[PATCH] Use the empty string for note names with zero size (without any data).

[Mark Wielaard]

ELF notes can have a zero sized name. In which case there is no data at
all (so also no zero terminator). Make sure to use the empty string for
such notes if the code does not otherwise explicitly check n_namesz.

https://sourceware.org/bugzilla/show_bug.cgi?id=21300

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 libdwfl/ChangeLog           | 6 ++++++
 libdwfl/linux-core-attach.c | 9 ++++++---
 src/ChangeLog               | 6 ++++++
 src/elfcmp.c                | 6 ++++--
 src/readelf.c               | 2 +-
 5 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 4c9f4f6..ede6d47 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,9 @@
+2017-03-24  Mark Wielaard  <mark@klomp.org>
+
+	* linux-core-attach.c (core_next_thread): If n_namesz == 0 then
+	the note name data is the empty string.
+	(dwfl_core_file_attach): Likewise.
+
 2017-02-15  Ulf Hermann  <ulf.hermann@qt.io>
 
 	* linux-kernel-modules.c: Include system.h.
diff --git a/libdwfl/linux-core-attach.c b/libdwfl/linux-core-attach.c
index 93d0e46..f82ed03 100644
--- a/libdwfl/linux-core-attach.c
+++ b/libdwfl/linux-core-attach.c
@@ -125,7 +125,8 @@ core_next_thread (Dwfl *dwfl __attribute__ ((unused)), void *dwfl_arg,
 							  &desc_offset)) > 0)
     {
       /* Do not check NAME for now, help broken Linux kernels.  */
-      const char *name = note_data->d_buf + name_offset;
+      const char *name = (nhdr.n_namesz == 0
+			  ? "" : note_data->d_buf + name_offset);
       const char *desc = note_data->d_buf + desc_offset;
       GElf_Word regs_offset;
       size_t nregloc;
@@ -178,7 +179,8 @@ core_set_initial_registers (Dwfl_Thread *thread, void *thread_arg_voidp)
   /* __libdwfl_attach_state_for_core already verified the note is there.  */
   assert (getnote_err != 0);
   /* Do not check NAME for now, help broken Linux kernels.  */
-  const char *name = note_data->d_buf + name_offset;
+  const char *name = (nhdr.n_namesz == 0
+		      ? "" : note_data->d_buf + name_offset);
   const char *desc = note_data->d_buf + desc_offset;
   GElf_Word regs_offset;
   size_t nregloc;
@@ -367,7 +369,8 @@ dwfl_core_file_attach (Dwfl *dwfl, Elf *core)
 				    &nhdr, &name_offset, &desc_offset)) > 0)
     {
       /* Do not check NAME for now, help broken Linux kernels.  */
-      const char *name = note_data->d_buf + name_offset;
+      const char *name = (nhdr.n_namesz == 0
+			  ? "" : note_data->d_buf + name_offset);
       const char *desc = note_data->d_buf + desc_offset;
       GElf_Word regs_offset;
       size_t nregloc;
diff --git a/src/ChangeLog b/src/ChangeLog
index 9dd76c0..41381aa 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,11 @@
 2017-03-24  Mark Wielaard  <mjw@redhat.com>
 
+	* elfcmp.c (main): If n_namesz == 0 then the note name data is the
+	empty string.
+	* readelf.c (handle_notes_data): Likewise.
+
+2017-03-24  Mark Wielaard  <mjw@redhat.com>
+
 	* readelf.c (handle_gnu_hash): Check inner < max_nsyms before
 	indexing into chain array.
 
diff --git a/src/elfcmp.c b/src/elfcmp.c
index 7673cf2..5046420 100644
--- a/src/elfcmp.c
+++ b/src/elfcmp.c
@@ -419,7 +419,8 @@ main (int argc, char *argv[])
 		   && (off1 = gelf_getnote (data1, off1, &note1,
 					    &name_offset, &desc_offset)) > 0)
 	      {
-		const char *name1 = data1->d_buf + name_offset;
+		const char *name1 = (note1.n_namesz == 0
+				     ? "" : data1->d_buf + name_offset);
 		const void *desc1 = data1->d_buf + desc_offset;
 		if (off2 >= data2->d_size)
 		  {
@@ -435,7 +436,8 @@ main (int argc, char *argv[])
 		  error (2, 0, gettext ("\
 cannot read note section [%zu] '%s' in '%s': %s"),
 			 elf_ndxscn (scn2), sname2, fname2, elf_errmsg (-1));
-		const char *name2 = data2->d_buf + name_offset;
+		const char *name2 = (note2.n_namesz == 0
+				     ? "" : data2->d_buf + name_offset);
 		const void *desc2 = data2->d_buf + desc_offset;
 
 		if (note1.n_namesz != note2.n_namesz
diff --git a/src/readelf.c b/src/readelf.c
index 490b6d5..97a43b0 100644
--- a/src/readelf.c
+++ b/src/readelf.c
@@ -9365,7 +9365,7 @@ handle_notes_data (Ebl *ebl, const GElf_Ehdr *ehdr,
 	 && (offset = gelf_getnote (data, offset,
 				    &nhdr, &name_offset, &desc_offset)) > 0)
     {
-      const char *name = data->d_buf + name_offset;
+      const char *name = nhdr.n_namesz == 0 ? "" : data->d_buf + name_offset;
       const char *desc = data->d_buf + desc_offset;
 
       char buf[100];
-- 
1.8.3.1