Add support for IMPORT_NAME_EXPORTAS in ILF (MSVC style) import libraries

This import name type is formally yet undocumented, but MSVC
produces/supports it, primarily for ARM64EC import libraries.

LLVM/LLD also supports this import name type. Since recently,
llvm-dlltool also uses this type for certain kinds of renamed imports
(that are easy to do in the long style import libraries produced by
GNU dlltool, but require this name type in short import libraries).

This name type contains a third string, in addition to the symbol
name and the DLL name, indicating the actual imported name to
reference in the import tables - which now can be distinct different
from the symbol name on the object file level.

8f23464a5d
and
7b275aa243
show how this import name type was added in LLVM.

Signed-off-by: Martin Storsjö <martin@martin.st>

bfd/peicode.h

@@ -409,7 +409,7 @@ pe_bfd_copy_private_bfd_data (bfd *ibfd, bfd *obfd)
 
       There will be two symbols for the imported value, one the symbol name
       and one with _imp__ prefixed.  Allowing for the terminating nul's this
-      is strlen (symbol_name) * 2 + 8 + 21 + strlen (source_dll).
+      is strlen (import_name) * 2 + 8 + 21 + strlen (source_dll).
 
       The strings in the string table must start STRING__SIZE_SIZE bytes into
       the table in order to for the string lookup code in coffgen/coffcode to
@@ -425,7 +425,7 @@ pe_bfd_copy_private_bfd_data (bfd *ibfd, bfd *obfd)
 #define SIZEOF_ILF_EXT_SYMS	 (NUM_ILF_SYMS * sizeof (* vars.esym_table))
 #define SIZEOF_ILF_RELOCS	 (NUM_ILF_RELOCS * sizeof (* vars.reltab))
 #define SIZEOF_ILF_INT_RELOCS	 (NUM_ILF_RELOCS * sizeof (* vars.int_reltab))
-#define SIZEOF_ILF_STRINGS	 (strlen (symbol_name) * 2 + 8 \
+#define SIZEOF_ILF_STRINGS	 (strlen (import_name) * 2 + 8 \
 					+ 21 + strlen (source_dll) \
 					+ NUM_ILF_SECTIONS * 9 \
 					+ STRING_SIZE_SIZE)
@@ -440,7 +440,7 @@ pe_bfd_copy_private_bfd_data (bfd *ibfd, bfd *obfd)
 #define SIZEOF_IDATA5		(1 * 4)
 #endif
 
-#define SIZEOF_IDATA6		(2 + strlen (symbol_name) + 1 + 1)
+#define SIZEOF_IDATA6		(2 + strlen (import_name) + 1 + 1)
 #define SIZEOF_IDATA7		(strlen (source_dll) + 1 + 1)
 #define SIZEOF_ILF_SECTIONS	(NUM_ILF_SECTIONS * sizeof (struct coff_section_tdata))
 
@@ -792,7 +792,8 @@ pe_ILF_build_a_bfd (bfd *	    abfd,
 		    char *	    symbol_name,
 		    char *	    source_dll,
 		    unsigned int    ordinal,
-		    unsigned int    types)
+		    unsigned int    types,
+		    char *          import_name)
 {
   bfd_byte *		   ptr;
   pe_ILF_vars		   vars;
@@ -834,6 +835,17 @@ pe_ILF_build_a_bfd (bfd *	    abfd,
     case IMPORT_NAME:
     case IMPORT_NAME_NOPREFIX:
     case IMPORT_NAME_UNDECORATE:
+      import_name = symbol_name;
+      break;
+
+    case IMPORT_NAME_EXPORTAS:
+      if (!import_name || !import_name[0])
+	{
+	  _bfd_error_handler (_("%pB: missing import name for "
+				"IMPORT_NAME_EXPORTAS for %s"),
+			      abfd, symbol_name);
+	  return false;
+	}
       break;
 
     default:
@@ -955,7 +967,7 @@ pe_ILF_build_a_bfd (bfd *	    abfd,
 	goto error_return;
 
       /* If necessary, trim the import symbol name.  */
-      symbol = symbol_name;
+      symbol = import_name;
 
       /* As used by MS compiler, '_', '@', and '?' are alternative
 	 forms of USER_LABEL_PREFIX, with '?' for c++ mangled names,
@@ -964,7 +976,8 @@ pe_ILF_build_a_bfd (bfd *	    abfd,
 	 IMPORT_NAME_NOPREFIX and IMPORT_NAME_UNDECORATE as per the
 	 PE COFF 6.0 spec (section 8.3, Import Name Type).  */
 
-      if (import_name_type != IMPORT_NAME)
+      if (import_name_type != IMPORT_NAME &&
+	  import_name_type != IMPORT_NAME_EXPORTAS)
 	{
 	  char c = symbol[0];
 
@@ -1179,6 +1192,7 @@ pe_ILF_object_p (bfd * abfd)
   bfd_byte *	  ptr;
   char *	  symbol_name;
   char *	  source_dll;
+  char *	  import_name;
   unsigned int	  machine;
   bfd_size_type	  size;
   unsigned int	  ordinal;
@@ -1340,9 +1354,24 @@ pe_ILF_object_p (bfd * abfd)
       return NULL;
     }
 
+  /* An ILF file may contain a third string, after source_dll; this is used
+   * for IMPORT_NAME_EXPORTAS. We know from above that the whole block of
+   * data is null terminated, ptr[size-1]==0, but we don't know how many
+   * individual null terminated strings we have in there.
+   *
+   * First find the end of source_dll. */
+  import_name = source_dll + strlen (source_dll) + 1;
+  if ((bfd_byte *) import_name >= ptr + size)
+    {
+      /* If this points at the end of the ptr+size block, we only had two
+       * strings. */
+      import_name = NULL;
+    }
+
   /* Now construct the bfd.  */
   if (! pe_ILF_build_a_bfd (abfd, magic, symbol_name,
-			    source_dll, ordinal, types))
+			    source_dll, ordinal, types,
+			    import_name))
     {
       bfd_release (abfd, ptr);
       return NULL;

include/coff/pe.h

@@ -338,6 +338,7 @@ typedef struct
 #define IMPORT_NAME		1
 #define IMPORT_NAME_NOPREFIX	2
 #define IMPORT_NAME_UNDECORATE	3
+#define IMPORT_NAME_EXPORTAS	4
 
 /* Weak external characteristics.  */
 #define IMAGE_WEAK_EXTERN_SEARCH_NOLIBRARY	1