commit 591a12a1d4c8843343eb999145d8bcc1efedf408

Author: Ulrich Weigand <ulrich.weigand@de.ibm.com>

Date:   Tue Feb 4 18:44:14 2014 +0100

    PowerPC64 ELFv2 ABI: skip global entry point code

    This patch handles another aspect of the ELFv2 ABI, which unfortunately

    requires common code changes.

    In ELFv2, functions may provide both a global and a local entry point.

    The global entry point (where the function symbol points to) is intended

    to be used for function-pointer or cross-module (PLT) calls, and requires

    r12 to be set up to the entry point address itself.   The local entry

    point (which is found at a fixed offset after the global entry point,

    as defined by bits in the symbol table entries' st_other field), instead

    expects r2 to be set up to the current TOC.

    Now, when setting a breakpoint on a function by name, you really want

    that breakpoint to trigger either way, no matter whether the function

    is called via its local or global entry point.  Since the global entry

    point will always fall through into the local entry point, the way to

    achieve that is to simply set the breakpoint at the local entry point.

    One way to do that would be to have prologue parsing skip the code

    sequence that makes up the global entry point.  Unfortunately, this

    does not work reliably, since -for optimized code- GDB these days

    will not actuall invoke the prologue parsing code but instead just

    set the breakpoint at the symbol address and rely on DWARF being

    correct at any point throughout the function ...

    Unfortunately, I don't really see any way to express the notion of

    local entry points with the current set of gdbarch callbacks.

    Thus this patch adds a new callback, skip_entrypoint, that is

    somewhat analogous to skip_prologue, but is called every time

    GDB needs to determine a function start address, even in those

    cases where GDB decides to not call skip_prologue.

    As a side effect, the skip_entrypoint implementation on ppc64

    does not need to perform any instruction parsing; it can simply

    rely on the local entry point flags in the symbol table entry.

    With this implemented, two test cases would still fail to set

    the breakpoint correctly, but that's because they use the construct:

     gdb_test "break *hello"

    Now, using "*hello" explicitly instructs GDB to set the breakpoint

    at the numerical value of "hello" treated as function pointer, so

    it will by definition only hit the global entry point.

    I think this behaviour is unavoidable, but acceptable -- most people

    do not use this construct, and if they do, they get what they

    asked for ...

    In one of those two test cases, use of this construct is really

    not appropriate.  I think this was added way back when as a means

    to work around prologue skipping problems on some platforms.  These

    days that shouldn't really be necessary any more ...

    For the other (step-bt), we really want to make sure backtracing

    works on the very first instruction of the routine.  To enable that

    test also on powerpc64le-linux, we can modify the code to call the

    test function via function pointer (which makes it use the global

    entry point in the ELFv2 ABI).

    gdb/ChangeLog:

    	* gdbarch.sh (skip_entrypoint): New callback.

    	* gdbarch.c, gdbarch.h: Regenerate.

    	* symtab.c (skip_prologue_sal): Call gdbarch_skip_entrypoint.

    	* infrun.c (fill_in_stop_func): Likewise.

    	* ppc-linux-tdep.c: Include "elf/ppc64.h".

    	(ppc_elfv2_elf_make_msymbol_special): New function.

    	(ppc_elfv2_skip_entrypoint): Likewise.

    	(ppc_linux_init_abi): Install them for ELFv2.

    gdb/testsuite/ChangeLog:

    	* gdb.base/sigbpt.exp: Do not use "*" when setting breakpoint

    	on a function.

    	* gdb.base/step-bt.c: Call hello via function pointer to make

    	sure its first instruction is executed on powerpc64le-linux.

Index: gdb-7.6.1/gdb/gdbarch.c

===================================================================

--- gdb-7.6.1.orig/gdb/gdbarch.c

+++ gdb-7.6.1/gdb/gdbarch.c

@@ -200,6 +200,7 @@ struct gdbarch

   gdbarch_return_in_first_hidden_param_p_ftype *return_in_first_hidden_param_p;

   gdbarch_skip_prologue_ftype *skip_prologue;

   gdbarch_skip_main_prologue_ftype *skip_main_prologue;

+  gdbarch_skip_entrypoint_ftype *skip_entrypoint;

   gdbarch_inner_than_ftype *inner_than;

   gdbarch_breakpoint_from_pc_ftype *breakpoint_from_pc;

   gdbarch_remote_breakpoint_from_pc_ftype *remote_breakpoint_from_pc;

@@ -371,6 +372,7 @@ struct gdbarch startup_gdbarch =

   default_return_in_first_hidden_param_p,  /* return_in_first_hidden_param_p */

   0,  /* skip_prologue */

   0,  /* skip_main_prologue */

+  0,  /* skip_entrypoint */

   0,  /* inner_than */

   0,  /* breakpoint_from_pc */

   default_remote_breakpoint_from_pc,  /* remote_breakpoint_from_pc */

@@ -672,6 +674,7 @@ verify_gdbarch (struct gdbarch *gdbarch)

   if (gdbarch->skip_prologue == 0)

     fprintf_unfiltered (log, "\n\tskip_prologue");

   /* Skip verify of skip_main_prologue, has predicate.  */

+  /* Skip verify of skip_entrypoint, has predicate.  */

   if (gdbarch->inner_than == 0)

     fprintf_unfiltered (log, "\n\tinner_than");

   if (gdbarch->breakpoint_from_pc == 0)

@@ -1285,6 +1288,12 @@ gdbarch_dump (struct gdbarch *gdbarch, s

                       "gdbarch_dump: single_step_through_delay = <%s>\n",

                       host_address_to_string (gdbarch->single_step_through_delay));

   fprintf_unfiltered (file,

+                      "gdbarch_dump: gdbarch_skip_entrypoint_p() = %d\n",

+                      gdbarch_skip_entrypoint_p (gdbarch));

+  fprintf_unfiltered (file,

+                      "gdbarch_dump: skip_entrypoint = <%s>\n",

+                      host_address_to_string (gdbarch->skip_entrypoint));

+  fprintf_unfiltered (file,

                       "gdbarch_dump: gdbarch_skip_main_prologue_p() = %d\n",

                       gdbarch_skip_main_prologue_p (gdbarch));

   fprintf_unfiltered (file,

@@ -2635,6 +2644,30 @@ set_gdbarch_skip_main_prologue (struct g

 }

 int

+gdbarch_skip_entrypoint_p (struct gdbarch *gdbarch)

+{

+  gdb_assert (gdbarch != NULL);

+  return gdbarch->skip_entrypoint != NULL;

+}

+

+CORE_ADDR

+gdbarch_skip_entrypoint (struct gdbarch *gdbarch, CORE_ADDR ip)

+{

+  gdb_assert (gdbarch != NULL);

+  gdb_assert (gdbarch->skip_entrypoint != NULL);

+  if (gdbarch_debug >= 2)

+    fprintf_unfiltered (gdb_stdlog, "gdbarch_skip_entrypoint called\n");

+  return gdbarch->skip_entrypoint (gdbarch, ip);

+}

+

+void

+set_gdbarch_skip_entrypoint (struct gdbarch *gdbarch,

+                             gdbarch_skip_entrypoint_ftype skip_entrypoint)

+{

+  gdbarch->skip_entrypoint = skip_entrypoint;

+}

+

+int

 gdbarch_inner_than (struct gdbarch *gdbarch, CORE_ADDR lhs, CORE_ADDR rhs)

 {

   gdb_assert (gdbarch != NULL);

Index: gdb-7.6.1/gdb/gdbarch.h

===================================================================

--- gdb-7.6.1.orig/gdb/gdbarch.h

+++ gdb-7.6.1/gdb/gdbarch.h

@@ -487,6 +487,24 @@ typedef CORE_ADDR (gdbarch_skip_main_pro

 extern CORE_ADDR gdbarch_skip_main_prologue (struct gdbarch *gdbarch, CORE_ADDR ip);

 extern void set_gdbarch_skip_main_prologue (struct gdbarch *gdbarch, gdbarch_skip_main_prologue_ftype *skip_main_prologue);

+/* On some platforms, a single function may provide multiple entry points,

+   e.g. one that is used for function-pointer calls and a different one

+   that is used for direct function calls.

+   In order to ensure that breakpoints set on the function will trigger

+   no matter via which entry point the function is entered, a platform

+   may provide the skip_entrypoint callback.  It is called with IP set

+   to the main entry point of a function (as determined by the symbol table),

+   and should return the address of the innermost entry point, where the

+   actual breakpoint needs to be set.  Note that skip_entrypoint is used

+   by GDB common code even when debugging optimized code, where skip_prologue

+   is not used. */

+

+extern int gdbarch_skip_entrypoint_p (struct gdbarch *gdbarch);

+

+typedef CORE_ADDR (gdbarch_skip_entrypoint_ftype) (struct gdbarch *gdbarch, CORE_ADDR ip);

+extern CORE_ADDR gdbarch_skip_entrypoint (struct gdbarch *gdbarch, CORE_ADDR ip);

+extern void set_gdbarch_skip_entrypoint (struct gdbarch *gdbarch, gdbarch_skip_entrypoint_ftype *skip_entrypoint);

+

 typedef int (gdbarch_inner_than_ftype) (CORE_ADDR lhs, CORE_ADDR rhs);

 extern int gdbarch_inner_than (struct gdbarch *gdbarch, CORE_ADDR lhs, CORE_ADDR rhs);

 extern void set_gdbarch_inner_than (struct gdbarch *gdbarch, gdbarch_inner_than_ftype *inner_than);

Index: gdb-7.6.1/gdb/gdbarch.sh

===================================================================

--- gdb-7.6.1.orig/gdb/gdbarch.sh

+++ gdb-7.6.1/gdb/gdbarch.sh

@@ -527,6 +527,19 @@ m:int:return_in_first_hidden_param_p:str

 m:CORE_ADDR:skip_prologue:CORE_ADDR ip:ip:0:0

 M:CORE_ADDR:skip_main_prologue:CORE_ADDR ip:ip

+# On some platforms, a single function may provide multiple entry points,

+# e.g. one that is used for function-pointer calls and a different one

+# that is used for direct function calls.

+# In order to ensure that breakpoints set on the function will trigger

+# no matter via which entry point the function is entered, a platform

+# may provide the skip_entrypoint callback.  It is called with IP set

+# to the main entry point of a function (as determined by the symbol table),

+# and should return the address of the innermost entry point, where the

+# actual breakpoint needs to be set.  Note that skip_entrypoint is used

+# by GDB common code even when debugging optimized code, where skip_prologue

+# is not used.

+M:CORE_ADDR:skip_entrypoint:CORE_ADDR ip:ip

+

 f:int:inner_than:CORE_ADDR lhs, CORE_ADDR rhs:lhs, rhs:0:0

 m:const gdb_byte *:breakpoint_from_pc:CORE_ADDR *pcptr, int *lenptr:pcptr, lenptr::0:

 # Return the adjusted address and kind to use for Z0/Z1 packets.

Index: gdb-7.6.1/gdb/infrun.c

===================================================================

--- gdb-7.6.1.orig/gdb/infrun.c

+++ gdb-7.6.1/gdb/infrun.c

@@ -3162,6 +3162,10 @@ fill_in_stop_func (struct gdbarch *gdbar

       ecs->stop_func_start

 	+= gdbarch_deprecated_function_start_offset (gdbarch);

+      if (gdbarch_skip_entrypoint_p (gdbarch))

+	ecs->stop_func_start = gdbarch_skip_entrypoint (gdbarch,

+							ecs->stop_func_start);

+

       ecs->stop_func_filled_in = 1;

     }

 }

Index: gdb-7.6.1/gdb/ppc-linux-tdep.c

===================================================================

--- gdb-7.6.1.orig/gdb/ppc-linux-tdep.c

+++ gdb-7.6.1/gdb/ppc-linux-tdep.c

@@ -44,6 +44,7 @@

 #include "observer.h"

 #include "auxv.h"

 #include "elf/common.h"

+#include "elf/ppc64.h"

 #include "exceptions.h"

 #include "arch-utils.h"

 #include "spu-tdep.h"

@@ -875,6 +876,55 @@ ppc_linux_core_read_description (struct

     }

 }

+

+/* Implementation of `gdbarch_elf_make_msymbol_special', as defined in

+   gdbarch.h.  This implementation is used for the ELFv2 ABI only.  */

+

+static void

+ppc_elfv2_elf_make_msymbol_special (asymbol *sym, struct minimal_symbol *msym)

+{

+  elf_symbol_type *elf_sym = (elf_symbol_type *)sym;

+

+  /* If the symbol is marked as having a local entry point, set a target

+     flag in the msymbol.  We currently only support local entry point

+     offsets of 8 bytes, which is the only entry point offset ever used

+     by current compilers.  If/when other offsets are ever used, we will

+     have to use additional target flag bits to store them.  */

+  switch (PPC64_LOCAL_ENTRY_OFFSET (elf_sym->internal_elf_sym.st_other))

+    {

+    default:

+      break;

+    case 8:

+      MSYMBOL_TARGET_FLAG_1 (msym) = 1;

+      break;

+    }

+}

+

+/* Implementation of `gdbarch_skip_entrypoint', as defined in

+   gdbarch.h.  This implementation is used for the ELFv2 ABI only.  */

+

+static CORE_ADDR

+ppc_elfv2_skip_entrypoint (struct gdbarch *gdbarch, CORE_ADDR pc)

+{

+  struct minimal_symbol *fun;

+  int local_entry_offset = 0;

+

+  fun = lookup_minimal_symbol_by_pc (pc);

+  if (fun == NULL)

+    return pc;

+

+  /* See ppc_elfv2_elf_make_msymbol_special for how local entry point

+     offset values are encoded.  */

+  if (MSYMBOL_TARGET_FLAG_1 (fun))

+    local_entry_offset = 8;

+

+  if (SYMBOL_VALUE_ADDRESS (fun) <= pc

+      && pc < SYMBOL_VALUE_ADDRESS (fun) + local_entry_offset)

+    return SYMBOL_VALUE_ADDRESS (fun) + local_entry_offset;

+

+  return pc;

+}

+

 /* Implementation of `gdbarch_stap_is_single_operand', as defined in

    gdbarch.h.  */

@@ -1341,6 +1391,13 @@ ppc_linux_init_abi (struct gdbarch_info

 	  set_gdbarch_elf_make_msymbol_special

 	    (gdbarch, ppc64_elf_make_msymbol_special);

 	}

+      else

+	{

+	  set_gdbarch_elf_make_msymbol_special

+	    (gdbarch, ppc_elfv2_elf_make_msymbol_special);

+

+	  set_gdbarch_skip_entrypoint (gdbarch, ppc_elfv2_skip_entrypoint);

+	}

       /* Shared library handling.  */

       set_gdbarch_skip_trampoline_code (gdbarch, ppc64_skip_trampoline_code);

Index: gdb-7.6.1/gdb/symtab.c

===================================================================

--- gdb-7.6.1.orig/gdb/symtab.c

+++ gdb-7.6.1/gdb/symtab.c

@@ -2872,6 +2872,8 @@ skip_prologue_sal (struct symtab_and_lin

       /* Skip "first line" of function (which is actually its prologue).  */

       pc += gdbarch_deprecated_function_start_offset (gdbarch);

+      if (gdbarch_skip_entrypoint_p (gdbarch))

+        pc = gdbarch_skip_entrypoint (gdbarch, pc);

       if (skip)

 	pc = gdbarch_skip_prologue (gdbarch, pc);

Index: gdb-7.6.1/gdb/testsuite/gdb.base/sigbpt.exp

===================================================================

--- gdb-7.6.1.orig/gdb/testsuite/gdb.base/sigbpt.exp

+++ gdb-7.6.1/gdb/testsuite/gdb.base/sigbpt.exp

@@ -82,7 +82,7 @@ gdb_test "break keeper"

 set bowler_addrs bowler

 set segv_addr none

 gdb_test {display/i $pc}

-gdb_test "advance *bowler" "bowler.*" "advance to the bowler"

+gdb_test "advance bowler" "bowler.*" "advance to the bowler"

 set test "stepping to fault"

 set signame "SIGSEGV"

 gdb_test_multiple "stepi" "$test" {

Index: gdb-7.6.1/gdb/testsuite/gdb.base/step-bt.c

===================================================================

--- gdb-7.6.1.orig/gdb/testsuite/gdb.base/step-bt.c

+++ gdb-7.6.1/gdb/testsuite/gdb.base/step-bt.c

@@ -23,10 +23,19 @@ hello (void)

   printf ("Hello world.\n");

 }

+/* The test case uses "break *hello" to make sure to step at the very

+   first instruction of the function.  This causes a problem running

+   the test on powerpc64le-linux, since the first instruction belongs

+   to the global entry point prologue, which is skipped when doing a

+   local direct function call.  To make sure that first instruction is

+   indeed being executed and the breakpoint hits, we make sure to call

+   the routine via an indirect call.  */

+void (*ptr) (void) = hello;

+

 int

 main (void)

 {

-  hello ();

+  ptr ();

   return 0;

 }