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 --git a/bfd/cofflink.c b/bfd/cofflink.c
index e5c8a987d699a399ee459aa4a458ff4b392cf6dc..eba0fad4c3f321f8185edba2292b35b319f23d41 100644 (file)
--- 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))