Re: [PATCH] libdwfl: Never relocate value against section zero (load address).

Re: [PATCH] libdwfl: Never relocate value against section zero (load address).

[Mark Wielaard]

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On Sun, 2014-12-07 at 23:45 +0100, Mark Wielaard wrote:
> This really is just a robustify patch in case section zero got the
> wrong section flags set. In that case __libdwfl_relocate_value might
> call dwfl_offline_section_address which might assert (because it isn't
> prepared to handle section zero). elf_nextscn will never see section zero.
> So be explicit in dwfl_offline_section_address and immediately assert
> when shndx is zero. And handle section zero immediately by not relocating
> the value in __libdwfl_relocate_value.
>
> +2014-12-07  Mark Wielaard  <mjw@redhat.com>
> +
> +	* offline.c (dwfl_offline_section_address): Assert shndx is not zero.
> +	* relocate.c (__libdwfl_relocate_value): Don't relocate against
> +	section zero.

I pushed this to master now.

[PATCH] libdwfl: Never relocate value against section zero (load address).

[Mark Wielaard]

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This really is just a robustify patch in case section zero got the
wrong section flags set. In that case __libdwfl_relocate_value might
call dwfl_offline_section_address which might assert (because it isn't
prepared to handle section zero). elf_nextscn will never see section zero.
So be explicit in dwfl_offline_section_address and immediately assert
when shndx is zero. And handle section zero immediately by not relocating
the value in __libdwfl_relocate_value.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
---
 libdwfl/ChangeLog  | 6 ++++++
 libdwfl/offline.c  | 1 +
 libdwfl/relocate.c | 6 ++++++
 3 files changed, 13 insertions(+)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 03faecf..6eec018 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,9 @@
+2014-12-07  Mark Wielaard  <mjw@redhat.com>
+
+	* offline.c (dwfl_offline_section_address): Assert shndx is not zero.
+	* relocate.c (__libdwfl_relocate_value): Don't relocate against
+	section zero.
+
 2014-11-29  Mark Wielaard  <mjw@redhat.com>
 
 	* relocate.c (relocate_section): Check relocation section and target
diff --git a/libdwfl/offline.c b/libdwfl/offline.c
index 28d2782..982ceab 100644
--- a/libdwfl/offline.c
+++ b/libdwfl/offline.c
@@ -48,6 +48,7 @@ dwfl_offline_section_address (Dwfl_Module *mod,
   assert (mod->e_type == ET_REL);
   assert (shdr->sh_addr == 0);
   assert (shdr->sh_flags & SHF_ALLOC);
+  assert (shndx != 0);
 
   if (mod->debug.elf == NULL)
     /* We are only here because sh_addr is zero even though layout is complete.
diff --git a/libdwfl/relocate.c b/libdwfl/relocate.c
index 6862189..fc8ae23 100644
--- a/libdwfl/relocate.c
+++ b/libdwfl/relocate.c
@@ -38,6 +38,12 @@ internal_function
 __libdwfl_relocate_value (Dwfl_Module *mod, Elf *elf, size_t *shstrndx,
 			  Elf32_Word shndx, GElf_Addr *value)
 {
+  /* No adjustment needed for section zero, it is never loaded.
+     Handle it first, just in case the ELF file has strange section
+     zero flags set.  */
+  if (shndx == 0)
+    return DWFL_E_NOERROR;
+
   Elf_Scn *refscn = elf_getscn (elf, shndx);
   GElf_Shdr refshdr_mem, *refshdr = gelf_getshdr (refscn, &refshdr_mem);
   if (refshdr == NULL)
-- 
1.9.3