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binutils-gdb/gdb/psymtab.h
Andrew Burgess 3dd9bb4620 gdb: Don't fault for 'maint print psymbols' when using an index 
I found that these tests:

  make check-gdb RUNTESTFLAGS="--target_board=cc-with-gdb-index gdb.base/maint.exp"
  make check-gdb RUNTESTFLAGS="--target_board=cc-with-debug-names gdb.base/maint.exp"

were causing GDB to segfault.  It turns out that this test runs this
command:

  maint print psymbols -pc main /path/to/some/file

which tries to lookup the partial_symtab for 'main'.  The problem is
that there is no partial_symtab for 'main' as we are using the
.gdb_index or .debug_names instead of partial_symtabs.

What happens is that maintenance_print_symbols calls
find_pc_sect_psymtab, which looks for the partial_symtab in the
objfile's objfile->partial_symtabs->psymtabs_addrmap.

This is a problem because when we are using the indexes
psymtabs_addrmap is reused to hold things other than partial_symtabs,
this can be seen in dwarf2read.c in create_addrmap_from_index and
create_addrmap_from_aranges.  If we then lookup in psymtabs_addrmap we
end up returning a pointer to something that isn't really a
partial_symtab, after which everything goes wrong.

Initially I simply added a check at the start of find_pc_sect_psymtab
that the objfile had some partial_symtabs, like:

  if (objfile->partial_symtabs->psymtabs == NULL)
    return NULL;

Figuring that if there were no partial_symtabs at all then this
function should always return NULL, however, this caused a failure in
the test gdb.python/py-event.exp which I didn't dig into too deeply,
but seems to be that in this tests there are initially no psymtabs,
but the second part of find_pc_sect_psymtab does manage to read some
in from somewhere, with the check I added the test fails as we
returned NULL here and this caused GDB to load in the full symtabs
earlier than was expected.

Instead I chose to guard only the access to psymtabs_addrmap with a
check that the function has some psymtabs.  This allows my original
tests to pass, and the py-event.exp test to pass too.

Now, a good argument can be made that we simply should never call
find_pc_sect_psymtab on an objfile that is using indexes instead of
partial_symtabs.  I did consider this approach, we could easily add an
assert into find_pc_sect_psymtab that if we find a partial_symtab in
psymtabs_addrmap then the psymtabs pointer must be non-null.  The
responsibility would then be on the user of find_pc_sect_psymtab to
ensure that the objfile being checked is suitable.  In the end I
didn't take this approach as the check in find_pc_sect_psymtab is
cheap and this ensures that any future miss-uses of the function will
not cause problems.

I also extended the comment on psymtabs_addrmap to indicate that it
holds more than just partial_symtabs as this was not at all clear from
the original comment, and caused me some confusion when I was
initially debugging this problem.

gdb/ChangeLog:

	* psymtab.c (find_pc_sect_psymtab): Move baseaddr local into more
	inner scope, add check that the objfile has psymtabs before
	checking psymtabs_addrmap.
	* psymtab.h (psymtab_storage) <psymtabs_addrmap>: Extend comment.
2019-09-12 20:31:29 -04:00

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/* Public partial symbol table definitions.
Copyright (C) 2009-2019 Free Software Foundation, Inc.
This file is part of GDB.
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/>. */
#ifndef PSYMTAB_H
#define PSYMTAB_H
#include "gdb_obstack.h"
#include "symfile.h"
#include "gdbsupport/next-iterator.h"
#include "bcache.h"
struct partial_symbol;
/* An instance of this class manages the partial symbol tables and
partial symbols for a given objfile.
The core psymtab functions -- those in psymtab.c -- arrange for
nearly all psymtab- and psymbol-related allocations to happen
either in the psymtab_storage object (either on its obstack or in
other memory managed by this class), or on the per-BFD object. The
only link from the psymtab storage object back to the objfile (or
objfile_obstack) that is made by the core psymtab code is the
compunit_symtab member in the psymtab.
However, it is up to each symbol reader to maintain this invariant
in other ways, if it wants to reuse psymtabs across multiple
objfiles. The main issue here is ensuring that read_symtab_private
does not point into objfile_obstack. */
class psymtab_storage
{
public:
psymtab_storage ();
~psymtab_storage ();
DISABLE_COPY_AND_ASSIGN (psymtab_storage);
/* Discard all partial symbol tables starting with "psymtabs" and
proceeding until "to" has been discarded. */
void discard_psymtabs_to (struct partial_symtab *to)
{
while (psymtabs != to)
discard_psymtab (psymtabs);
}
/* Discard the partial symbol table. */
void discard_psymtab (struct partial_symtab *pst);
/* Return the obstack that is used for storage by this object. */
struct obstack *obstack ()
{
if (!m_obstack.has_value ())
m_obstack.emplace ();
return &*m_obstack;
}
/* Allocate storage for the "dependencies" field of a psymtab.
NUMBER says how many dependencies there are. */
struct partial_symtab **allocate_dependencies (int number)
{
return OBSTACK_CALLOC (obstack (), number, struct partial_symtab *);
}
/* Allocate a new psymtab on the psymtab obstack. The new psymtab
will be linked in to the "psymtabs" list, but otherwise all other
fields will be zero. */
struct partial_symtab *allocate_psymtab ();
typedef next_adapter<struct partial_symtab> partial_symtab_range;
/* A range adapter that makes it possible to iterate over all
psymtabs in one objfile. */
partial_symtab_range range ()
{
return partial_symtab_range (psymtabs);
}
/* Each objfile points to a linked list of partial symtabs derived from
this file, one partial symtab structure for each compilation unit
(source file). */
struct partial_symtab *psymtabs = nullptr;
/* Map addresses to the entries of PSYMTABS. It would be more efficient to
have a map per the whole process but ADDRMAP cannot selectively remove
its items during FREE_OBJFILE. This mapping is already present even for
PARTIAL_SYMTABs which still have no corresponding full SYMTABs read.
The DWARF parser reuses this addrmap to store things other than
psymtabs in the cases where debug information is being read from, for
example, the .debug-names section. */
struct addrmap *psymtabs_addrmap = nullptr;
/* A byte cache where we can stash arbitrary "chunks" of bytes that
will not change. */
struct bcache psymbol_cache;
/* Vectors of all partial symbols read in from file. The actual data
is stored in the objfile_obstack. */
std::vector<partial_symbol *> global_psymbols;
std::vector<partial_symbol *> static_psymbols;
private:
/* List of freed partial symtabs, available for re-use. */
struct partial_symtab *free_psymtabs = nullptr;
/* The obstack where allocations are made. This is lazily allocated
so that we don't waste memory when there are no psymtabs. */
gdb::optional<auto_obstack> m_obstack;
};
extern const struct quick_symbol_functions psym_functions;
extern const struct quick_symbol_functions dwarf2_gdb_index_functions;
extern const struct quick_symbol_functions dwarf2_debug_names_functions;
/* Ensure that the partial symbols for OBJFILE have been loaded. If
VERBOSE is non-zero, then this will print a message when symbols
are loaded. This function returns a range adapter suitable for
iterating over the psymtabs of OBJFILE. */
extern psymtab_storage::partial_symtab_range require_partial_symbols
(struct objfile *objfile, int verbose);
#endif /* PSYMTAB_H */
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