Re: [PATCH v2 28/42] Remove dwarf2_per_cu_data::objfile ()

[Simon Marchi <simark@simark.ca>]

On 2020-05-27 6:16 p.m., Simon Marchi via Gdb-patches wrote:
> Ok, I reproduced it with opensuse/tumbleweed, after installing glibc's debug info.  I'll
> look into it.

Quick update to say that I understand a bit more what happens.  It turns out that call sites
can refer indirectly to other objfiles.  Here, we deal with an indirect call (through a function
pointer probably).  In that case, the call site doesn't know at compile time what it calls.  It
provides a DWARF expression to compute the called address. The DIE corresponding to the call site
is:

0x0091aa10:     DW_TAG_GNU_call_site
                  DW_AT_low_pc [DW_FORM_addr]   (0x00000000001398e0)
                  DW_AT_GNU_call_site_target [DW_FORM_exprloc]  (DW_OP_fbreg -272, DW_OP_deref)
                  DW_AT_sibling [DW_FORM_ref4]  (0x0091aa2b)

This call site is in the _dl_catch_exception function of libc (which indeed takes a function pointer
as argument).  In our case, the function pointer is a function in ld-linux.so.  So this is how the
two objfiles can be different.

For comparison, when the function being called is known at compile time, the call site DIE will refer
directly to the function being called, like in this case:

0x0091a9f2:     DW_TAG_GNU_call_site
                  DW_AT_low_pc [DW_FORM_addr]   (0x00000000001398cd)
                  DW_AT_abstract_origin [DW_FORM_ref4]  (0x0091ad45 "__GI___sigsetjmp")
                  DW_AT_sibling [DW_FORM_ref4]  (0x0091aa10)

In that case, I presume that it's not really possible for the caller and calle to be in different
objfiles (this is the scenarion I had in my mind, I didn't know that indirect call sites could be
described as well).

See the patch below (still missing the commit message).  I think we only need to put and restore the
caller's per_objfile in this->per_objfile.  Alternatively, we could maybe just create a new
dwarf_evaluate_loc_desc local object, instead of setting and restoring all the fields.  I'll try that
tomorrow.

Simon


From 2381d562b3130b94ad8e266dd034431f80275c8b Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@efficios.com>
Date: Wed, 27 May 2020 21:54:45 -0400
Subject: [PATCH] gdb: use caller objfile in
 dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value

Change-Id: Ib227d767ce525c10607ab6621a373aaae982c67a
---
 gdb/dwarf2/loc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index 7953361adeed..1aab1a4f51bc 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -726,8 +726,6 @@ class dwarf_evaluate_loc_desc : public dwarf_expr_context
     data_src = deref_size == -1 ? parameter->value : parameter->data_value;
     size = deref_size == -1 ? parameter->value_size : parameter->data_value_size;

-    gdb_assert (this->per_objfile == caller_per_objfile);
-
     /* DEREF_SIZE size is not verified here.  */
     if (data_src == NULL)
       throw_error (NO_ENTRY_VALUE_ERROR,
@@ -739,11 +737,13 @@ class dwarf_evaluate_loc_desc : public dwarf_expr_context
 						      caller_per_cu);
     scoped_restore save_obj_addr = make_scoped_restore (&this->obj_address,
 							(CORE_ADDR) 0);
+    scoped_restore save_per_objfile = make_scoped_restore (&this->per_objfile,
+							   caller_per_objfile);

     scoped_restore save_arch = make_scoped_restore (&this->gdbarch);
     this->gdbarch = this->per_objfile->objfile->arch ();
     scoped_restore save_addr_size = make_scoped_restore (&this->addr_size);
-    this->addr_size = per_cu->addr_size ();
+    this->addr_size = this->per_cu->addr_size ();

     this->eval (data_src, size);
   }
-- 
2.26.2