[binutils-gdb] PE-COFF: Fix weak external symbol resolution when strong undef is seen first

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=2e2848327d8f8490f77af1a8aebf8aa3a2de588c

commit 2e2848327d8f8490f77af1a8aebf8aa3a2de588c
Author: Peter Damianov <[email protected]>
Date:   Tue Jun 23 17:38:04 2026 -0400

    PE-COFF: Fix weak external symbol resolution when strong undef is seen first
    
    When linking PE-COFF objects, a weak external symbol (C_NT_WEAK with an
    aux record specifying a fallback alias) may fail to resolve if a strong
    undefined reference to the same symbol is encountered before the weak
    definition.  This causes "undefined reference" errors for symbols like
    operator new or personality routines that GCC emits as weak externals
    with a fallback to a default implementation.
    
    There are two problems:
    
    1. In coff_link_add_symbols, when the generic linker resolves a weak
       undefined against an existing strong undefined (NOACT in the action
       table), the COFF-specific symbol_class and aux record were not stored
       because the existing hash entry already had non-null class/type from
       the first (strong) object file.
    
    2. In _bfd_coff_generic_relocate_section, the weak alias fallback only
       triggered for bfd_link_hash_undefweak symbols.  When a strong undef
       is seen first, the hash type stays bfd_link_hash_undefined (the
       generic linker does not downgrade it), so the fallback was skipped.
    
    Fix by extending the condition in coff_link_add_symbols to also update
    symbol_class and aux when the incoming symbol is a PE weak external
    with aux and the existing hash is still undefined.  Also extend the
    relocation handler to resolve the weak alias fallback for
    bfd_link_hash_undefined symbols that carry C_NT_WEAK class and have
    an aux record.
    
    bfd/
            * cofflink.c (coff_link_add_symbols): Also store symbol_class
            and aux record when a PE weak external with aux meets an
            existing undefined hash entry.
            (_bfd_coff_generic_relocate_section): Also resolve weak alias
            fallback for undefined symbols with C_NT_WEAK class and aux.

Diff:
---
 bfd/cofflink.c | 55 ++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 42 insertions(+), 13 deletions(-)

diff --git a/bfd/cofflink.c b/bfd/cofflink.c
index e5c8a987d69..eba0fad4c3f 100644
--- a/bfd/cofflink.c
+++ b/bfd/cofflink.c
@@ -477,13 +477,20 @@ coff_link_add_symbols (bfd *abfd,
 	      /* If we don't have any symbol information currently in
 		 the hash table, or if we are looking at a symbol
 		 definition, then update the symbol class and type in
-		 the hash table.  */
+		 the hash table.  Also update if the incoming symbol is
+		 a weak external with an aux record (PE COFF weak alias)
+		 and the existing symbol is still undefined, so the
+		 fallback alias information is preserved for the linker's
+		 relocation resolution.  */
 	      if (((*sym_hash)->symbol_class == C_NULL
 		   && (*sym_hash)->type == T_NULL)
 		  || sym.n_scnum != 0
 		  || (sym.n_value != 0
 		      && (*sym_hash)->root.type != bfd_link_hash_defined
-		      && (*sym_hash)->root.type != bfd_link_hash_defweak))
+		      && (*sym_hash)->root.type != bfd_link_hash_defweak)
+		  || (IS_WEAK_EXTERNAL (abfd, sym)
+		      && sym.n_numaux > 0
+		      && (*sym_hash)->root.type == bfd_link_hash_undefined))
 		{
 		  (*sym_hash)->symbol_class = sym.n_sclass;
 		  if (sym.n_type != T_NULL)
@@ -521,10 +528,9 @@ coff_link_add_symbols (bfd *abfd,
 		      union internal_auxent *iaux;
 
 		      (*sym_hash)->numaux = sym.n_numaux;
-		      alloc = ((union internal_auxent *)
-			       bfd_hash_allocate (&info->hash->table,
-						  (sym.n_numaux
-						   * sizeof (*alloc))));
+		      alloc = bfd_hash_allocate (&info->hash->table,
+						 (sym.n_numaux
+						  * sizeof (*alloc)));
 		      if (alloc == NULL)
 			goto error_return;
 		      for (i = 0, eaux = esym + symesz, iaux = alloc;
@@ -3065,14 +3071,24 @@ _bfd_coff_generic_relocate_section (bfd *output_bfd,
 		     + sec->output_offset);
 	    }
 
-	  else if (h->root.type == bfd_link_hash_undefweak)
+	  else if (h->root.type == bfd_link_hash_undefweak
+		   || (h->root.type == bfd_link_hash_undefined
+		       && h->symbol_class == C_NT_WEAK && h->numaux == 1))
 	    {
-	      if (h->symbol_class == C_NT_WEAK && h->numaux == 1)
+	      /* Weak undefined symbol: either GNU weak (no aux record) or
+		 PE COFF weak external (C_NT_WEAK with aux record).
+		 Also handles strong undefined symbols that carry PE weak
+		 external metadata (when strong undef is seen before weak def,
+		 the hash type stays bfd_link_hash_undefined but we preserve
+		 the weak external class and aux for later resolution).  */
+
+	      bool is_pe_weak = (h->symbol_class == C_NT_WEAK && h->numaux == 1);
+
+	      if (is_pe_weak)
 		{
-		  /* See _Microsoft Portable Executable and Common Object
+		  /* PE COFF weak external: resolve via fallback alias.
+		     See _Microsoft Portable Executable and Common Object
 		     File Format Specification_, section 5.5.3.
-		     Note that weak symbols without aux records are a GNU
-		     extension.
 		     FIXME: All weak externals are treated as having
 		     characteristic IMAGE_WEAK_EXTERN_SEARCH_NOLIBRARY (1).
 		     These behave as per SVR4 ABI:  A library member
@@ -3081,24 +3097,37 @@ _bfd_coff_generic_relocate_section (bfd *output_bfd,
 		     See also linker.c: generic_link_check_archive_element. */
 		  struct coff_link_hash_entry *h2 = NULL;
 		  unsigned long symndx2 = h->aux->x_sym.x_tagndx.u32;
+
 		  if (symndx2 < obj_raw_syment_count (h->auxbfd))
 		    h2 = obj_coff_sym_hashes (h->auxbfd)[symndx2];
 
 		  if (!h2 || h2->root.type == bfd_link_hash_undefined)
 		    {
+		      /* Fallback alias not found or still undefined.
+			 Resolve to NULL.  */
 		      sec = bfd_abs_section_ptr;
 		      val = 0;
 		    }
 		  else
 		    {
+		      /* Use fallback alias target.  */
 		      sec = h2->root.u.def.section;
 		      val = h2->root.u.def.value
 			+ sec->output_section->vma + sec->output_offset;
 		    }
 		}
 	      else
-		/* This is a GNU extension.  */
-		val = 0;
+		{
+		  /* GNU extension: ELF-style weak symbol in COFF without
+		     PE weak external aux record.  COFF has no native support
+		     for weak symbols (unlike ELF where they're part of the
+		     format).  PE COFF adds them via C_NT_WEAK storage class
+		     with an aux record pointing to a fallback symbol.  GNU ld
+		     extends this by allowing __attribute__((weak)) in COFF
+		     objects even without the PE aux structure, treating them
+		     like ELF weak symbols: resolve to NULL if not defined.  */
+		  val = 0;
+		}
 	    }
 
 	  else if (! bfd_link_relocatable (info))