[PATCH] Handle function aliases better (PR gdb/19487, errno printing)

[Pedro Alves <palves at redhat dot com>]

(Ref: https://sourceware.org/ml/gdb/2017-06/msg00048.html)

This patch improves GDB support for function aliases defined with
__attribute__ alias.  For example, in the test added by this commit,
there is no reference to "func_alias" in the debug info at all, only
to "func"'s definition:

 $ nm  ./testsuite/outputs/gdb.base/symbol-alias/symbol-alias  | grep " func"
 00000000004005ae t func
 00000000004005ae T func_alias

 $ readelf -w ./testsuite/outputs/gdb.base/symbol-alias/symbol-alias | grep func -B 1 -A 8
 <1><db>: Abbrev Number: 5 (DW_TAG_subprogram)
    <dc>   DW_AT_name        : (indirect string, offset: 0x111): func
    <e0>   DW_AT_decl_file   : 1
    <e1>   DW_AT_decl_line   : 27
    <e2>   DW_AT_prototyped  : 1
    <e2>   DW_AT_type        : <0xf8>
    <e6>   DW_AT_low_pc      : 0x4005ae
    <ee>   DW_AT_high_pc     : 0xb
    <f6>   DW_AT_frame_base  : 1 byte block: 9c         (DW_OP_call_frame_cfa)
    <f8>   DW_AT_GNU_all_call_sites: 1

So all GDB knows about "func_alias" is from the minsym (elf symbol):

 (gdb) p func_alias
 $1 = {<text variable, no debug info>} 0x4005ae <func>
 (gdb) ptype func_alias
 type = int ()

 (gdb) p func
 $2 = {struct S *(void)} 0x4005ae <func>
 (gdb) ptype func
 type = struct S {
     int field1;
     int field2;
 } *(void)

The result is that calling func_alias from the command line produces
incorrect results.

This is similar (though not exactly the same) to the glibc
errno/__errno_location/__GI___errno_location situation.  On glibc,
errno is defined like this:

  extern int *__errno_location (void);
  #define errno (*__errno_location ())

with __GI___errno_location being an internal alias for
__errno_location.  On my system's libc (F23), I do see debug info for
__errno_location, in the form of name vs linkage name:

 <1><95a5>: Abbrev Number: 18 (DW_TAG_subprogram)
    <95a6>   DW_AT_external    : 1
    <95a6>   DW_AT_name        : (indirect string, offset: 0x2c26): __errno_location
    <95aa>   DW_AT_decl_file   : 1
    <95ab>   DW_AT_decl_line   : 24
    <95ac>   DW_AT_linkage_name: (indirect string, offset: 0x2c21): __GI___errno_location
    <95b0>   DW_AT_prototyped  : 1
    <95b0>   DW_AT_type        : <0x9206>
    <95b4>   DW_AT_low_pc      : 0x20f40
    <95bc>   DW_AT_high_pc     : 0x11
    <95c4>   DW_AT_frame_base  : 1 byte block: 9c       (DW_OP_call_frame_cfa)
    <95c6>   DW_AT_GNU_all_call_sites: 1

however that doesn't matter in practice, because GDB doesn't record
demangled names anyway, and so we end up with the exact same situation
covered by the testcase.

So the fix is to make the expression parser find a debug symbol for
the same address as the just-found minsym, when a lookup by name
didn't find a debug symbol by name.  We now get:

 (gdb) p func_alias
 $1 = {struct S *(void)} 0x4005ae <func>
 (gdb) p __errno_location
 $2 = {int *(void)} 0x7ffff6e92830 <__errno_location>

I've made the test exercise variable aliases too, for completeness.
Those already work correctly, because unlike for function aliases, GCC
emits debug information for variable aliases.

Tested on GNU/Linux.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	PR gdb/19487
	* c-exp.y (variable production): Handle function aliases.
	* minsyms.c (msymbol_is_text): New function.
	* minsyms.h (msymbol_is_text): Declare.
	* symtab.c (find_function_alias_target): New function.
	* symtab.h (find_function_alias_target): Declare.

gdb/testsuite/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	PR gdb/19487
	* gdb.base/symbol-alias.c: New.
	* gdb.base/symbol-alias2.c: New.
	* gdb.base/symbol-alias.exp: New.
---
 gdb/c-exp.y                             | 35 ++++++++++++++++++++++++-------
 gdb/minsyms.c                           | 17 +++++++++++++++
 gdb/minsyms.h                           |  4 ++++
 gdb/symtab.c                            | 19 +++++++++++++++++
 gdb/symtab.h                            |  7 +++++++
 gdb/testsuite/gdb.base/symbol-alias.c   | 31 +++++++++++++++++++++++++++
 gdb/testsuite/gdb.base/symbol-alias.exp | 37 +++++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.base/symbol-alias2.c  | 34 ++++++++++++++++++++++++++++++
 8 files changed, 176 insertions(+), 8 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/symbol-alias.c
 create mode 100644 gdb/testsuite/gdb.base/symbol-alias.exp
 create mode 100644 gdb/testsuite/gdb.base/symbol-alias2.c

diff --git a/gdb/c-exp.y b/gdb/c-exp.y
index bdcd51f..650ae10 100644
--- a/gdb/c-exp.y
+++ b/gdb/c-exp.y
@@ -1041,15 +1041,34 @@ variable:	name_not_typename
 			      struct bound_minimal_symbol msymbol;
 			      char *arg = copy_name ($1.stoken);
 
-			      msymbol =
-				lookup_bound_minimal_symbol (arg);
-			      if (msymbol.minsym != NULL)
-				write_exp_msymbol (pstate, msymbol);
-			      else if (!have_full_symbols () && !have_partial_symbols ())
-				error (_("No symbol table is loaded.  Use the \"file\" command."));
+			      msymbol = lookup_bound_minimal_symbol (arg);
+			      if (msymbol.minsym == NULL)
+				{
+				  if (!have_full_symbols () && !have_partial_symbols ())
+				    error (_("No symbol table is loaded.  Use the \"file\" command."));
+				  else
+				    error (_("No symbol \"%s\" in current context."),
+					   copy_name ($1.stoken));
+				}
+
+			      /* This minsym might be an alias for
+				 another function.  See if we can find
+				 the debug symbol for the target, and
+				 if so, use it instead, since it has
+				 prototype info.  This is important
+				 for example for "p *__errno_location()".  */
+			      symbol *alias_target
+				= find_function_alias_target (msymbol);
+			      if (alias_target != NULL)
+				{
+				  write_exp_elt_opcode (pstate, OP_VAR_VALUE);
+				  write_exp_elt_block
+				    (pstate, SYMBOL_BLOCK_VALUE (alias_target));
+				  write_exp_elt_sym (pstate, alias_target);
+				  write_exp_elt_opcode (pstate, OP_VAR_VALUE);
+				}
 			      else
-				error (_("No symbol \"%s\" in current context."),
-				       copy_name ($1.stoken));
+				write_exp_msymbol (pstate, msymbol);
 			    }
 			}
 	;
diff --git a/gdb/minsyms.c b/gdb/minsyms.c
index c93eaa3..69b9e40 100644
--- a/gdb/minsyms.c
+++ b/gdb/minsyms.c
@@ -52,6 +52,23 @@
 #include "cli/cli-utils.h"
 #include "symbol.h"
 
+/* See minsyms.h.  */
+
+bool
+msymbol_is_text (minimal_symbol *msymbol)
+{
+  switch (MSYMBOL_TYPE (msymbol))
+    {
+    case mst_text:
+    case mst_text_gnu_ifunc:
+    case mst_solib_trampoline:
+    case mst_file_text:
+      return true;
+    default:
+      return false;
+    }
+}
+
 /* Accumulate the minimal symbols for each objfile in bunches of BUNCH_SIZE.
    At the end, copy them all into one newly allocated location on an objfile's
    per-BFD storage obstack.  */
diff --git a/gdb/minsyms.h b/gdb/minsyms.h
index b82a22a..e763f62 100644
--- a/gdb/minsyms.h
+++ b/gdb/minsyms.h
@@ -157,6 +157,10 @@ void terminate_minimal_symbol_table (struct objfile *objfile);
 
 
 
+/* Return whether MSYMBOL is a function/method.  */
+
+bool msymbol_is_text (minimal_symbol *msymbol);
+
 /* Compute a hash code for the string argument.  */
 
 unsigned int msymbol_hash (const char *);
diff --git a/gdb/symtab.c b/gdb/symtab.c
index 497d520..4269267 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3827,6 +3827,25 @@ skip_prologue_using_sal (struct gdbarch *gdbarch, CORE_ADDR func_addr)
     /* Don't return END_PC, which is past the end of the function.  */
     return prologue_sal.pc;
 }
+
+/* See symtab.h.  */
+
+symbol *
+find_function_alias_target (bound_minimal_symbol msymbol)
+{
+  if (!msymbol_is_text (msymbol.minsym))
+    return NULL;
+
+  CORE_ADDR addr = BMSYMBOL_VALUE_ADDRESS (msymbol);
+  symbol *sym = find_pc_function (addr);
+  if (sym != NULL
+      && SYMBOL_CLASS (sym) == LOC_BLOCK
+      && BLOCK_START (SYMBOL_BLOCK_VALUE (sym)) == addr)
+    return sym;
+
+  return NULL;
+}
+
 
 /* If P is of the form "operator[ \t]+..." where `...' is
    some legitimate operator text, return a pointer to the
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 341deca..1b48611 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1537,6 +1537,13 @@ extern CORE_ADDR skip_prologue_using_sal (struct gdbarch *gdbarch,
 extern struct symbol *fixup_symbol_section (struct symbol *,
 					    struct objfile *);
 
+/* If MSYMBOL is an text symbol, look for a function debug symbol with
+   the same address.  Returns NULL if not found.  This is necessary in
+   case a function is an alias to some other function, because debug
+   information is only emitted for the alias target function's
+   definition, not for the alias.  */
+extern symbol *find_function_alias_target (bound_minimal_symbol msymbol);
+
 /* Symbol searching */
 /* Note: struct symbol_search, search_symbols, et.al. are declared here,
    instead of making them local to symtab.c, for gdbtk's sake.  */
diff --git a/gdb/testsuite/gdb.base/symbol-alias.c b/gdb/testsuite/gdb.base/symbol-alias.c
new file mode 100644
index 0000000..cc5d104
--- /dev/null
+++ b/gdb/testsuite/gdb.base/symbol-alias.c
@@ -0,0 +1,31 @@
+/* This test is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+struct S
+{
+  int field1;
+  int field2;
+};
+
+extern struct S *func_alias (void);
+
+int
+main (void)
+{
+  struct S *s = func_alias ();
+  return s->field1 - s->field1;
+}
diff --git a/gdb/testsuite/gdb.base/symbol-alias.exp b/gdb/testsuite/gdb.base/symbol-alias.exp
new file mode 100644
index 0000000..dc63eb1
--- /dev/null
+++ b/gdb/testsuite/gdb.base/symbol-alias.exp
@@ -0,0 +1,37 @@
+# Test for printing alias symbols.
+# Copyright 2017 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+standard_testfile symbol-alias.c symbol-alias2.c
+
+if { [prepare_for_testing "failed to prepare" ${testfile} [list $srcfile $srcfile2]] } {
+    return -1
+}
+
+if ![runto_main] then {
+    fail "can't run to main"
+    continue
+}
+
+# Functions.
+foreach f {"func" "func_alias"} {
+    gdb_test "p $f" " = {struct S \\*\\(void\\)} $hex <func>"
+    gdb_test "p *${f}()" "= {field1 = 1, field2 = 2}"
+}
+
+# Variables.
+foreach v {"g_var_s" "g_var_s_alias"} {
+    gdb_test "p $v" "= {field1 = 1, field2 = 2}"
+}
diff --git a/gdb/testsuite/gdb.base/symbol-alias2.c b/gdb/testsuite/gdb.base/symbol-alias2.c
new file mode 100644
index 0000000..aedd520
--- /dev/null
+++ b/gdb/testsuite/gdb.base/symbol-alias2.c
@@ -0,0 +1,34 @@
+/* This test is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+struct S
+{
+  int field1;
+  int field2;
+};
+
+struct S g_var_s = { 1, 2 };
+
+static struct S *
+func (void)
+{
+  return &g_var_s;
+}
+
+struct S *func_alias (void) __attribute__ ((alias ("func")));
+
+extern struct S g_var_s_alias __attribute__ ((alias ("g_var_s")));
-- 
2.5.5