Re: [PATCH v2 1/2] gdb/c++: validate 'using' directives based on the current line

[Bruno Larsen <blarsen@redhat.com>]

On 16/11/2022 17:14, Lancelot SIX wrote:
> Hi Bruno,
>
> I have included comments inlined in the patch.
>
> On Wed, Nov 16, 2022 at 03:13:36PM +0100, Bruno Larsen via Gdb-patches wrote:
>> When asking GDB to print a variable from an imported namespace, we only
>> want to see variables imported in lines that the inferior has already
>> gone through, as is being tested last in gdb.cp/nsusing.exp. However
>> with the proposed change to gdb.cp/nsusing.exp, we get the following
>> failures:
>>
>> (gdb) PASS: gdb.cp/nsusing.exp: continue to breakpoint: marker10 stop
>> print x
>> $9 = 911
>> (gdb) FAIL: gdb.cp/nsusing.exp: print x, before using statement
>> next
>> 15        y += x;
>> (gdb) PASS: gdb.cp/nsusing.exp: using namespace M
>> print x
>> $10 = 911
>> (gdb) PASS: gdb.cp/nsusing.exp: print x, only using M
>>
>> Showing that the feature wasn't functioning properly, it just so
>> happened that gcc ordered the namespaces in a convenient way.
>> This happens because GDB doesn't take into account the line where the
>> "using namespace" directive is written. So long as it shows up in the
>> current scope, we assume it is valid.
>>
>> To fix this, add a new member to struct using_direct, that stores the
>> line where the directive was written, and a new function that informs if
>> the using directive is valid already.
>>
>> Unfortunately, due to a GCC bug, the failure still shows up. Compilers
>> that set the declaration line of the using directive correctly (such as
>> Clang) do not show such a bug, so the test includes an XFAIL for gcc
>> code.
>>
>> Finally, because the final test of gdb.cp/nsusing.exp has turned into
>> multiple that all would need XFAILs for older GCCs (<= 4.3), and that
>> GCC is very old, if it is detected, the test just exits early.
>> ---
>>   gdb/cp-namespace.c               | 15 ++++++++++++---
>>   gdb/dwarf2/read.c                | 25 ++++++++++++++++++++++++-
>>   gdb/namespace.c                  | 25 +++++++++++++++++++++++++
>>   gdb/namespace.h                  | 16 +++++++++++++++-
>>   gdb/testsuite/gdb.cp/nsusing.cc  |  3 ++-
>>   gdb/testsuite/gdb.cp/nsusing.exp | 16 +++++++++++++---
>>   6 files changed, 91 insertions(+), 9 deletions(-)
>>
>> diff --git a/gdb/cp-namespace.c b/gdb/cp-namespace.c
>> index 634dab6ada0..6ecb29fb1ac 100644
>> --- a/gdb/cp-namespace.c
>> +++ b/gdb/cp-namespace.c
>> @@ -93,10 +93,12 @@ cp_scan_for_anonymous_namespaces (struct buildsym_compunit *compunit,
>>   	      /* We've found a component of the name that's an
>>   		 anonymous namespace.  So add symbols in it to the
>>   		 namespace given by the previous component if there is
>> -		 one, or to the global namespace if there isn't.  */
>> +		 one, or to the global namespace if there isn't.
>> +		 The declared line of this using directive can be set
>> +		 to 0, this way it is always considered valid.  */
>>   	      std::vector<const char *> excludes;
>>   	      add_using_directive (compunit->get_local_using_directives (),
>> -				   dest, src, NULL, NULL, excludes,
>> +				   dest, src, NULL, NULL, excludes, 0,
>>   				   1, &objfile->objfile_obstack);
>>   	    }
>>   	  /* The "+ 2" is for the "::".  */
>> @@ -392,16 +394,23 @@ cp_lookup_symbol_via_imports (const char *scope,
>>     if (sym.symbol != NULL)
>>       return sym;
>>   
>> +  /* Due to a GCC bug, we need to know the boundaries of the current block
>> +     to know if a certain using directive is valid.  */
>> +  symtab_and_line boundary_sal = find_pc_line (block->end () - 1, 0);
>> +
>>     /* Go through the using directives.  If any of them add new names to
>>        the namespace we're searching in, see if we can find a match by
>>        applying them.  */
>> -
>>     for (current = block_using (block);
>>          current != NULL;
>>          current = current->next)
>>       {
>>         const char **excludep;
>>   
>> +      /* If the using directive was below the place we are stopped at,
>> +	 do not use this directive.  */
>> +      if (!current->valid_line (boundary_sal.line))
>> +	continue;
>>         len = strlen (current->import_dest);
>>         directive_match = (search_parents
>>   			 ? (startswith (scope, current->import_dest)
>> diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
>> index 60e120a9d76..68e3149a4bb 100644
>> --- a/gdb/dwarf2/read.c
>> +++ b/gdb/dwarf2/read.c
>> @@ -9299,6 +9299,26 @@ using_directives (struct dwarf2_cu *cu)
>>       return cu->get_builder ()->get_local_using_directives ();
>>   }
>>   
>> +/* Read the DW_ATTR_decl_line attribute for the given DIE in the
>> +   given CU.  If the format is not recognized or the attribute is
>> +   not present, set it to 0.  */
>> +
>> +static unsigned int
>> +read_decl_line (struct die_info *die, struct dwarf2_cu *cu)
>> +{
>> +
>> +  struct attribute *decl_line = dwarf2_attr (die, DW_AT_decl_line, cu);
>> +  if (decl_line == nullptr)
>> +    return 0;
>> +  if (decl_line->form_is_constant ())
>> +    return decl_line->constant_value (0);
> This is probably me being pedantic here, but constant_value return a
> LONGEST (i.e. long on x86_64) while read_decl_line returns an unsigned
> int.
>
> I really do not expect any realistic scenario where a line number goes
> above UINT_MAX, but I can easily imagine a buggy producer giving a
> negative value which would end up trash after the cast to unsigned int.
> Should we check that "0 <= decl_line->constant_value (0) <= UINT_MAX" ?
Sure, I could add a check and complaint here.
>
>> +  else if (decl_line->form_is_unsigned ())
> I do not see when this case should be possible.  The DW_AT_decl_line
> attribute is of class "constant" (so one of DW_FORM_data[1,2,4,8,16],
> DW_FORM_[s,u]data] or DW_FORM_implicit_const.  The only case not covered
> by form_is_constant is DW_FORM_data16 and it is not covered by
> form_is_unsigned either.  I do believe that this is more a problem in
> attribute::form_is_constant / attribute::constant_value.  Of course, the
> problem is that attribute::constant_value signature does not allow to
> return a 128bits value, but this is a question out of scope of this
> patch.
>
> Calling form_is_unsigned can return true if the form is DW_FORM_ref_addr
> or DW_FORM_sec_offset which would not make much sense in my opinion.
>
> I think I would remove this "else if" block completely as getting there
> would imply invalid DWARF.  In such situation, I think returning 0 would
> the right thing to do.

Well, now I'm very confused... When I was just starting out, I needed 
this to make the patch work with clang, but now it doesn't seem 
necessary anymore. Thanks for double checking it!

-- 
Cheers,
Bruno

>
> Best,
> Lancelot.
>
>> +    return decl_line->as_unsigned ();
>> +
>> +  complaint (_("Declared line for using directive is of incorrect format"));
>> +  return 0;
>> +}
>> +
>>   /* Read the import statement specified by the given die and record it.  */
>>   
>>   static void
>> @@ -9441,6 +9461,7 @@ read_import_statement (struct die_info *die, struct dwarf2_cu *cu)
>>   		       import_alias,
>>   		       imported_declaration,
>>   		       excludes,
>> +		       read_decl_line (die, cu),
>>   		       0,
>>   		       &objfile->objfile_obstack);
>>   }
>> @@ -16078,7 +16099,9 @@ read_namespace (struct die_info *die, struct dwarf2_cu *cu)
>>   	  std::vector<const char *> excludes;
>>   	  add_using_directive (using_directives (cu),
>>   			       previous_prefix, type->name (), NULL,
>> -			       NULL, excludes, 0, &objfile->objfile_obstack);
>> +			       NULL, excludes,
>> +			       read_decl_line (die, cu),
>> +			       0, &objfile->objfile_obstack);
>>   	}
>>       }
>>   
>> diff --git a/gdb/namespace.c b/gdb/namespace.c
>> index 0c39c921a3e..b2cca5a1da4 100644
>> --- a/gdb/namespace.c
>> +++ b/gdb/namespace.c
>> @@ -18,6 +18,7 @@
>>   
>>   #include "defs.h"
>>   #include "namespace.h"
>> +#include "frame.h"
>>   
>>   /* Add a using directive to USING_DIRECTIVES.  If the using directive
>>      in question has already been added, don't add it twice.
>> @@ -40,6 +41,7 @@ add_using_directive (struct using_direct **using_directives,
>>   		     const char *alias,
>>   		     const char *declaration,
>>   		     const std::vector<const char *> &excludes,
>> +		     unsigned int decl_line,
>>   		     int copy_names,
>>   		     struct obstack *obstack)
>>   {
>> @@ -76,6 +78,9 @@ add_using_directive (struct using_direct **using_directives,
>>         if (ix < excludes.size () || current->excludes[ix] != NULL)
>>   	continue;
>>   
>> +      if (decl_line != current->decl_line)
>> +	continue;
>> +
>>         /* Parameters exactly match CURRENT.  */
>>         return;
>>       }
>> @@ -111,6 +116,26 @@ add_using_directive (struct using_direct **using_directives,
>>   	    excludes.size () * sizeof (*newobj->excludes));
>>     newobj->excludes[excludes.size ()] = NULL;
>>   
>> +  newobj->decl_line = decl_line;
>> +
>>     newobj->next = *using_directives;
>>     *using_directives = newobj;
>>   }
>> +
>> +/* See namespace.h.  */
>> +
>> +bool
>> +using_direct::valid_line (unsigned int boundary) const
>> +{
>> +  try
>> +    {
>> +      CORE_ADDR curr_pc = get_frame_pc (get_selected_frame (nullptr));
>> +      symtab_and_line curr_sal = find_pc_line (curr_pc, 0);
>> +      return (decl_line <= curr_sal.line)
>> +	     || (decl_line >= boundary);
>> +    }
>> +  catch (const gdb_exception &ex)
>> +    {
>> +      return true;
>> +    }
>> +}
>> diff --git a/gdb/namespace.h b/gdb/namespace.h
>> index dc052a44e42..b46806684c8 100644
>> --- a/gdb/namespace.h
>> +++ b/gdb/namespace.h
>> @@ -30,7 +30,8 @@
>>      string representing the alias.  Otherwise, ALIAS is NULL.
>>      DECLARATION is the name of the imported declaration, if this import
>>      statement represents one.  Otherwise DECLARATION is NULL and this
>> -   import statement represents a namespace.
>> +   import statement represents a namespace.  DECL_LINE is the line
>> +   where the using directive is written in the source code.
>>   
>>      C++:      using namespace A;
>>      Fortran:  use A
>> @@ -96,6 +97,11 @@ struct using_direct
>>   
>>     struct using_direct *next;
>>   
>> +  /* The line where the using directive was declared on the source file.
>> +     This is used to check if the using directive is already active at the
>> +     point where the inferior is stopped.  */
>> +  unsigned int decl_line;
>> +
>>     /* Used during import search to temporarily mark this node as
>>        searched.  */
>>     int searched;
>> @@ -103,6 +109,13 @@ struct using_direct
>>     /* USING_DIRECT has variable allocation size according to the number of
>>        EXCLUDES entries, the last entry is NULL.  */
>>     const char *excludes[1];
>> +
>> +  /* Returns true if the using_direcive USING_DIR is valid in CURR_LINE.
>> +     Because current GCC (at least version 12.2) sets the decl_line as
>> +     the last line in the current block, we need to take this into
>> +     consideration when checking the validity, by comparing it to
>> +     BOUNDARY, the last line of the current block.  */
>> +  bool valid_line (unsigned int boundary) const;
>>   };
>>   
>>   extern void add_using_directive (struct using_direct **using_directives,
>> @@ -111,6 +124,7 @@ extern void add_using_directive (struct using_direct **using_directives,
>>   				 const char *alias,
>>   				 const char *declaration,
>>   				 const std::vector<const char *> &excludes,
>> +				 const unsigned int decl_line,
>>   				 int copy_names,
>>   				 struct obstack *obstack);
>>   
>> diff --git a/gdb/testsuite/gdb.cp/nsusing.cc b/gdb/testsuite/gdb.cp/nsusing.cc
>> index fa5c9d01f59..dcf0ba99e22 100644
>> --- a/gdb/testsuite/gdb.cp/nsusing.cc
>> +++ b/gdb/testsuite/gdb.cp/nsusing.cc
>> @@ -10,8 +10,9 @@ namespace N
>>   
>>   int marker10 ()
>>   {
>> +  int y = 1; // marker10 stop
>>     using namespace M;
>> -  int y = x + 1; // marker10 stop
>> +  y += x;
>>     using namespace N;
>>     return y;
>>   }
>> diff --git a/gdb/testsuite/gdb.cp/nsusing.exp b/gdb/testsuite/gdb.cp/nsusing.exp
>> index 2835207a21e..b79f3d26084 100644
>> --- a/gdb/testsuite/gdb.cp/nsusing.exp
>> +++ b/gdb/testsuite/gdb.cp/nsusing.exp
>> @@ -120,8 +120,18 @@ gdb_continue_to_breakpoint "marker10 stop"
>>   
>>   if { [test_compiler_info {gcc-[0-3]-*}] ||
>>        [test_compiler_info {gcc-4-[0-3]-*}]} {
>> -    setup_xfail *-*-*
>> +    return
>>   }
>>   
>> -# Assert that M::x is printed and not N::x
>> -gdb_test "print x" "= 911" "print x (from M::x)"
>> +gdb_test_multiple "print x" "print x, before using statement" {
>> +    -re -wrap "No symbol .x. in current context.*" {
>> +	pass $gdb_test_name
>> +    }
>> +    -re -wrap "= 911.*" {
>> +	# GCC doesn't properly set the decl_line for namespaces, so GDB believes
>> +	# that the "using namespace M" line has already passed at this point.
>> +	xfail $gdb_test_name
>> +    }
>> +}
>> +gdb_test "next" ".*" "using namespace M"
>> +gdb_test "print x" "= 911" "print x, only using M"
>> -- 
>> 2.38.1
>>