Message ID | 20170822224425.GA29673@gmail.com |
---|---|
State | New |
Headers | show |
On 08/23/2017 12:44 AM, H.J. Lu wrote: > On i386, when multi-arch is enabled, all external functions must be > called via PIC PLT in PIE, which requires setting up EBX register, > since they may be IFUNC functions. This explanation does not make sense to me. Could you elaborate further? I suspect the actual problem is something else (probably a linker/ELF limitation). Thanks, Florian
On Wed, Aug 23, 2017 at 2:23 AM, Florian Weimer <fweimer@redhat.com> wrote: > On 08/23/2017 12:44 AM, H.J. Lu wrote: >> On i386, when multi-arch is enabled, all external functions must be >> called via PIC PLT in PIE, which requires setting up EBX register, >> since they may be IFUNC functions. > > This explanation does not make sense to me. Could you elaborate further? When a linker detects a call to a hidden IFUNC function, foo, it will create a PLT entry and resolve call foo to call foo@PLT On i386, there are 2 kinds of PLT: 1. PLT: 080499a0 <strstr@plt>: 80499a0: ff 25 0c 00 22 08 jmp *0x822000c 80499a6: 68 00 00 00 00 push $0x0 80499ab: e9 e0 ff ff ff jmp 8049990 <.plt> This is used for non-PIC binary since the address of GOT is fixed. 2. PIC PLT: 000178a0 <_Unwind_Find_FDE@plt>: 178a0: ff a3 0c 00 00 00 jmp *0xc(%ebx) 178a6: 68 00 00 00 00 push $0x0 178ab: e9 e0 ff ff ff jmp 17890 <.plt> This is used for PIC binary since the address of GOT is in EBX. For non-PIC binary, it is OK to turn call foo to call foo@PLT since GOT is at the fixed address. But for PIE, it isn't OK to turn call foo to call foo@PLT since PIE uses PIC PLT and EBX doesn't have the address of GOT.
On 8/23/17, H.J. Lu <hjl.tools@gmail.com> wrote: > On Wed, Aug 23, 2017 at 2:23 AM, Florian Weimer <fweimer@redhat.com> wrote: >> On 08/23/2017 12:44 AM, H.J. Lu wrote: >>> On i386, when multi-arch is enabled, all external functions must be >>> called via PIC PLT in PIE, which requires setting up EBX register, >>> since they may be IFUNC functions. >> >> This explanation does not make sense to me. Could you elaborate further? > > When a linker detects a call to a hidden IFUNC function, foo, it will create > a > PLT entry and resolve > > call foo > > to > > call foo@PLT > > On i386, there are 2 kinds of PLT: > > 1. PLT: > > 080499a0 <strstr@plt>: > 80499a0: ff 25 0c 00 22 08 jmp *0x822000c > 80499a6: 68 00 00 00 00 push $0x0 > 80499ab: e9 e0 ff ff ff jmp 8049990 <.plt> > > This is used for non-PIC binary since the address of GOT is fixed. > > 2. PIC PLT: > > 000178a0 <_Unwind_Find_FDE@plt>: > 178a0: ff a3 0c 00 00 00 jmp *0xc(%ebx) > 178a6: 68 00 00 00 00 push $0x0 > 178ab: e9 e0 ff ff ff jmp 17890 <.plt> > > This is used for PIC binary since the address of GOT is in EBX. > > For non-PIC binary, it is OK to turn > > call foo > > to > > call foo@PLT > > since GOT is at the fixed address. But for PIE, it isn't OK to > turn > > call foo > > to > > call foo@PLT > > since PIE uses PIC PLT and EBX doesn't have the address of > GOT. > > I am checking it in.
diff --git a/config.h.in b/config.h.in index 26ed7865ef..d397eeae1a 100644 --- a/config.h.in +++ b/config.h.in @@ -86,6 +86,10 @@ #undef PI_STATIC_AND_HIDDEN /* Define this to disable the 'hidden_proto' et al macros in + include/libc-symbols.h that avoid PLT slots in PIE. */ +#undef NO_HIDDEN_EXTERN_FUNC_IN_PIE + +/* Define this to disable the 'hidden_proto' et al macros in include/libc-symbols.h that avoid PLT slots in the shared objects. */ #undef NO_HIDDEN diff --git a/include/libc-symbols.h b/include/libc-symbols.h index fe3571af52..4668902d5f 100644 --- a/include/libc-symbols.h +++ b/include/libc-symbols.h @@ -514,6 +514,7 @@ for linking") #else # ifndef __ASSEMBLER__ # if !defined SHARED && IS_IN (libc) && !defined LIBC_NONSHARED \ + && (!defined PIC || !defined NO_HIDDEN_EXTERN_FUNC_IN_PIE) \ && !defined NO_HIDDEN # define __hidden_proto_hiddenattr(attrs...) \ __attribute__ ((visibility ("hidden"), ##attrs)) diff --git a/sysdeps/i386/configure b/sysdeps/i386/configure index 4d6685f7c2..4cf968d8bc 100644 --- a/sysdeps/i386/configure +++ b/sysdeps/i386/configure @@ -26,7 +26,7 @@ libc_compiler_builtin_inlined=no cat > conftest.c <<EOF int _start (void) { int a, b, c; __sync_val_compare_and_swap (&a, b, c); return 0; } EOF -if ! { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS +if ! { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp -O0 -nostdlib -nostartfiles -S conftest.c -o - | fgrep "__sync_val_compare_and_swap" 1>&5' @@ -79,3 +79,8 @@ fi $as_echo "#define PI_STATIC_AND_HIDDEN 1" >>confdefs.h + +if test x"$multi_arch" != xno; then + $as_echo "#define NO_HIDDEN_EXTERN_FUNC_IN_PIE 1" >>confdefs.h + +fi diff --git a/sysdeps/i386/configure.ac b/sysdeps/i386/configure.ac index f7766ad2b8..b598b120bc 100644 --- a/sysdeps/i386/configure.ac +++ b/sysdeps/i386/configure.ac @@ -48,3 +48,9 @@ fi dnl It is always possible to access static and hidden symbols in an dnl position independent way. AC_DEFINE(PI_STATIC_AND_HIDDEN) + +dnl When multi-arch is enabled, all external functions must be called +dnl via PIC PLT in PIE, which requires setting up EBX register. +if test x"$multi_arch" != xno; then + AC_DEFINE(NO_HIDDEN_EXTERN_FUNC_IN_PIE) +fi