d, * testsuite/ld-powerpc/ambiguousv1b.d, * testsuite/ld-powerpc/ambiguousv2.d, * testsuite/ld-powerpc/ambiguousv2b.d, * testsuite/ld-powerpc/tlsexe.d, * testsuite/ld-powerpc/tlsexe.r, * testsuite/ld-powerpc/tlsexe32.d, * testsuite/ld-powerpc/tlsexe32.g, * testsuite/ld-powerpc/tlsexe32.r, * testsuite/ld-powerpc/tlsexetoc.d, * testsuite/ld-powerpc/tlsexetoc.r, * testsuite/ld-powerpc/tlsso.d, * testsuite/ld-powerpc/tlsso.r, * testsuite/ld-powerpc/tlsso32.g, * testsuite/ld-powerpc/tlsso32.r, * testsuite/ld-powerpc/tlstocso.d, * testsuite/ld-powerpc/tlstocso.g, * testsuite/ld-powerpc/tlstocso.r, * testsuite/ld-s390/gotreloc_31-1.dd, * testsuite/ld-s390/tlsbin.dd, * testsuite/ld-s390/tlsbin.rd, * testsuite/ld-s390/tlsbin_64.dd, * testsuite/ld-s390/tlsbin_64.rd, * testsuite/ld-s390/tlspic.rd, * testsuite/ld-s390/tlspic_64.rd, * testsuite/ld-sh/tlsbin-2.d, * testsuite/ld-sh/tlspic-2.d, * testsuite/ld-sparc/gotop32.rd, * testsuite/ld-sparc/gotop64.rd, * testsuite/ld-sparc/tlssunbin32.rd, * testsuite/ld-sparc/tlssunbin64.rd, * testsuite/ld-sparc/tlssunnopic32.rd, * testsuite/ld-sparc/tlssunnopic64.rd, * testsuite/ld-sparc/tlssunpic32.rd, * testsuite/ld-sparc/tlssunpic64.rd, * testsuite/ld-x86-64/bnd-branch-1-now.d, * testsuite/ld-x86-64/bnd-ifunc-1-now.d, * testsuite/ld-x86-64/bnd-ifunc-2-now.d, * testsuite/ld-x86-64/bnd-ifunc-2.d, * testsuite/ld-x86-64/bnd-plt-1-now.d, * testsuite/ld-x86-64/bnd-plt-1.d, * testsuite/ld-x86-64/ibt-plt-1-x32.d, * testsuite/ld-x86-64/ibt-plt-1.d, * testsuite/ld-x86-64/ibt-plt-2a-x32.d, * testsuite/ld-x86-64/ibt-plt-2a.d, * testsuite/ld-x86-64/ibt-plt-2b-x32.d, * testsuite/ld-x86-64/ibt-plt-2b.d, * testsuite/ld-x86-64/ibt-plt-2c-x32.d, * testsuite/ld-x86-64/ibt-plt-2c.d, * testsuite/ld-x86-64/ibt-plt-2d-x32.d, * testsuite/ld-x86-64/ibt-plt-2d.d, * testsuite/ld-x86-64/ibt-plt-3a-x32.d, * testsuite/ld-x86-64/ibt-plt-3a.d, * testsuite/ld-x86-64/ibt-plt-3b-x32.d, * testsuite/ld-x86-64/ibt-plt-3b.d, * testsuite/ld-x86-64/ibt-plt-3c-x32.d, * testsuite/ld-x86-64/ibt-plt-3c.d, * testsuite/ld-x86-64/ibt-plt-3d-x32.d, * testsuite/ld-x86-64/ibt-plt-3d.d, * testsuite/ld-x86-64/ilp32-4-nacl.d, * testsuite/ld-x86-64/ilp32-4.d, * testsuite/ld-x86-64/load1c-nacl.d, * testsuite/ld-x86-64/load1c.d, * testsuite/ld-x86-64/load1d-nacl.d, * testsuite/ld-x86-64/load1d.d, * testsuite/ld-x86-64/mpx3n.dd, * testsuite/ld-x86-64/mpx4.dd, * testsuite/ld-x86-64/mpx4n.dd, * testsuite/ld-x86-64/plt2.dd, * testsuite/ld-x86-64/pr14207.d, * testsuite/ld-x86-64/pr19162.d, * testsuite/ld-x86-64/pr20253-1f.d, * testsuite/ld-x86-64/pr20253-1l.d, * testsuite/ld-x86-64/pr20830a-now.d, * testsuite/ld-x86-64/pr20830a.d, * testsuite/ld-x86-64/pr20830b-now.d, * testsuite/ld-x86-64/pr20830b.d, * testsuite/ld-x86-64/pr21038a-now.d, * testsuite/ld-x86-64/pr21038a.d, * testsuite/ld-x86-64/pr21038b-now.d, * testsuite/ld-x86-64/pr21038b.d, * testsuite/ld-x86-64/pr21038c-now.d, * testsuite/ld-x86-64/pr21038c.d, * testsuite/ld-x86-64/tlsbin-nacl.rd, * testsuite/ld-x86-64/tlsbin.rd, * testsuite/ld-x86-64/tlsbin2-nacl.rd, * testsuite/ld-x86-64/tlsbin2.rd, * testsuite/ld-x86-64/tlsbindesc-nacl.rd, * testsuite/ld-x86-64/tlsbindesc.rd, * testsuite/ld-x86-64/tlsdesc-nacl.rd, * testsuite/ld-x86-64/tlsdesc.rd, * testsuite/ld-x86-64/tlsgdesc-nacl.rd, * testsuite/ld-x86-64/tlsgdesc.rd, * testsuite/ld-x86-64/tlspic-nacl.rd, * testsuite/ld-x86-64/tlspic.rd, * testsuite/ld-x86-64/tlspic2-nacl.rd, * testsuite/ld-x86-64/tlspic2.rd: Update.
The master branch has been updated by H.J. Lu <hjl@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=ecf99cc09236316db27fd8ffaedca864f81ea081 commit ecf99cc09236316db27fd8ffaedca864f81ea081 Author: H.J. Lu <hjl.tools@gmail.com> Date: Fri Jun 8 12:41:31 2018 -0700 ld/x86: Remove hidden _edata, __bss_start, and _end There is no need to put hidden _edata, __bss_start, and _end in dynamic symbol table in shared libraries. bfd/ PR ld/23161 * elfxx-x86.c (elf_x86_hide_linker_defined): New function. (_bfd_x86_elf_link_check_relocs): Use it to hide hidden __bss_start, _end and _edata in shared libraries. ld/ PR ld/23161 * testsuite/ld-elf/pr23161d.rd: Remove local _edata, __bss_start, and _end from dynamic symbol table.
Fixed for 2.31.