[PATCH 19/20] abg-writer.cc: fix write_elf_symbol_reference loop

[Matthias Maennich <maennich@google.com>]

From: Giuliano Procida <gprocida@google.com>

The function write_elf_symbol_reference iterates through aliases,
looking for an unsuppressed alias to use. The existing code went wrong
in the case when aliases are present. In the case of all symbols
suppressed, it would also have selected the last alias, rather than
the first, if the data structure invariants had matched the code's
expectations.

The main symbol is always distinguished. When aliases are absent, the
sole symbol's next pointer is null, but when aliases are present, they
form a circular list. This makes traversal of aliases a bit tricky.

The code now picks the first symbol from the following:

- the given symbol, if unsuppressed
- the main symbol, if unsuppressed
- the unsuppressed aliases in the remainder of the alias chain
- the main symbol (suppressed)

The given symbol, which need not be the same as the main symbol, will
be tested twice, if suppressed, but addressing this would make the
code even more elaborate and fragile.

The last case may be unreachable if symbol suppression triggers when
all aliases are suppressed.

I left this change stand-alone for easier review and to credit Giuliano for his
work on it, though it fixes a previous commit.

	* src/abg-writer.cc (write_elf_symbol_reference): Check main
	symbol and aliases with more care.

Fixes: commmit ("dwarf-reader/writer: consider aliases when dealing with suppressions")
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Matthias Maennich <maennich@google.com>
---
 src/abg-writer.cc | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/src/abg-writer.cc b/src/abg-writer.cc
index c2fdd8f0a043..4985174d3abd 100644
--- a/src/abg-writer.cc
+++ b/src/abg-writer.cc
@@ -1728,10 +1728,28 @@ write_elf_symbol_aliases(const elf_symbol& sym, ostream& out)
 static bool
 write_elf_symbol_reference(const elf_symbol& sym, ostream& o)
 {
-  auto actual_sym = &sym;
-  while (actual_sym->is_suppressed() && actual_sym->has_aliases())
-    actual_sym = actual_sym->get_next_alias().get();
-  o << " elf-symbol-id='" << actual_sym->get_id_string() << "'";
+  const elf_symbol* main = sym.get_main_symbol().get();
+  const elf_symbol* alias = &sym;
+  bool found = !alias->is_suppressed();
+  // If the symbol itself is suppressed, check the alias chain.
+  if (!found)
+    {
+      alias = main;
+      found = !alias->is_suppressed();
+    }
+  // If the main symbol is suppressed, search the remainder of the chain.
+  while (!found)
+    {
+      alias = alias->get_next_alias().get();
+      // Two separate termination conditions at present.
+      if (!alias || alias == main)
+        break;
+      found = !alias->is_suppressed();
+    }
+  // If all aliases are suppressed, just stick with the main symbol.
+  if (!found)
+    alias = main;
+  o << " elf-symbol-id='" << alias->get_id_string() << "'";
   return true;
 }
 
-- 
2.30.0.280.ga3ce27912f-goog