[RFA] DLLs without debugging symbols (repost)

Raoul Gough
2003-02-06T22:55:53Z

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Apparently my copyright assignment forms have now been processed, so
my patches for extracting minimal symbols from DLLs could now be
entered into CVS. This message includes up-to-date diffs with the
latest versions of the relevant files, as well as some documentation
that I've added to gdb.texinfo (basically, there's a new node under
the Cygwin Native node entitled "Non-debug DLL symbols" which explains
some of the tricks involved).

Regards,
Raoul Gough.

[-- Attachment #2: coffread.diff.txt --]
[-- Type: text/plain, Size: 880 bytes --]

Index: coffread.c
===================================================================
RCS file: /cvs/src/src/gdb/coffread.c,v
retrieving revision 1.33
diff -c -p -r1.33 coffread.c
*** coffread.c 4 Feb 2003 18:07:00 -0000 1.33
--- coffread.c 6 Feb 2003 19:39:27 -0000
***************
*** 45,50 ****
--- 45,52 ----
#include "target.h"
#include "gdb_assert.h"

+ #include "coff-pe-read.h"
+
extern void _initialize_coffread (void);

struct coff_symfile_info
*************** coff_symtab_read (long symtab_offset, un
*** 1084,1089 ****
--- 1086,1098 ----
process_coff_symbol (cs, &main_aux, objfile);
break;
}
+ }
+
+ if ((nsyms == 0) && (pe_file))
+ {
+ /* We've got no debugging symbols, but it's is a portable
+ executable, so try to read the export table */
+ read_pe_exported_syms (objfile);
}

if (last_source_file)

[-- Attachment #3: Makefile.in.diff.txt --]
[-- Type: text/plain, Size: 3072 bytes --]

Index: Makefile.in
===================================================================
RCS file: /cvs/src/src/gdb/Makefile.in,v
retrieving revision 1.325
diff -c -p -r1.325 Makefile.in
*** Makefile.in 6 Feb 2003 05:30:16 -0000 1.325
--- Makefile.in 6 Feb 2003 19:39:49 -0000
*************** SFILES = ada-exp.y ada-lang.c ada-typepr
*** 511,517 ****
ax-general.c ax-gdb.c \
bcache.c blockframe.c breakpoint.c buildsym.c builtin-regs.c \
c-exp.y c-lang.c c-typeprint.c c-valprint.c \
! charset.c cli-out.c coffread.c complaints.c completer.c corefile.c \
cp-abi.c cp-support.c cp-valprint.c \
dbxread.c demangle.c disasm.c doublest.c \
dummy-frame.c dwarfread.c dwarf2read.c \
--- 511,518 ----
ax-general.c ax-gdb.c \
bcache.c blockframe.c breakpoint.c buildsym.c builtin-regs.c \
c-exp.y c-lang.c c-typeprint.c c-valprint.c \
! charset.c cli-out.c coffread.c coff-pe-read.c \
! complaints.c completer.c corefile.c \
cp-abi.c cp-support.c cp-valprint.c \
dbxread.c demangle.c disasm.c doublest.c \
dummy-frame.c dwarfread.c dwarf2read.c \
*************** call_cmds_h = call-cmds.h
*** 612,617 ****
--- 613,619 ----
ch_lang_h = ch-lang.h
cli_out_h = cli-out.h
coff_solib_h = coff-solib.h
+ coff_pe_read_h = coff-pe-read.h
command_h = command.h
complaints_h = complaints.h
completer_h = completer.h
*************** COMMON_OBS = version.o blockframe.o brea
*** 833,839 ****
kod.o kod-cisco.o \
gdb-events.o \
exec.o bcache.o objfiles.o minsyms.o maint.o demangle.o \
! dbxread.o coffread.o elfread.o \
dwarfread.o dwarf2read.o mipsread.o stabsread.o corefile.o \
c-lang.o f-lang.o \
ui-out.o cli-out.o \
--- 835,841 ----
kod.o kod-cisco.o \
gdb-events.o \
exec.o bcache.o objfiles.o minsyms.o maint.o demangle.o \
! dbxread.o coffread.o coff-pe-read.o elfread.o \
dwarfread.o dwarf2read.o mipsread.o stabsread.o corefile.o \
c-lang.o f-lang.o \
ui-out.o cli-out.o \
*************** coffread.o: coffread.c $(defs_h) $(symta
*** 1564,1570 ****
$(breakpoint_h) $(bfd_h) $(gdb_obstack_h) $(gdb_string_h) \
$(coff_internal_h) $(libcoff_h) $(symfile_h) $(objfiles_h) \
$(buildsym_h) $(gdb_stabs_h) $(stabsread_h) $(complaints_h) \
! $(target_h) $(gdb_assert_h)
complaints.o: complaints.c $(defs_h) $(complaints_h) $(gdb_assert_h) \
$(command_h) $(gdbcmd_h)
completer.o: completer.c $(defs_h) $(symtab_h) $(gdbtypes_h) $(expression_h) \
--- 1566,1574 ----
$(breakpoint_h) $(bfd_h) $(gdb_obstack_h) $(gdb_string_h) \
$(coff_internal_h) $(libcoff_h) $(symfile_h) $(objfiles_h) \
$(buildsym_h) $(gdb_stabs_h) $(stabsread_h) $(complaints_h) \
! $(target_h) $(gdb_assert_h) $(coff_pe_read_h)
! coff-pe-read.o: coff-pe-read.c $(bfd_h) $(defs_h) $(symtab_h) \
! $(gdbtypes_h) $(symfile_h) $(objfiles_h) $(coff_pe_read_h)
complaints.o: complaints.c $(defs_h) $(complaints_h) $(gdb_assert_h) \
$(command_h) $(gdbcmd_h)
completer.o: completer.c $(defs_h) $(symtab_h) $(gdbtypes_h) $(expression_h) \

[-- Attachment #4: coff-pe-read.h --]
[-- Type: application/octet-stream, Size: 1207 bytes --]

/* Interface to coff-pe-read.c (portable-executable-specific symbol reader).

Copyright 2003 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 2 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, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.

Contributed by Raoul M. Gough (RaoulGough@yahoo.co.uk). */

#if !defined (COFF_PE_READ_H)
#define COFF_PE_READ_H

struct objfile;

/* Read the export table and convert it to minimal symbol table entries */
void read_pe_exported_syms (struct objfile *objfile);

#endif /* !defined (COFF_PE_READ_H) */

[-- Attachment #5: coff-pe-read.c --]
[-- Type: application/octet-stream, Size: 10412 bytes --]

/* Read the export table symbols from a portable executable and
convert to internal format, for GDB. Used as a last resort if no
debugging symbols recognized.

Copyright 2003 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 2 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, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.

Contributed by Raoul M. Gough (RaoulGough@yahoo.co.uk). */

#include "coff-pe-read.h"

#include "bfd.h"

#include "defs.h"
#include "gdbtypes.h"

#include "symtab.h"
#include "symfile.h"
#include "objfiles.h"

/* Internal section information */

struct read_pe_section_data
{
CORE_ADDR vma_offset; /* Offset to loaded address of section.*/
unsigned long rva_start; /* Start offset within the pe. */
unsigned long rva_end; /* End offset within the pe. */
enum minimal_symbol_type ms_type; /* Type to assign symbols in section. */
};

#define PE_SECTION_INDEX_TEXT 0
#define PE_SECTION_INDEX_DATA 1
#define PE_SECTION_INDEX_BSS 2
#define PE_SECTION_TABLE_SIZE 3
#define PE_SECTION_INDEX_INVALID -1
\f
/* Get the index of the named section in our own array, which contains
text, data and bss in that order. Return PE_SECTION_INDEX_INVALID
if passed an unrecognised section name. */

static int
read_pe_section_index (const char *section_name)
{
if (strcmp (section_name, ".text") == 0)
{
return PE_SECTION_INDEX_TEXT;
}

else if (strcmp (section_name, ".data") == 0)
{
return PE_SECTION_INDEX_DATA;
}

else if (strcmp (section_name, ".bss") == 0)
{
return PE_SECTION_INDEX_BSS;
}

else
{
return PE_SECTION_INDEX_INVALID;
}
}

/* Record the virtual memory address of a section. */

static void
get_section_vmas (bfd *abfd, asection *sectp, void *context)
{
struct read_pe_section_data *sections = context;
int sectix = read_pe_section_index (sectp->name);

if (sectix != PE_SECTION_INDEX_INVALID)
{
/* Data within the section start at rva_start in the pe and at
bfd_get_section_vma() within memory. Store the offset. */

sections[sectix].vma_offset
= bfd_get_section_vma (abfd, sectp) - sections[sectix].rva_start;
}
}
\f
/* Create a minimal symbol entry for an exported symbol. */

static void
add_pe_exported_sym (char *sym_name,
unsigned long func_rva,
const struct read_pe_section_data *section_data,
const char *dll_name,
struct objfile *objfile)
{
/* Add the stored offset to get the loaded address of the symbol. */

CORE_ADDR vma = func_rva + section_data->vma_offset;

char *qualified_name = 0;
int dll_name_len = strlen (dll_name);
int count;

/* Generate a (hopefully unique) qualified name using the first part
of the dll name, e.g. KERNEL32!AddAtomA. This matches the style
used by windbg from the "Microsoft Debugging Tools for Windows". */

qualified_name = xmalloc (dll_name_len + strlen (sym_name) + 2);

strncpy (qualified_name, dll_name, dll_name_len);
qualified_name[dll_name_len] = '!';
strcpy (qualified_name + dll_name_len + 1, sym_name);

prim_record_minimal_symbol (qualified_name,
vma,
section_data->ms_type,
objfile);

xfree (qualified_name);

/* Enter the plain name as well, which might not be unique. */
prim_record_minimal_symbol (sym_name,
vma,
section_data->ms_type,
objfile);
}

/* Truncate a dll_name at the first dot character. */

static void
read_pe_truncate_name (char *dll_name)
{
while (*dll_name)
{
if ((*dll_name) == '.')
{
*dll_name = '\0'; /* truncates and causes loop exit. */
}

else
{
++dll_name;
}
}
}
\f
/* Low-level support functions, direct from the ld module pe-dll.c. */
static unsigned int
pe_get16 (bfd *abfd, int where)
{
unsigned char b[2];

bfd_seek (abfd, (file_ptr) where, SEEK_SET);
bfd_bread (b, (bfd_size_type) 2, abfd);
return b[0] + (b[1] << 8);
}

static unsigned int
pe_get32 (bfd *abfd, int where)
{
unsigned char b[4];

bfd_seek (abfd, (file_ptr) where, SEEK_SET);
bfd_bread (b, (bfd_size_type) 4, abfd);
return b[0] + (b[1] << 8) + (b[2] << 16) + (b[3] << 24);
}

static unsigned int
pe_as32 (void *ptr)
{
unsigned char *b = ptr;

return b[0] + (b[1] << 8) + (b[2] << 16) + (b[3] << 24);
}
\f
/* Read the (non-debug) export symbol table from a portable
executable. Code originally lifted from the ld function
pe_implied_import_dll in pe-dll.c. */

void
read_pe_exported_syms (struct objfile *objfile)
{
bfd *dll = objfile->obfd;
unsigned long pe_header_offset, opthdr_ofs, num_entries, i;
unsigned long export_rva, export_size, nsections, secptr, expptr;
unsigned long exp_funcbase;
unsigned char *expdata, *erva;
unsigned long name_rvas, ordinals, nexp, ordbase;
char *dll_name;

/* Array elements are for text, data and bss in that order
Initialization with start_rva > end_rva guarantees that
unused sections won't be matched. */
struct read_pe_section_data section_data[PE_SECTION_TABLE_SIZE]
= { {0, 1, 0, mst_text},
{0, 1, 0, mst_data},
{0, 1, 0, mst_bss} };

struct cleanup *back_to = 0;

char const *target = bfd_get_target (objfile->obfd);

if ((strcmp (target, "pe-i386") != 0) && (strcmp (target, "pei-i386") != 0))
{
/* This is not an i386 format file. Abort now, because the code
is untested on anything else. *FIXME* test on further
architectures and loosen or remove this test. */
return;
}

/* Get pe_header, optional header and numbers of export entries. */
pe_header_offset = pe_get32 (dll, 0x3c);
opthdr_ofs = pe_header_offset + 4 + 20;
num_entries = pe_get32 (dll, opthdr_ofs + 92);

if (num_entries < 1) /* No exports. */
{
return;
}

export_rva = pe_get32 (dll, opthdr_ofs + 96);
export_size = pe_get32 (dll, opthdr_ofs + 100);
nsections = pe_get16 (dll, pe_header_offset + 4 + 2);
secptr = (pe_header_offset + 4 + 20 +
pe_get16 (dll, pe_header_offset + 4 + 16));
expptr = 0;

/* Get the rva and size of the export section. */
for (i = 0; i < nsections; i++)
{
char sname[8];
unsigned long secptr1 = secptr + 40 * i;
unsigned long vaddr = pe_get32 (dll, secptr1 + 12);
unsigned long vsize = pe_get32 (dll, secptr1 + 16);
unsigned long fptr = pe_get32 (dll, secptr1 + 20);

bfd_seek (dll, (file_ptr) secptr1, SEEK_SET);
bfd_bread (sname, (bfd_size_type) 8, dll);

if (vaddr <= export_rva && vaddr + vsize > export_rva)
{
expptr = fptr + (export_rva - vaddr);
if (export_rva + export_size > vaddr + vsize)
export_size = vsize - (export_rva - vaddr);
break;
}
}

if (export_size == 0)
{
/* Empty export table. */
return;
}

/* Scan sections and store the base and size of the relevant sections. */
for (i = 0; i < nsections; i++)
{
unsigned long secptr1 = secptr + 40 * i;
unsigned long vsize = pe_get32 (dll, secptr1 + 8);
unsigned long vaddr = pe_get32 (dll, secptr1 + 12);
unsigned long flags = pe_get32 (dll, secptr1 + 36);
char sec_name[9];
int sectix;

sec_name[8] = '\0';
bfd_seek (dll, (file_ptr) secptr1 + 0, SEEK_SET);
bfd_bread (sec_name, (bfd_size_type) 8, dll);

sectix = read_pe_section_index (sec_name);

if (sectix != PE_SECTION_INDEX_INVALID)
{
section_data[sectix].rva_start = vaddr;
section_data[sectix].rva_end = vaddr + vsize;
}
}

expdata = (unsigned char *) xmalloc (export_size);
back_to = make_cleanup (xfree, expdata);

bfd_seek (dll, (file_ptr) expptr, SEEK_SET);
bfd_bread (expdata, (bfd_size_type) export_size, dll);
erva = expdata - export_rva;

nexp = pe_as32 (expdata + 24);
name_rvas = pe_as32 (expdata + 32);
ordinals = pe_as32 (expdata + 36);
ordbase = pe_as32 (expdata + 16);
exp_funcbase = pe_as32 (expdata + 28);

/* Use internal dll name instead of full pathname. */
dll_name = pe_as32 (expdata + 12) + erva;

bfd_map_over_sections (dll, get_section_vmas, section_data);

/* Adjust the vma_offsets in case this PE got relocated. This
assumes that *all* sections share the same relocation offset
as the text section. */
for (i = 0; i < PE_SECTION_TABLE_SIZE; i++)
{
section_data[i].vma_offset
+= ANOFFSET (objfile->section_offsets, SECT_OFF_TEXT (objfile));
}

printf_filtered ("Minimal symbols from %s...", dll_name);
wrap_here ("");

/* Truncate name at first dot. Should maybe also convert to all
lower case for convenience on Windows. */
read_pe_truncate_name (dll_name);

/* Iterate through the list of symbols. */
for (i = 0; i < nexp; i++)
{
/* Pointer to the names vector. */
unsigned long name_rva = pe_as32 (erva + name_rvas + i * 4);

/* Pointer to the function address vector. */
unsigned long func_rva = pe_as32 (erva + exp_funcbase + i * 4);

/* Find this symbol's section in our own array. */
int sectix = 0;

for (sectix = 0; sectix < PE_SECTION_TABLE_SIZE; ++sectix)
{
if ((func_rva >= section_data[sectix].rva_start)
&& (func_rva < section_data[sectix].rva_end))
{
add_pe_exported_sym (erva + name_rva,
func_rva,
section_data + sectix,
dll_name,
objfile);
break;
}
}
}

/* discard expdata. */
do_cleanups (back_to);
}

[-- Attachment #6: ChangeLog_entry.txt --]
[-- Type: text/plain, Size: 506 bytes --]

2003-02-06 Raoul Gough <RaoulGough@yahoo.co.uk>

* coff-pe-read.c: New file - support reading of minimal symbols
from a portable executable using the export table.
* coff-pe-read.h: New file
* coffread.c: #include coff-pe-read.h
(coff_symtab_read): call read_pe_exported_syms iff no recognized
debugging symbols found.
* Makefile.in (SFILES): add coff-pe-read.o
(coff_pe_read_h): define
(COMMON_OBS): add coff-pe-read.o
(coffread.o): add coff_pe_read_h dependency
(coff-pe-read.o): New target

[-- Attachment #7: doc_changelog_entry.txt --]
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2003-02-06 Raoul Gough <RaoulGough@yahoo.co.uk>

* gdb.texinfo (Cygwin Native): Links to Non-debug DLL symbols.
(Non-debug DLL symbols): New node, describing the minimal
symbols loaded from DLLs without real debugging symbols.

[-- Attachment #8: gdb.texinfo.diff.txt --]
[-- Type: text/plain, Size: 6381 bytes --]

Index: gdb.texinfo
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v
retrieving revision 1.147
diff -c -p -r1.147 gdb.texinfo
*** gdb.texinfo 4 Feb 2003 22:52:51 -0000 1.147
--- gdb.texinfo 6 Feb 2003 22:54:28 -0000
*************** This command is supported only with some
*** 11142,11150 ****
@cindex native Cygwin debugging
@cindex Cygwin-specific commands

! @value{GDBN} supports native debugging of MS Windows programs, and
! defines a few commands specific to the Cygwin port. This
! subsection describes those commands.

@table @code
@kindex info w32
--- 11142,11153 ----
@cindex native Cygwin debugging
@cindex Cygwin-specific commands

! @value{GDBN} supports native debugging of MS Windows programs, including
! DLLs with and without symbolic debugging information. There are various
! additional Cygwin-specific commands, described in this subsection. The
! subsubsection @ref{Non-debug DLL symbols} describes working with DLLs
! that have no debugging symbols.
!

@table @code
@kindex info w32
*************** via a shell or directly (default value i
*** 11221,11226 ****
--- 11224,11350 ----
Displays if the debuggee will be started with a shell.

@end table
+
+ @menu
+ * Non-debug DLL symbols:: Support for DLLs without debugging symbols
+ @end menu
+
+ @node Non-debug DLL symbols
+ @subsubsection Support for DLLs without debugging symbols
+ @cindex DLLs with no debugging symbols
+
+ Very often on windows, some of the DLLs that your program relies on do
+ not include symbolic debugging information (for example,
+ kernel32.dll). When @value{GDBN} doesn't recognize any debugging symbols
+ in a DLL, it relies on the minimal amount of symbolic information
+ contained in the DLL's export table. This subsubsection describes
+ working with such symbols, known internally to @value{GDBN} as ``minimal
+ symbols''.
+
+ Note that before the debugged program has started execution, no DLLs
+ will have been loaded. The easiest way around this problem is simply to
+ start the program - either by setting a breakpoint or letting the
+ program run once to completion. It is also possible to force
+ @value{GDBN} to load a particular DLL before starting the executable -
+ see the shared library information in @ref{Files} or the
+ @code{dll-symbols} command in @ref{Cygwin Native}. Currently, explicitly
+ loading symbols from a DLL with no debugging information will cause the
+ same symbols to be duplicated in @value{GDBN}'s symbol table, which may
+ adversely affect symbol lookup performance.
+
+ @subsubsection DLL name prefixes
+
+ In keeping with the naming conventions used by the Microsoft debugging
+ tools, DLL export symbols are made available with a prefix based on the
+ DLL name, for instance ``KERNEL32!CreateFileA''. The plain name is also
+ entered into the symbol table, so ``CreateFileA'' is often
+ sufficient. In some cases there will be name clashes within a program
+ (particularly if the executable itself includes full debugging symbols)
+ necessitating the use of the fully qualified name. Use single-quotes
+ around the name to avoid the exclamation mark (``!'') being interpreted
+ as a language operator.
+
+ Note that the internal name of the DLL may be all upper-case, even
+ though the file name of the DLL is lower-case, or vice-versa. Since
+ symbols within @value{GDBN} are @emph{case-sensitive} this may cause
+ some confusion. If in doubt, try the @code{info functions} and
+ @code{info variables} commands or even @code{maint print msymbols} (see
+ @ref{Symbols}). Here's an example:
+
+ @smallexample
+ (gdb) info function CreateFileA
+ All functions matching regular expression "CreateFileA":
+
+ Non-debugging symbols:
+ 0x77e885f4 CreateFileA
+ 0x77e885f4 KERNEL32!CreateFileA
+ @end smallexample
+
+ @smallexample
+ (gdb) info function !
+ All functions matching regular expression "!":
+
+ Non-debugging symbols:
+ 0x6100114c cygwin1!__assert
+ 0x61004034 cygwin1!_dll_crt0@@0
+ 0x61004240 cygwin1!dll_crt0(per_process *)
+ [etc...]
+ @end smallexample
+
+ @subsubsection Working with minimal symbols
+
+ Symbols extracted from a DLL's export table do not contain very much
+ type information. All that @value{GDBN} can do is guess whether a symbol
+ refers to a function or variable depending on the linker section that
+ contains the symbol. Also note that the actual contents of the memory
+ contained in a DLL are not available unless the program is running. This
+ means that you cannot examine the contents of a variable or disassemble
+ a function within a DLL without a running program.
+
+ Variables are generally treated as pointers and dereferenced
+ automatically. For this reason, it is often necessary to prefix a
+ variable name with an ampersand (``&'') and provide explicit type
+ information in the command. Here's an example of the type of problem:
+
+ @smallexample
+ (gdb) print 'cygwin1!__argv'
+ $1 = 268572168
+ @end smallexample
+
+ @smallexample
+ (gdb) x 'cygwin1!__argv'
+ 0x10021610: "\230y\""
+ @end smallexample
+
+ And two possible solutions:
+
+ @smallexample
+ (gdb) print ((char **)'cygwin1!__argv')[0]
+ $2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
+ @end smallexample
+
+ @smallexample
+ (gdb) x/2x &'cygwin1!__argv'
+ 0x610c0aa8 <cygwin1!__argv>: 0x10021608 0x00000000
+ (gdb) x/x 0x10021608
+ 0x10021608: 0x0022fd98
+ (gdb) x/s 0x0022fd98
+ 0x22fd98: "/cygdrive/c/mydirectory/myprogram"
+ @end smallexample
+
+ Setting a break point within a DLL is possible even before the program
+ starts execution. However, under these circumstances, @value{GDBN} can't
+ examine the initial instructions of the function in order to skip the
+ function's frame set-up code. You can work around this by using ``*&''
+ to set the breakpoint at a raw memory address:
+
+ @smallexample
+ (gdb) break *&'python22!PyOS_Readline'
+ Breakpoint 1 at 0x1e04eff0
+ @end smallexample
+
+ The author of these extensions is not entirely convinced that setting a
+ break point within a shared DLL like kernel32.dll is completely safe.

@node Embedded OS
@section Embedded Operating Systems