inconsistent module relocation in elfutils

inconsistent module relocation in elfutils

[Frank Ch. Eigler]

Hi, Roland -

As a part of my work with but #1329, I've found that different kernel
builds work differently with the same elfutils/systemtap code with
respect to module relocation.  The 'kernel' module appears to work
consistently.

Specifically, with some kernel versions (e.g. 2.6.13-1.1532_FC4) I
find that the GElf_Shdr sh_addr fields of .text-like sections are
correctly relocated, and for other kernel versions (2.6.9-22.EL), that
field is left at zero.  Can you suggest an explanation or workaround?

Test thusly (cvs systemtap, elfutils .116):
% stap -p2 -e 'probe module("nfs").function("init_nfs_fs") {}' -v |& grep skip

One can put a breakpoint at tapsets.cxx:1673 and look at variable "*shdr".


- FChE

Re: inconsistent module relocation in elfutils

[Roland McGrath]

> Specifically, with some kernel versions (e.g. 2.6.13-1.1532_FC4) I
> find that the GElf_Shdr sh_addr fields of .text-like sections are
> correctly relocated, and for other kernel versions (2.6.9-22.EL), that
> field is left at zero.  Can you suggest an explanation or workaround?

You should not have any particular expectations about section headers in
ET_REL modules.  The fact that you see an sh_addr you can use there is more
an artifact of the implementation than something you should be relying on.
When using the style of module callback now in use (-k), the only sections
whose addresses are noted are those needed to relocate the debug info.

> One can put a breakpoint at tapsets.cxx:1673 and look at variable "*shdr".

I don't think that code is the right way to do that.  Instead use
dwfl_module_relocate_address and dwfl_module_relocation_info to get the
section name.

RE: inconsistent module relocation in elfutils

[Mao, Bibo]

[-- Attachment #1: Type: text/plain, Size: 1758 bytes --]



>-----Original Message-----
>From: systemtap-owner@sourceware.org [mailto:systemtap-owner@sourceware.org]
>On Behalf Of Roland McGrath
>Sent: 2005年11月5日 4:37
>To: Frank Ch. Eigler
>Cc: systemtap@sources.redhat.com
>Subject: Re: inconsistent module relocation in elfutils
>
>> Specifically, with some kernel versions (e.g. 2.6.13-1.1532_FC4) I
>> find that the GElf_Shdr sh_addr fields of .text-like sections are
>> correctly relocated, and for other kernel versions (2.6.9-22.EL), that
>> field is left at zero.  Can you suggest an explanation or workaround?
>
>You should not have any particular expectations about section headers in
>ET_REL modules.  The fact that you see an sh_addr you can use there is more
>an artifact of the implementation than something you should be relying on.
>When using the style of module callback now in use (-k), the only sections
>whose addresses are noted are those needed to relocate the debug info.
>
>> One can put a breakpoint at tapsets.cxx:1673 and look at variable "*shdr".
>
>I don't think that code is the right way to do that.  Instead use
>dwfl_module_relocate_address and dwfl_module_relocation_info to get the
>section name.

In IA64 platform, we test elfutils-0.116 by test case ./dwflmodtest -k command, we find that there will be "Unsupported relocation type". In IA64 platform, it use default default_reloc_simple_type function, and this function return ELF_T_NUM value. It will be regarded as unknown relocated type.
So I think in IA64 platform there should be some function like i386_reloc_simple_type, x86_64_reloc_simple_type. Here in the attachment it defines relocation type for IA64. This patch only defines little endian type, also it need define big endian type.

[-- Attachment #2: patch_elfutils_ia64_relocate_type --]
[-- Type: application/octet-stream, Size: 2318 bytes --]

diff -urN elfutils-0.115.org/libebl/ia64_init.c elfutils-0.115/libebl/ia64_init.c
--- elfutils-0.115.org/libebl/ia64_init.c	2005-11-01 03:10:41.000000000 +0800
+++ elfutils-0.115/libebl/ia64_init.c	2005-11-01 03:11:57.000000000 +0800
@@ -34,6 +34,7 @@
   eh->name = "Intel IA-64";
   eh->reloc_type_name = ia64_reloc_type_name;
   eh->reloc_type_check = ia64_reloc_type_check;
+  eh->reloc_simple_type = ia64_reloc_simple_type;
   eh->segment_type_name = ia64_segment_type_name;
   eh->dynamic_tag_name = ia64_dynamic_tag_name;
   eh->copy_reloc_p = ia64_copy_reloc_p;
diff -urN elfutils-0.115.org/libebl/ia64_symbol.c elfutils-0.115/libebl/ia64_symbol.c
--- elfutils-0.115.org/libebl/ia64_symbol.c	2005-11-01 03:10:41.000000000 +0800
+++ elfutils-0.115/libebl/ia64_symbol.c	2005-11-01 03:12:08.000000000 +0800
@@ -131,6 +131,42 @@
 }
 
 
+/* Check for the simple reloc types.  */
+Elf_Type 
+ia64_reloc_simple_type (Elf *elf __attribute__ ((unused)), int type)
+{
+  switch (type)
+    {
+    case R_IA64_SECREL64LSB:
+    case R_IA64_SEGREL64LSB:
+    case R_IA64_DIR64LSB:
+    case R_IA64_REL64LSB:
+    case R_IA64_GPREL64LSB:
+    case R_IA64_FPTR64LSB:
+    case R_IA64_PCREL64LSB:
+    case R_IA64_LTOFF_FPTR64LSB:
+    case R_IA64_LTV64LSB:
+    case R_IA64_DTPREL64LSB:	
+      return ELF_T_XWORD;
+
+    case R_IA64_SECREL32LSB:
+    case R_IA64_SEGREL32LSB:
+    case R_IA64_DIR32LSB:	
+    case R_IA64_REL32LSB:
+    case R_IA64_GPREL32LSB:
+    case R_IA64_FPTR32LSB:
+    case R_IA64_PCREL32LSB:
+    case R_IA64_LTOFF_FPTR32LSB:
+    case R_IA64_LTV32LSB:
+    case R_IA64_DTPREL32LSB:
+      return ELF_T_WORD;
+
+    default:
+      return ELF_T_NUM;
+    }
+}
+
+
 /* Check for correct relocation type.  */
 bool
 ia64_reloc_type_check (int type)
diff -urN elfutils-0.115.org/libebl/libebl_ia64.h elfutils-0.115/libebl/libebl_ia64.h
--- elfutils-0.115.org/libebl/libebl_ia64.h	2005-11-01 03:10:41.000000000 +0800
+++ elfutils-0.115/libebl/libebl_ia64.h	2005-11-01 03:12:26.000000000 +0800
@@ -31,6 +31,9 @@
 /* Check relocation type.  */
 extern bool ia64_reloc_type_check (int type);
 
+/* Check for simple reloc types */
+extern Elf_Type ia64_reloc_simple_type(Elf *elf, int type);
+
 /* Name of segment type.  */
 extern const char *ia64_segment_type_name (int segment, char *buf, size_t len);