[PATCH 2/4] Handle copy relocations

[Tom Tromey <tromey at adacore dot com>]

In ELF, if a data symbol is defined in a shared library and used by
the main program, it will be subject to a "copy relocation".  In this
scenario, the main program has a copy of the symbol in question, and a
relocation that tells ld.so to copy the data from the shared library.
Then the symbol in the main program is used to satisfy all references.

This patch changes gdb to handle this scenario.  Data symbols coming
from ELF shared libraries get a special flag that indicates that the
symbol's address may be subject to copy relocation.

I looked briefly into handling copy relocations by looking at the
actual relocations in the main program, but this seemed difficult to
do with BFD.

Note that no caching is done here.  Perhaps this could be changed if
need be; I wanted to avoid possible problems with either objfile
lifetimes and changes, or conflicts with the long-term (vapor-ware)
objfile splitting project.

gdb/ChangeLog
2019-06-27  Tom Tromey  <tromey@adacore.com>

	* symmisc.c (dump_msymbols): Don't use MSYMBOL_VALUE_ADDRESS.
	* ada-lang.c (lesseq_defined_than): Handle
	LOC_STATIC.
	* dwarf2read.c (dwarf2_per_objfile): Add can_copy
	parameter.
	(dwarf2_has_info): Likewise.
	(new_symbol): Set maybe_copied on symbol when
	appropriate.
	* dwarf2read.h (dwarf2_per_objfile): Add can_copy
	parameter.
	<can_copy>: New member.
	* elfread.c (record_minimal_symbol): Set maybe_copied
	on symbol when appropriate.
	(elf_symfile_read): Update call to dwarf2_has_info.
	* minsyms.c (lookup_minimal_symbol_linkage): New
	function.
	* minsyms.h (lookup_minimal_symbol_linkage): Declare.
	* symtab.c (get_symbol_address, get_msymbol_address):
	New functions.
	* symtab.h (get_symbol_address, get_msymbol_address):
	Declare.
	(SYMBOL_VALUE_ADDRESS, MSYMBOL_VALUE_ADDRESS): Handle
	maybe_copied.
	(struct symbol, struct minimal_symbol) <maybe_copied>:
	New member.
---
 gdb/ChangeLog    | 28 ++++++++++++++++++++++++++++
 gdb/ada-lang.c   |  9 +++++++++
 gdb/dwarf2read.c | 36 ++++++++++++++++++------------------
 gdb/dwarf2read.h | 10 ++++++++--
 gdb/elfread.c    | 16 +++++++++++-----
 gdb/minsyms.c    | 20 ++++++++++++++++++++
 gdb/minsyms.h    |  7 +++++++
 gdb/symfile.h    |  3 ++-
 gdb/symmisc.c    | 10 +++++++---
 gdb/symtab.c     | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 gdb/symtab.h     | 42 +++++++++++++++++++++++++++++++++++++++---
 11 files changed, 196 insertions(+), 32 deletions(-)

diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 1e996557b6e..5a055fced7b 100644
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -4734,6 +4734,15 @@ lesseq_defined_than (struct symbol *sym0, struct symbol *sym1)
     case LOC_CONST:
       return SYMBOL_VALUE (sym0) == SYMBOL_VALUE (sym1)
         && equiv_types (SYMBOL_TYPE (sym0), SYMBOL_TYPE (sym1));
+
+    case LOC_STATIC:
+      {
+        const char *name0 = SYMBOL_LINKAGE_NAME (sym0);
+        const char *name1 = SYMBOL_LINKAGE_NAME (sym1);
+        return (strcmp (name0, name1) == 0
+                && SYMBOL_VALUE_ADDRESS (sym0) == SYMBOL_VALUE_ADDRESS (sym1));
+      }
+
     default:
       return 0;
     }
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 2945a2d090d..eec6eea9085 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -2126,8 +2126,10 @@ attr_value_as_address (struct attribute *attr)
 /* See declaration.  */
 
 dwarf2_per_objfile::dwarf2_per_objfile (struct objfile *objfile_,
-					const dwarf2_debug_sections *names)
-  : objfile (objfile_)
+					const dwarf2_debug_sections *names,
+					bool can_copy_)
+  : objfile (objfile_),
+    can_copy (can_copy_)
 {
   if (names == NULL)
     names = &dwarf2_elf_names;
@@ -2202,11 +2204,14 @@ private:
 /* Try to locate the sections we need for DWARF 2 debugging
    information and return true if we have enough to do something.
    NAMES points to the dwarf2 section names, or is NULL if the standard
-   ELF names are used.  */
+   ELF names are used.  CAN_COPY is true for formats where symbol
+   interposition is possible and so symbol values must follow copy
+   relocation rules.  */
 
 int
 dwarf2_has_info (struct objfile *objfile,
-                 const struct dwarf2_debug_sections *names)
+                 const struct dwarf2_debug_sections *names,
+		 bool can_copy)
 {
   if (objfile->flags & OBJF_READNEVER)
     return 0;
@@ -2216,7 +2221,8 @@ dwarf2_has_info (struct objfile *objfile,
 
   if (dwarf2_per_objfile == NULL)
     dwarf2_per_objfile = dwarf2_objfile_data_key.emplace (objfile, objfile,
-							  names);
+							  names,
+							  can_copy);
 
   return (!dwarf2_per_objfile->info.is_virtual
 	  && dwarf2_per_objfile->info.s.section != NULL
@@ -21610,19 +21616,13 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 		}
 	      else if (attr2 && (DW_UNSND (attr2) != 0))
 		{
-		  /* Workaround gfortran PR debug/40040 - it uses
-		     DW_AT_location for variables in -fPIC libraries which may
-		     get overriden by other libraries/executable and get
-		     a different address.  Resolve it by the minimal symbol
-		     which may come from inferior's executable using copy
-		     relocation.  Make this workaround only for gfortran as for
-		     other compilers GDB cannot guess the minimal symbol
-		     Fortran mangling kind.  */
-		  if (cu->language == language_fortran && die->parent
-		      && die->parent->tag == DW_TAG_module
-		      && cu->producer
-		      && startswith (cu->producer, "GNU Fortran"))
-		    SYMBOL_ACLASS_INDEX (sym) = LOC_UNRESOLVED;
+		  if (SYMBOL_CLASS (sym) == LOC_STATIC
+		      && (objfile->flags & OBJF_MAINLINE) == 0)
+		    {
+		      /* A global static variable might be subject to
+			 copy relocation.  */
+		      sym->maybe_copied = dwarf2_per_objfile->can_copy;
+		    }
 
 		  /* A variable with DW_AT_external is never static,
 		     but it may be block-scoped.  */
diff --git a/gdb/dwarf2read.h b/gdb/dwarf2read.h
index 9251bde1259..9c56a57b6bb 100644
--- a/gdb/dwarf2read.h
+++ b/gdb/dwarf2read.h
@@ -104,9 +104,12 @@ struct dwarf2_per_objfile
 {
   /* Construct a dwarf2_per_objfile for OBJFILE.  NAMES points to the
      dwarf2 section names, or is NULL if the standard ELF names are
-     used.  */
+     used.  CAN_COPY is true for formats where symbol
+     interposition is possible and so symbol values must follow copy
+     relocation rules.  */
   dwarf2_per_objfile (struct objfile *objfile,
-		      const dwarf2_debug_sections *names);
+		      const dwarf2_debug_sections *names,
+		      bool can_copy);
 
   ~dwarf2_per_objfile ();
 
@@ -206,6 +209,9 @@ public:
      original data was compressed using 'dwz -m'.  */
   std::unique_ptr<struct dwz_file> dwz_file;
 
+  /* Whether copy relocations are supported by this object format.  */
+  bool can_copy;
+
   /* A flag indicating whether this objfile has a section loaded at a
      VMA of 0.  */
   bool has_section_at_zero = false;
diff --git a/gdb/elfread.c b/gdb/elfread.c
index 630550b80dc..b2fade4124b 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -203,10 +203,16 @@ record_minimal_symbol (minimal_symbol_reader &reader,
       || ms_type == mst_text_gnu_ifunc)
     address = gdbarch_addr_bits_remove (gdbarch, address);
 
-  return reader.record_full (name, name_len, copy_name, address,
-			     ms_type,
-			     gdb_bfd_section_index (objfile->obfd,
-						    bfd_section));
+  struct minimal_symbol *result
+    = reader.record_full (name, name_len, copy_name, address,
+			  ms_type,
+			  gdb_bfd_section_index (objfile->obfd,
+						 bfd_section));
+  if ((objfile->flags & OBJF_MAINLINE) == 0
+      && (ms_type == mst_data || ms_type == mst_bss))
+    result->maybe_copied = 1;
+
+  return result;
 }
 
 /* Read the symbol table of an ELF file.
@@ -1239,7 +1245,7 @@ elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
 				bfd_section_size (abfd, str_sect));
     }
 
-  if (dwarf2_has_info (objfile, NULL))
+  if (dwarf2_has_info (objfile, NULL, true))
     {
       dw_index_kind index_kind;
 
diff --git a/gdb/minsyms.c b/gdb/minsyms.c
index e64e5df1a18..d9abce851e4 100644
--- a/gdb/minsyms.c
+++ b/gdb/minsyms.c
@@ -519,6 +519,26 @@ iterate_over_minimal_symbols
 
 /* See minsyms.h.  */
 
+struct minimal_symbol *
+lookup_minimal_symbol_linkage (const char *name, struct objfile *objf)
+{
+  unsigned int hash = msymbol_hash (name) % MINIMAL_SYMBOL_HASH_SIZE;
+
+  for (minimal_symbol *msymbol = objf->per_bfd->msymbol_hash[hash];
+       msymbol != NULL;
+       msymbol = msymbol->hash_next)
+    {
+      if (strcmp (MSYMBOL_LINKAGE_NAME (msymbol), name) == 0
+	  && (MSYMBOL_TYPE (msymbol) == mst_data
+	      || MSYMBOL_TYPE (msymbol) == mst_bss))
+	return msymbol;
+    }
+
+  return nullptr;
+}
+
+/* See minsyms.h.  */
+
 struct bound_minimal_symbol
 lookup_minimal_symbol_text (const char *name, struct objfile *objf)
 {
diff --git a/gdb/minsyms.h b/gdb/minsyms.h
index bb43165620d..fae6cd25496 100644
--- a/gdb/minsyms.h
+++ b/gdb/minsyms.h
@@ -193,6 +193,13 @@ struct bound_minimal_symbol lookup_minimal_symbol (const char *,
 
 struct bound_minimal_symbol lookup_bound_minimal_symbol (const char *);
 
+/* Look through the minimal symbols in OBJF for a global (not
+   file-local) minsym whose linkage name is NAME.  Returns a minimal
+   symbol that matches, or nullptr otherwise.  */
+
+struct minimal_symbol *lookup_minimal_symbol_linkage
+  (const char *name, struct objfile *objf);
+
 /* Look through all the current minimal symbol tables and find the
    first minimal symbol that matches NAME and has text type.  If OBJF
    is non-NULL, limit the search to that objfile.  Returns a bound
diff --git a/gdb/symfile.h b/gdb/symfile.h
index daddd2e21ab..35c7f9e4d87 100644
--- a/gdb/symfile.h
+++ b/gdb/symfile.h
@@ -579,7 +579,8 @@ struct dwarf2_debug_sections {
 };
 
 extern int dwarf2_has_info (struct objfile *,
-                            const struct dwarf2_debug_sections *);
+                            const struct dwarf2_debug_sections *,
+			    bool = false);
 
 /* Dwarf2 sections that can be accessed by dwarf2_get_section_info.  */
 enum dwarf2_section_enum {
diff --git a/gdb/symmisc.c b/gdb/symmisc.c
index 7666de390cd..db93d716704 100644
--- a/gdb/symmisc.c
+++ b/gdb/symmisc.c
@@ -236,9 +236,13 @@ dump_msymbols (struct objfile *objfile, struct ui_file *outfile)
 	  break;
 	}
       fprintf_filtered (outfile, "[%2d] %c ", index, ms_type);
-      fputs_filtered (paddress (gdbarch, MSYMBOL_VALUE_ADDRESS (objfile,
-								msymbol)),
-		      outfile);
+
+      /* Use the relocated address as shown in the symbol here -- do
+	 not try to respect copy relocations.  */
+      CORE_ADDR addr = (msymbol->value.address
+			+ ANOFFSET (objfile->section_offsets,
+				    msymbol->section));
+      fputs_filtered (paddress (gdbarch, addr), outfile);
       fprintf_filtered (outfile, " %s", MSYMBOL_LINKAGE_NAME (msymbol));
       if (section)
 	{
diff --git a/gdb/symtab.c b/gdb/symtab.c
index 4920d94a247..4906e9cfb9b 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -6003,6 +6003,53 @@ symbol_set_symtab (struct symbol *symbol, struct symtab *symtab)
   symbol->owner.symtab = symtab;
 }
 
+/* See symtab.h.  */
+
+CORE_ADDR
+get_symbol_address (const struct symbol *sym)
+{
+  gdb_assert (sym->maybe_copied);
+  gdb_assert (SYMBOL_CLASS (sym) == LOC_STATIC);
+
+  const char *linkage_name = SYMBOL_LINKAGE_NAME (sym);
+
+  for (objfile *objfile : current_program_space->objfiles ())
+    {
+      if ((objfile->flags & OBJF_MAINLINE) != 0)
+	{
+	  minimal_symbol *minsym
+	    = lookup_minimal_symbol_linkage (linkage_name, objfile);
+	  if (minsym != nullptr)
+	    return MSYMBOL_VALUE_ADDRESS (objfile, minsym);
+	}
+    }
+  return sym->ginfo.value.address;
+}
+
+/* See symtab.h.  */
+
+CORE_ADDR
+get_msymbol_address (struct objfile *objf, const struct minimal_symbol *minsym)
+{
+  gdb_assert (minsym->maybe_copied);
+  gdb_assert ((objf->flags & OBJF_MAINLINE) == 0);
+
+  const char *linkage_name = MSYMBOL_LINKAGE_NAME (minsym);
+
+  for (objfile *objfile : current_program_space->objfiles ())
+    {
+      if ((objfile->flags & OBJF_MAINLINE) != 0)
+	{
+	  minimal_symbol *found
+	    = lookup_minimal_symbol_linkage (linkage_name, objfile);
+	  if (found != nullptr)
+	    return MSYMBOL_VALUE_ADDRESS (objfile, found);
+	}
+    }
+  return (minsym->value.address
+	  + ANOFFSET (objf->section_offsets, minsym->section));
+}
+
 
 
 void
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 7b3bb5d3775..4455bb09b83 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -454,6 +454,14 @@ extern const char *symbol_get_demangled_name
 
 extern CORE_ADDR symbol_overlayed_address (CORE_ADDR, struct obj_section *);
 
+/* Return the address of SYM.  The MAYBE_COPIED flag must be set on
+   SYM.  If SYM appears in the main program's minimal symbols, then
+   that minsym's address is returned; otherwise, SYM's address is
+   returned.  This should generally only be used via the
+   SYMBOL_VALUE_ADDRESS macro.  */
+
+extern CORE_ADDR get_symbol_address (const struct symbol *sym);
+
 /* Note that all the following SYMBOL_* macros are used with the
    SYMBOL argument being either a partial symbol or
    a full symbol.  Both types have a ginfo field.  In particular
@@ -463,7 +471,9 @@ extern CORE_ADDR symbol_overlayed_address (CORE_ADDR, struct obj_section *);
    field only, instead of the SYMBOL parameter.  */
 
 #define SYMBOL_VALUE(symbol)		(symbol)->ginfo.value.ivalue
-#define SYMBOL_VALUE_ADDRESS(symbol)	((symbol)->ginfo.value.address + 0)
+#define SYMBOL_VALUE_ADDRESS(symbol)			      \
+  (((symbol)->maybe_copied) ? get_symbol_address (symbol)     \
+   : ((symbol)->ginfo.value.address))
 #define SET_SYMBOL_VALUE_ADDRESS(symbol, new_value)	\
   ((symbol)->ginfo.value.address = (new_value))
 #define SYMBOL_VALUE_BYTES(symbol)	(symbol)->ginfo.value.bytes
@@ -671,6 +681,14 @@ struct minimal_symbol : public general_symbol_info
      the object file format may not carry that piece of information.  */
   unsigned int has_size : 1;
 
+  /* For data symbols only, if this is set, then the symbol might be
+     subject to copy relocation.  In this case, a minimal symbol
+     matching the symbol's linkage name is first looked for in the
+     main objfile.  If found, then that address is used; otherwise the
+     address in this symbol is used.  */
+
+  unsigned maybe_copied : 1;
+
   /* Minimal symbols with the same hash key are kept on a linked
      list.  This is the link.  */
 
@@ -690,6 +708,15 @@ struct minimal_symbol : public general_symbol_info
   bool text_p () const;
 };
 
+/* Return the address of MINSYM, which comes from OBJF.  The
+   MAYBE_COPIED flag must be set on MINSYM.  If MINSYM appears in the
+   main program's minimal symbols, then that minsym's address is
+   returned; otherwise, MINSYM's address is returned.  This should
+   generally only be used via the MSYMBOL_VALUE_ADDRESS macro.  */
+
+extern CORE_ADDR get_msymbol_address (struct objfile *objf,
+				      const struct minimal_symbol *minsym);
+
 #define MSYMBOL_TARGET_FLAG_1(msymbol)  (msymbol)->target_flag_1
 #define MSYMBOL_TARGET_FLAG_2(msymbol)  (msymbol)->target_flag_2
 #define MSYMBOL_SIZE(msymbol)		((msymbol)->size + 0)
@@ -708,8 +735,9 @@ struct minimal_symbol : public general_symbol_info
 /* The relocated address of the minimal symbol, using the section
    offsets from OBJFILE.  */
 #define MSYMBOL_VALUE_ADDRESS(objfile, symbol)				\
-  ((symbol)->value.address					\
-   + ANOFFSET ((objfile)->section_offsets, ((symbol)->section)))
+  (((symbol)->maybe_copied) ? get_msymbol_address (objfile, symbol)	\
+   : ((symbol)->value.address						\
+      + ANOFFSET ((objfile)->section_offsets, ((symbol)->section))))
 /* For a bound minsym, we can easily compute the address directly.  */
 #define BMSYMBOL_VALUE_ADDRESS(symbol) \
   MSYMBOL_VALUE_ADDRESS ((symbol).objfile, (symbol).minsym)
@@ -1112,6 +1140,14 @@ struct symbol
   /* Whether this is an inlined function (class LOC_BLOCK only).  */
   unsigned is_inlined : 1;
 
+  /* For LOC_STATIC only, if this is set, then the symbol might be
+     subject to copy relocation.  In this case, a minimal symbol
+     matching the symbol's linkage name is first looked for in the
+     main objfile.  If found, then that address is used; otherwise the
+     address in this symbol is used.  */
+
+  unsigned maybe_copied : 1;
+
   /* The concrete type of this symbol.  */
 
   ENUM_BITFIELD (symbol_subclass_kind) subclass : 2;
-- 
2.20.1