elf: add support for GNU indirect function symbol type (gnu_ifunc)

doc/outfmt.src

@@ -1415,6 +1415,15 @@ Declaring the type and size of global symbols is necessary when
 writing shared library code. For more information, see
 \k{picglobal}.
 
+NASM supports the GNU indirect function symbol type using the keyword
+\c{gnu_ifunc}. This marks the symbol as \c{STT_GNU_IFUNC} and causes the
+dynamic loader to call the symbol as a resolver at runtime. For example:
+
+\c    global  func_ifunc:function
+\c
+\c    global  func:gnu_ifunc
+\c    func    equ func_ifunc
+
 
 \S{elfextrn} \c{elf} Extensions to the \c{EXTERN} Directive\I{EXTERN,
 elf extensions to}\I{EXTERN, elf extensions to}

output/elf.h

@@ -232,6 +232,7 @@
 #define STT_COMMON	5	/* Symbol is a common data object */
 #define STT_TLS		6	/* Symbol is thread-local data object */
 #define STT_NUM		7	/* Number of defined types */
+#define STT_GNU_IFUNC   10      /* Symbol is indirect code object */
 
 /* Symbol visibilities */
 #define STV_DEFAULT	0	/* Default symbol visibility rules */

output/outelf.c

@@ -886,6 +886,8 @@ static void elf_deflabel(char *name, int32_t segment, int64_t offset,
             case 9:
                 if (!nasm_strnicmp(spcword, "protected", wlen))
                     sym->other = STV_PROTECTED;
+                else if (!nasm_strnicmp(spcword, "gnu_ifunc", wlen))
+                    type = STT_GNU_IFUNC;
                 else
                     ok = false;
                 break;

travis/test/gnu_ifunc.asm

@@ -0,0 +1,14 @@
+section .text
+
+global __foo_ifunc:function
+global __foo_impl:function
+
+__foo_impl:
+	ret
+
+__foo_ifunc:
+    lea rax, [rel __foo_impl]
+    ret
+
+global foo:gnu_ifunc
+foo equ __foo_ifunc

travis/test/gnu_ifunc.json

@@ -0,0 +1,11 @@
+[
+    {
+        "description": "Test for GNU indirect function (STT_GNU_IFUNC) symbol emission",
+        "id": "gnu_ifunc",
+        "format": "elf64",
+        "source": "gnu_ifunc.asm",
+        "target": [
+            { "output": "gnu_ifunc.o" }
+        ]
+    }
+]