[Patch] Don't relocate compressed sections

[Mark Wielaard <mjw at redhat dot com>]

Hi,

As pointed out in https://bugzilla.redhat.com/show_bug.cgi?id=807053
relocating a compressed section will corrupt it. To more properly
support compressed sections we should finish the roland/relocate branch
for lazy relocations work. The best we can do for now is to not corrupt
the contents of the compressed sections and not crash when we don't have
all Dwarf debug section data. The attached two patches do that.

Cheers,

Mark

From ce12994c215b08758e5c79fcd1af8b8b3d069acc Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mjw@redhat.com>
Date: Wed, 28 Mar 2012 11:20:36 +0200
Subject: [PATCH 1/2] readelf.c: Check debug section data before trying to
 print abbrev or str.

	* readelf.c (print_debug_abbrev_section): Check there is Dwarf
	section data.
	(print_debug_str_section): Likewise.
---
 src/ChangeLog |    6 ++++++
 src/readelf.c |    8 ++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index 6281756..30be3e1 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,9 @@
+2012-03-28  Mark Wielaard  <mjw@redhat.com>
+
+	* readelf.c (print_debug_abbrev_section): Check there is Dwarf
+	section data.
+	(print_debug_str_section): Likewise.
+
 2012-03-21  Mark Wielaard  <mjw@redhat.com>
 
 	* readelf.c (print_gdb_index_section): Accept version 6.
diff --git a/src/readelf.c b/src/readelf.c
index f9f56aa..b70779d 100644
--- a/src/readelf.c
+++ b/src/readelf.c
@@ -4566,13 +4566,16 @@ print_debug_abbrev_section (Dwfl_Module *dwflmod __attribute__ ((unused)),
 			    Ebl *ebl, GElf_Ehdr *ehdr,
 			    Elf_Scn *scn, GElf_Shdr *shdr, Dwarf *dbg)
 {
+  const size_t sh_size = (dbg->sectiondata[IDX_debug_abbrev] ?
+			  dbg->sectiondata[IDX_debug_abbrev]->d_size : 0);
+
   printf (gettext ("\nDWARF section [%2zu] '%s' at offset %#" PRIx64 ":\n"
 		   " [ Code]\n"),
 	  elf_ndxscn (scn), section_name (ebl, ehdr, shdr),
 	  (uint64_t) shdr->sh_offset);
 
   Dwarf_Off offset = 0;
-  while (offset < dbg->sectiondata[IDX_debug_abbrev]->d_size)
+  while (offset < sh_size)
     {
       printf (gettext ("\nAbbreviation section at offset %" PRIu64 ":\n"),
 	      offset);
@@ -6781,7 +6784,8 @@ print_debug_str_section (Dwfl_Module *dwflmod __attribute__ ((unused)),
 			 Ebl *ebl, GElf_Ehdr *ehdr,
 			 Elf_Scn *scn, GElf_Shdr *shdr, Dwarf *dbg)
 {
-  const size_t sh_size = dbg->sectiondata[IDX_debug_str]->d_size;
+  const size_t sh_size = (dbg->sectiondata[IDX_debug_str] ?
+			  dbg->sectiondata[IDX_debug_str]->d_size : 0);
 
   /* Compute floor(log16(shdr->sh_size)).  */
   GElf_Addr tmp = sh_size;
-- 
1.7.7.6

From 342eb23d0f9cca93ac1e42e2ff78bc034ec4aa27 Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mjw@redhat.com>
Date: Wed, 28 Mar 2012 11:38:34 +0200
Subject: [PATCH 2/2] relocate.c (relocate_section): Don't touch compressed
 sections.

Compressed section can only be relocated after decompression, don't touch it
or we will corrupt it.
---
 libdwfl/ChangeLog  |    4 ++++
 libdwfl/relocate.c |    6 ++++++
 2 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 87a0555..bbd51d7 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,7 @@
+2012-03-28  Mark Wielaard  <mjw@redhat.com>
+
+	* relocate.c (relocate_section): Don't touch compressed sections.
+
 2011-12-02  Roland McGrath  <roland@hack.frob.com>
 
 	* elf-from-memory.c (elf_from_remote_memory): Fix ELFCLASS64 case
diff --git a/libdwfl/relocate.c b/libdwfl/relocate.c
index 95206f4..effef44 100644
--- a/libdwfl/relocate.c
+++ b/libdwfl/relocate.c
@@ -319,6 +319,12 @@ relocate_section (Dwfl_Module *mod, Elf *relocated, const GElf_Ehdr *ehdr,
   if (tdata == NULL)
     return DWFL_E_LIBELF;
 
+  if (tname[0] == '.' && tname[1] == 'z' && tdata->d_size >= 4 + 8
+      && memcmp (tdata->d_buf, "ZLIB", 4) == 0)
+    /* Compressed section can only be relocated after decompression,
+       don't touch it or we will corrupt it. */
+    return DWFL_E_NOERROR;
+
   /* Apply one relocation.  Returns true for any invalid data.  */
   Dwfl_Error relocate (GElf_Addr offset, const GElf_Sxword *addend,
 		       int rtype, int symndx)
-- 
1.7.7.6