Calling ifunc functions when target has no debug info but resolver has

After the previous patch, on Fedora 27 (glibc 2.26), if you try
calling strlen in the inferior, you now get:

  (top-gdb) p strlen ("hello")
  '__strlen_avx2' has unknown return type; cast the call to its declared return type

This is correct, because __strlen_avx2 is written in assembly.

We can improve on this though -- if the final ifunc resolved/target
function has no debug info, but the ifunc _resolver_ does have debug
info, we can try extracting the final function's type from the type
that the resolver returns.  E.g.,:

  typedef size_t (*strlen_t) (const char*);

  size_t my_strlen (const char *) { /* some implementation */ }
  strlen_t strlen_resolver (unsigned long hwcap) { return my_strlen; }

  extern size_t strlen (const char *s);
  __typeof (strlen) strlen __attribute__ ((ifunc ("strlen_resolver")));

In the strlen example above, the resolver returns strlen_t, which is a
typedef for pointer to a function that returns size_t.  "strlen_t" is
the type of both the user-visible "strlen", and of the the target
function that implements it.

This patch teaches GDB to extract that type.

This is done for actual inferior function calls (in infcall.c), and
for ptype (in eval_call).  By the time we get to either of these
places, we've already lost the original symbol/minsym, and only have
values and types to work with.  Hence the changes to c-exp.y and
evaluate_var_msym_value, to ensure that we propagate the ifunc
minsymbol's info.

The change to make ifunc symbols have no/unknown return type exposes a
latent problem -- gdb.compile/compile-ifunc.exp calls a no-debug-info
function, but we did not warn about it.  The test is fixed by this
commit too.

gdb/ChangeLog:
2018-04-26  Pedro Alves  <palves@redhat.com>

	* blockframe.c (find_gnu_ifunc_target_type): New function.
	(find_function_type): New.
	* eval.c (evaluate_var_msym_value): For GNU ifunc types, always
	return a value with a memory address.
	(eval_call): For calls to GNU ifunc functions, try to find the
	type of the target function from the type that the resolver
	returns.
	* gdbtypes.c (objfile_type): Don't install a return type for ifunc
	symbols.
	* infcall.c (find_function_return_type): Delete.
	(find_function_addr): Add 'function_type' parameter.  For calls to
	GNU ifunc functions, try to find the type of the target function
	from the type that the resolver returns, and return it via
	FUNCTION_TYPE.
	(call_function_by_hand_dummy): Adjust to use the function type
	returned by find_function_addr.
	(find_function_addr): Add 'function_type' parameter and move
	description here.
	* symtab.h (find_function_type, find_gnu_ifunc_target_type): New
	declarations.

gdb/testsuite/ChangeLog:
2018-04-26  Pedro Alves  <palves@redhat.com>

	* gdb.compile/compile-ifunc.exp: Also expect "function has unknown
	return type" warnings.

9 files changed, 140 insertions, 51 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e23eeee..8db2d8a 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,28 @@
 2018-04-26  Pedro Alves  <palves@redhat.com>
 
+	* blockframe.c (find_gnu_ifunc_target_type): New function.
+	(find_function_type): New.
+	* eval.c (evaluate_var_msym_value): For GNU ifunc types, always
+	return a value with a memory address.
+	(eval_call): For calls to GNU ifunc functions, try to find the
+	type of the target function from the type that the resolver
+	returns.
+	* gdbtypes.c (objfile_type): Don't install a return type for ifunc
+	symbols.
+	* infcall.c (find_function_return_type): Delete.
+	(find_function_addr): Add 'function_type' parameter.  For calls to
+	GNU ifunc functions, try to find the type of the target function
+	from the type that the resolver returns, and return it via
+	FUNCTION_TYPE.
+	(call_function_by_hand_dummy): Adjust to use the function type
+	returned by find_function_addr.
+	(find_function_addr): Add 'function_type' parameter and move
+	description here.
+	* symtab.h (find_function_type, find_gnu_ifunc_target_type): New
+	declarations.
+
+2018-04-26  Pedro Alves  <palves@redhat.com>
+
 	* c-exp.y (variable production): Skip finding an alias for ifunc
 	symbols.
 
diff --git a/gdb/blockframe.c b/gdb/blockframe.c
index 9be8871..e6938a3 100644
--- a/gdb/blockframe.c
+++ b/gdb/blockframe.c
@@ -323,6 +323,45 @@ find_pc_partial_function (CORE_ADDR pc, const char **name, CORE_ADDR *address,
   return find_pc_partial_function_gnu_ifunc (pc, name, address, endaddr, NULL);
 }
 
+/* See symtab.h.  */
+
+struct type *
+find_function_type (CORE_ADDR pc)
+{
+  struct symbol *sym = find_pc_function (pc);
+
+  if (sym != NULL && BLOCK_START (SYMBOL_BLOCK_VALUE (sym)) == pc)
+    return SYMBOL_TYPE (sym);
+
+  return NULL;
+}
+
+/* See symtab.h.  */
+
+struct type *
+find_gnu_ifunc_target_type (CORE_ADDR resolver_funaddr)
+{
+  struct type *resolver_type = find_function_type (resolver_funaddr);
+  if (resolver_type != NULL)
+    {
+      /* Get the return type of the resolver.  */
+      struct type *resolver_ret_type
+	= check_typedef (TYPE_TARGET_TYPE (resolver_type));
+
+      /* If we found a pointer to function, then the resolved type
+	 is the type of the pointed-to function.  */
+      if (TYPE_CODE (resolver_ret_type) == TYPE_CODE_PTR)
+	{
+	  struct type *resolved_type
+	    = TYPE_TARGET_TYPE (resolver_ret_type);
+	  if (TYPE_CODE (check_typedef (resolved_type)) == TYPE_CODE_FUNC)
+	    return resolved_type;
+	}
+    }
+
+  return NULL;
+}
+
 /* Return the innermost stack frame that is executing inside of BLOCK and is
    at least as old as the selected frame. Return NULL if there is no
    such frame.  If BLOCK is NULL, just return NULL.  */
diff --git a/gdb/eval.c b/gdb/eval.c
index b6fbfcf..ad66f7c 100644
--- a/gdb/eval.c
+++ b/gdb/eval.c
@@ -734,17 +734,13 @@ value *
 evaluate_var_msym_value (enum noside noside,
 			 struct objfile *objfile, minimal_symbol *msymbol)
 {
-  if (noside == EVAL_AVOID_SIDE_EFFECTS)
-    {
-      type *the_type = find_minsym_type_and_address (msymbol, objfile, NULL);
-      return value_zero (the_type, not_lval);
-    }
+  CORE_ADDR address;
+  type *the_type = find_minsym_type_and_address (msymbol, objfile, &address);
+
+  if (noside == EVAL_AVOID_SIDE_EFFECTS && !TYPE_GNU_IFUNC (the_type))
+    return value_zero (the_type, not_lval);
   else
-    {
-      CORE_ADDR address;
-      type *the_type = find_minsym_type_and_address (msymbol, objfile, &address);
-      return value_at_lazy (the_type, address);
-    }
+    return value_at_lazy (the_type, address);
 }
 
 /* Helper for returning a value when handling EVAL_SKIP.  */
@@ -797,6 +793,15 @@ eval_call (expression *exp, enum noside noside,
       else if (TYPE_CODE (ftype) == TYPE_CODE_FUNC
 	       || TYPE_CODE (ftype) == TYPE_CODE_METHOD)
 	{
+	  if (TYPE_GNU_IFUNC (ftype))
+	    {
+	      CORE_ADDR address = value_address (argvec[0]);
+	      type *resolved_type = find_gnu_ifunc_target_type (address);
+
+	      if (resolved_type != NULL)
+		ftype = resolved_type;
+	    }
+
 	  type *return_type = TYPE_TARGET_TYPE (ftype);
 
 	  if (return_type == NULL)
diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index b3a0379..2efd126 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -5443,10 +5443,6 @@ objfile_type (struct objfile *objfile)
   objfile_type->nodebug_text_gnu_ifunc_symbol
     = init_type (objfile, TYPE_CODE_FUNC, TARGET_CHAR_BIT,
 		 "<text gnu-indirect-function variable, no debug info>");
-  /* Ifunc resolvers return a function address.  */
-  TYPE_TARGET_TYPE (objfile_type->nodebug_text_gnu_ifunc_symbol)
-    = init_integer_type (objfile, gdbarch_addr_bit (gdbarch), 1,
-			 "__IFUNC_RESOLVER_RET");
   TYPE_GNU_IFUNC (objfile_type->nodebug_text_gnu_ifunc_symbol) = 1;
   objfile_type->nodebug_got_plt_symbol
     = init_pointer_type (objfile, gdbarch_addr_bit (gdbarch),
diff --git a/gdb/infcall.c b/gdb/infcall.c
index 9f02674..b233e36 100644
--- a/gdb/infcall.c
+++ b/gdb/infcall.c
@@ -229,26 +229,12 @@ value_arg_coerce (struct gdbarch *gdbarch, struct value *arg,
   return value_cast (type, arg);
 }
 
-/* Return the return type of a function with its first instruction exactly at
-   the PC address.  Return NULL otherwise.  */
-
-static struct type *
-find_function_return_type (CORE_ADDR pc)
-{
-  struct symbol *sym = find_pc_function (pc);
-
-  if (sym != NULL && BLOCK_START (SYMBOL_BLOCK_VALUE (sym)) == pc
-      && SYMBOL_TYPE (sym) != NULL)
-    return TYPE_TARGET_TYPE (SYMBOL_TYPE (sym));
-
-  return NULL;
-}
-
-/* Determine a function's address and its return type from its value.
-   Calls error() if the function is not valid for calling.  */
+/* See infcall.h.  */
 
 CORE_ADDR
-find_function_addr (struct value *function, struct type **retval_type)
+find_function_addr (struct value *function,
+		    struct type **retval_type,
+		    struct type **function_type)
 {
   struct type *ftype = check_typedef (value_type (function));
   struct gdbarch *gdbarch = get_type_arch (ftype);
@@ -275,17 +261,33 @@ find_function_addr (struct value *function, struct type **retval_type)
   if (TYPE_CODE (ftype) == TYPE_CODE_FUNC
       || TYPE_CODE (ftype) == TYPE_CODE_METHOD)
     {
-      value_type = TYPE_TARGET_TYPE (ftype);
-
       if (TYPE_GNU_IFUNC (ftype))
 	{
-	  funaddr = gnu_ifunc_resolve_addr (gdbarch, funaddr);
+	  CORE_ADDR resolver_addr = funaddr;
 
-	  /* Skip querying the function symbol if no RETVAL_TYPE has been
-	     asked for.  */
-	  if (retval_type)
-	    value_type = find_function_return_type (funaddr);
+	  /* Resolve the ifunc.  Note this may call the resolver
+	     function in the inferior.  */
+	  funaddr = gnu_ifunc_resolve_addr (gdbarch, resolver_addr);
+
+	  /* Skip querying the function symbol if no RETVAL_TYPE or
+	     FUNCTION_TYPE have been asked for.  */
+	  if (retval_type != NULL || function_type != NULL)
+	    {
+	      type *target_ftype = find_function_type (funaddr);
+	      /* If we don't have debug info for the target function,
+		 see if we can instead extract the target function's
+		 type from the type that the resolver returns.  */
+	      if (target_ftype == NULL)
+		target_ftype = find_gnu_ifunc_target_type (resolver_addr);
+	      if (target_ftype != NULL)
+		{
+		  value_type = TYPE_TARGET_TYPE (check_typedef (target_ftype));
+		  ftype = target_ftype;
+		}
+	    }
 	}
+      else
+	value_type = TYPE_TARGET_TYPE (ftype);
     }
   else if (TYPE_CODE (ftype) == TYPE_CODE_INT)
     {
@@ -320,6 +322,8 @@ find_function_addr (struct value *function, struct type **retval_type)
 
   if (retval_type != NULL)
     *retval_type = value_type;
+  if (function_type != NULL)
+    *function_type = ftype;
   return funaddr + gdbarch_deprecated_function_start_offset (gdbarch);
 }
 
@@ -719,15 +723,13 @@ call_function_by_hand_dummy (struct value *function,
 			     void *dummy_dtor_data)
 {
   CORE_ADDR sp;
-  struct type *values_type, *target_values_type;
+  struct type *target_values_type;
   unsigned char struct_return = 0, hidden_first_param_p = 0;
   CORE_ADDR struct_addr = 0;
   struct infcall_control_state *inf_status;
   struct cleanup *inf_status_cleanup;
   struct infcall_suspend_state *caller_state;
-  CORE_ADDR funaddr;
   CORE_ADDR real_pc;
-  struct type *ftype = check_typedef (value_type (function));
   CORE_ADDR bp_addr;
   struct frame_id dummy_id;
   struct frame_info *frame;
@@ -738,9 +740,6 @@ call_function_by_hand_dummy (struct value *function,
   char name_buf[RAW_FUNCTION_ADDRESS_SIZE];
   bool stack_temporaries = thread_stack_temporaries_enabled_p (inferior_ptid);
 
-  if (TYPE_CODE (ftype) == TYPE_CODE_PTR)
-    ftype = check_typedef (TYPE_TARGET_TYPE (ftype));
-
   if (!target_has_execution)
     noprocess ();
 
@@ -864,7 +863,10 @@ call_function_by_hand_dummy (struct value *function,
       }
   }
 
-  funaddr = find_function_addr (function, &values_type);
+  type *ftype;
+  type *values_type;
+  CORE_ADDR funaddr = find_function_addr (function, &values_type, &ftype);
+
   if (values_type == NULL)
     values_type = default_return_type;
   if (values_type == NULL)
diff --git a/gdb/infcall.h b/gdb/infcall.h
index a3861fb..8b21950 100644
--- a/gdb/infcall.h
+++ b/gdb/infcall.h
@@ -25,8 +25,15 @@
 struct value;
 struct type;
 
+/* Determine a function's address and its return type from its value.
+   If the function is a GNU ifunc, then return the address of the
+   target function, and set *FUNCTION_TYPE to the target function's
+   type, and *RETVAL_TYPE to the target function's return type.
+   Calls error() if the function is not valid for calling.  */
+
 extern CORE_ADDR find_function_addr (struct value *function, 
-				     struct type **retval_type);
+				     struct type **retval_type,
+				     struct type **function_type = NULL);
 
 /* Perform a function call in the inferior.
 
diff --git a/gdb/symtab.h b/gdb/symtab.h
index f9d52e7..83ff6f2 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1675,6 +1675,17 @@ extern int find_pc_partial_function_gnu_ifunc (CORE_ADDR pc, const char **name,
 extern int find_pc_partial_function (CORE_ADDR, const char **, CORE_ADDR *,
 				     CORE_ADDR *);
 
+/* Return the type of a function with its first instruction exactly at
+   the PC address.  Return NULL otherwise.  */
+
+extern struct type *find_function_type (CORE_ADDR pc);
+
+/* See if we can figure out the function's actual type from the type
+   that the resolver returns.  RESOLVER_FUNADDR is the address of the
+   ifunc resolver.  */
+
+extern struct type *find_gnu_ifunc_target_type (CORE_ADDR resolver_funaddr);
+
 extern void clear_pc_function_cache (void);
 
 /* Expand symtab containing PC, SECTION if not already expanded.  */
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 9ee7606..7f7a92f 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2018-04-26  Pedro Alves  <palves@redhat.com>
+
+	* gdb.compile/compile-ifunc.exp: Also expect "function has unknown
+	return type" warnings.
+
 2018-04-25  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/hook-stop.exp: Expect "killed" instead of "has been
diff --git a/gdb/testsuite/gdb.compile/compile-ifunc.exp b/gdb/testsuite/gdb.compile/compile-ifunc.exp
index ed700e4..979e391 100644
--- a/gdb/testsuite/gdb.compile/compile-ifunc.exp
+++ b/gdb/testsuite/gdb.compile/compile-ifunc.exp
@@ -37,7 +37,9 @@ with_test_prefix "nodebug" {
     }
 
     gdb_test "compile code resultvar = gnu_ifunc (10);" \
-	"warning: variable has unknown type; assuming int"
+	[multi_line \
+	     "warning: variable has unknown type; assuming int" \
+	     "warning: function has unknown return type; assuming int"]
 
     gdb_test "p (int) resultvar" " = 11"
 
@@ -52,10 +54,9 @@ with_test_prefix "debug" {
     if ![runto_main] {
 	return -1
     }
-
     # gnu_ifunc (10): error: too many arguments to function 'gnu_ifunc'
-    gdb_test_no_output "compile code resultvar = gnu_ifunc_alias (10);"
-
+    gdb_test "compile code resultvar = gnu_ifunc_alias (10);" \
+	"warning: function has unknown return type; assuming int"
     gdb_test "p resultvar" " = 11"
 
 }