[Bug dynamic-link/14843] New: Can't audit IFUNC resolver functions

["hjl.tools at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14843

             Bug #: 14843
           Summary: Can't audit IFUNC resolver functions
           Product: glibc
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
        AssignedTo: unassigned@sourceware.org
        ReportedBy: hjl.tools@gmail.com
                CC: amonakov@gmail.com, hjl.tools@gmail.com
        Depends on: 14831
    Classification: Unclassified


+++ This bug was initially created as a clone of Bug #14831 +++

Using the audit mechanism to redirect library lookups by implementing
la_objsearch and returning a library that depends on libm.so (or libm.so
itself) results in a subsequent segfault in the loader.  I have attempted to
create a standalone testcase, but did not succeed (I suspect the bug has to do
with how IRELATIVE relocations are processed, but a simple testcase with
IRELATIVE reloc works fine).  Attaching a small testcase that depends on
libm.so (and assumes it has IRELATIVE relocations).

$ gdb --args /tmp/glibc-build/elf/ld.so --audit ./libaudit.so ./main
GNU gdb (GDB) 7.4.1

(gdb) r
Starting program: /tmp/glibc-build/elf/ld.so --audit ./libaudit.so ./main
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?

Program received signal SIGSEGV, Segmentation fault.
_dl_profile_fixup (l=0x7ffff7a33508, reloc_arg=4, retaddr=140737345060825,
regs=0x7fffffffd1b0, framesizep=0x7fffffffd508) at ../elf/dl-runtime.c:176
176      DL_FIXUP_VALUE_TYPE value = *resultp;
(gdb) bt
#0  _dl_profile_fixup (l=0x7ffff7a33508, reloc_arg=4, retaddr=140737345060825,
regs=0x7fffffffd1b0, framesizep=0x7fffffffd508) at ../elf/dl-runtime.c:176
#1  0x0000555555568306 in _dl_runtime_profile () at
../sysdeps/x86_64/dl-trampoline.h:48
#2  0x00007ffff7757fd9 in ?? ()
#3  0x00007fffffffd650 in ?? ()
#4  0x000055555555f5d1 in elf_machine_lazy_rel (skip_ifunc=<optimized out>,
reloc=0x7ffff773e210, l_addr=140737344933888, map=0x7ffff7a33508) at
../sysdeps/x86_64/dl-machine.h:535
#5  elf_dynamic_do_Rela (skip_ifunc=<optimized out>, lazy=<optimized out>,
nrelative=<optimized out>, relsize=<optimized out>, reladdr=<optimized out>,
map=0x7ffff7a33508) at do-rel.h:85
#6  _dl_relocate_object (scope=0x7ffff7a33860, reloc_mode=<optimized out>,
consider_profiling=1, consider_profiling@entry=0) at dl-reloc.c:265
#7  0x0000555555557ad2 in dl_main (phdr=<optimized out>,
phdr@entry=0x555555554040, phnum=4160734848, phnum@entry=7,
user_entry=user_entry@entry=0x7fffffffd7d8, auxv=0x555555777801) at rtld.c:2299
#8  0x0000555555568afc in _dl_sysdep_start
(start_argptr=start_argptr@entry=0x7fffffffd890,
dl_main=dl_main@entry=0x555555555ae0 <dl_main>) at ../elf/dl-sysdep.c:242
#9  0x0000555555558d0e in _dl_start_final (arg=0x7fffffffd890) at rtld.c:337
#10 _dl_start (arg=0x7fffffffd890) at rtld.c:563
#11 0x00005555555555a8 in _start () from /tmp/glibc-build/elf/ld.so
(gdb) p l.l_reloc_result 
$1 = (struct reloc_result *) 0x0
(gdb) f 4
#4  0x000055555555f5d1 in elf_machine_lazy_rel (skip_ifunc=<optimized out>,
reloc=0x7ffff773e210, l_addr=140737344933888, map=0x7ffff7a33508) at
../sysdeps/x86_64/dl-machine.h:535
535        value = ((ElfW(Addr) (*) (void)) value) ();
(gdb) list
530        }
531      else if (__builtin_expect (r_type == R_X86_64_IRELATIVE, 0))
532        {
533          ElfW(Addr) value = map->l_addr + reloc->r_addend;
534          if (__builtin_expect (!skip_ifunc, 1))
535        value = ((ElfW(Addr) (*) (void)) value) ();
536          *reloc_addr = value;
537        }
538      else
539        _dl_reloc_bad_type (map, r_type, 1);
(gdb)

This patch fixes the crash

---
diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
index e6968a4..e3c8b26 100644
--- a/elf/dl-reloc.c
+++ b/elf/dl-reloc.c
@@ -262,8 +262,6 @@ _dl_relocate_object (struct link_map *l, struct
r_scope_elem *scope[],

 #include "dynamic-link.h"

-    ELF_DYNAMIC_RELOCATE (l, lazy, consider_profiling, skip_ifunc);
-
 #ifndef PROF
     if (__builtin_expect (consider_profiling, 0))
       {
@@ -290,6 +288,8 @@ _dl_relocate_object (struct link_map *l, struct
r_scope_elem *scope[],
       }
       }
 #endif
+
+    ELF_DYNAMIC_RELOCATE (l, lazy, consider_profiling, skip_ifunc);
   }

   /* Mark the object so we know this work has been done.  */
---

However it causes tst-audit2 failure:

{abcdef72, d8675309} != {d8675309, abcdef72}

This test is added from:

http://www.sourceware.org/ml/libc-alpha/2005-09/msg00075.html
http://www.sourceware.org/ml/libc-alpha/2006-03/msg00138.html

>From ELF_DYNAMIC_RELOCATE we call elf_machine_rela, which calls
CHECK_STATIC_TLS, which eventually allocates the static TLS *before*
the first call to any malloc-esque hooks.  That means we can't
audit IFUNC resolver functions because of this ordering issue.

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