[binutils-gdb] gdb: fix nullptr dereference in block::ranges()

[binutils-gdb] gdb: fix nullptr dereference in block::ranges()

[Andrew Burgess]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c42dd30d73ec441ed9cab207597c7f5ce88ee231

commit c42dd30d73ec441ed9cab207597c7f5ce88ee231
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Thu Apr 28 11:37:51 2022 +0100

    gdb: fix nullptr dereference in block::ranges()
    
    This commit:
    
      commit f5cb8afdd297dd68273d98a10fbfd350dff918d8
      Date:   Sun Feb 6 22:27:53 2022 -0500
    
          gdb: remove BLOCK_RANGES macro
    
    introduces a potential nullptr dereference in block::ranges, this is
    breaking most tests, e.g. gdb.base/break.exp is failing for me.
    
    In the above patch BLOCK_CONTIGUOUS_P is changed from this:
    
      #define BLOCK_CONTIGUOUS_P(bl)  (BLOCK_RANGES (bl) == nullptr \
                                       || BLOCK_NRANGES (bl) <= 1)
    
    to this:
    
      #define BLOCK_CONTIGUOUS_P(bl)  ((bl)->ranges ().size () == 0 \
                                       || (bl)->ranges ().size () == 1)
    
    So, before the commit we checked for the block ranges being nullptr,
    but afterwards we just call block::ranges() in all cases.
    
    The problem is that block::ranges() looks like this:
    
      /* Return a view on this block's ranges.  */
      gdb::array_view<blockrange> ranges ()
      { return gdb::make_array_view (m_ranges->range, m_ranges->nranges); }
    
    where m_ranges is:
    
      struct blockranges *m_ranges;
    
    And so, we see that the nullptr check has been lost, and we might end
    up dereferencing a nullptr.
    
    My proposed fix is to move the nullptr check into block::ranges, and
    return an explicit empty array_view if m_ranges is nullptr.
    
    After this, everything seems fine again.

Diff:
---
 gdb/block.h | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/gdb/block.h b/gdb/block.h
index b9f4e974c04..038ce7bd2f3 100644
--- a/gdb/block.h
+++ b/gdb/block.h
@@ -157,11 +157,21 @@ struct block
 
   /* Return a view on this block's ranges.  */
   gdb::array_view<blockrange> ranges ()
-  { return gdb::make_array_view (m_ranges->range, m_ranges->nranges); }
+  {
+    if (m_ranges == nullptr)
+      return {};
+    else
+      return gdb::make_array_view (m_ranges->range, m_ranges->nranges);
+  }
 
   /* Const version of the above.  */
   gdb::array_view<const blockrange> ranges () const
-  { return gdb::make_array_view (m_ranges->range, m_ranges->nranges); }
+  {
+    if (m_ranges == nullptr)
+      return {};
+    else
+      return gdb::make_array_view (m_ranges->range, m_ranges->nranges);
+  }
 
   /* Set this block's ranges array.  */
   void set_ranges (blockranges *ranges)