[binutils-gdb] Fix undefined behavior, don't pass NULL to fwrite

[Simon Marchi <simark at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=1f88d0c87c37d3a15fa6376335e8b0d1c79d85aa

commit 1f88d0c87c37d3a15fa6376335e8b0d1c79d85aa
Author: Simon Marchi <simon.marchi@ericsson.com>
Date:   Thu Oct 4 22:43:27 2018 -0400

    Fix undefined behavior, don't pass NULL to fwrite
    
    If a vector that we try to write using file_write is empty, we may end
    up passing NULL to fwrite, which triggers UBSan:
    
      .../gdb/dwarf-index-write.c:73:14: runtime error: null pointer passed as argument 1, which is declared to never be null
    
    Avoid it by skipping the write if the vector is empty.
    
    gdb/ChangeLog:
    
    	* dwarf-index-write.c (file_write): Don't write if the vector is
    	empty.

Diff:
---
 gdb/ChangeLog           | 5 +++++
 gdb/dwarf-index-write.c | 3 ++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 126deb7..0e5a3f4 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2018-10-04  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* dwarf-index-write.c (file_write): Don't write if the vector is
+	empty.
+
 2018-10-05  Tom de Vries  <tdevries@suse.de>
 
 	* python/py-progspace.c (pspy_solib_name): Fix type mismatch in
diff --git a/gdb/dwarf-index-write.c b/gdb/dwarf-index-write.c
index 2520321..d4585af 100644
--- a/gdb/dwarf-index-write.c
+++ b/gdb/dwarf-index-write.c
@@ -80,7 +80,8 @@ template<typename Elem, typename Alloc>
 static void
 file_write (FILE *file, const std::vector<Elem, Alloc> &vec)
 {
-  file_write (file, vec.data (), vec.size () * sizeof (vec[0]));
+  if (!vec.empty ())
+    file_write (file, vec.data (), vec.size () * sizeof (vec[0]));
 }
 
 /* In-memory buffer to prepare data to be written later to a file.  */