gdb add-symbol-file all sections and load address

I'm debugging a boot loader (syslinux) with gdb and the gdb-stub of qemu. At some point the main file load a shared object ldlinux.elf.

ldlinux.elf

I would like to add the symbols in gdb for that file. The command add-symbol-file seems like the way to go. However, as a relocatable file, I have to specify the memory address it has been loaded at. And here comes the problem.

add-symbol-file

Although I know the base address at which the LOAD segment has been loaded at, add-symbol-file works section-wise and want me to specify the address at which each section has been loaded.

LOAD

add-symbol-file

Can I tell gdb to load all the symbols of all the sections provided that I specify the base address of the file in memory?

Does the behavior of gdb make sens? The section headers aren't used for running an ELF and are even optional. I can't see a use case where specifying the load address of the sections would be useful.

Here are the program headers and section headers of the shared object.

If I try to load the file at the address 0x7fab000 then it will relocate the symbols so that the .text section starts at 0x7fab000.

0x7fab000

.text

And then all the symbols are off by 0x4c60 bytes.

2 Answers
2

So, finally, I made my own command with python and the readelf tool. It's not very clean since it runs readelf in a subprocess and parse its output instead of parsing the ELF file directly, but it works (for 32 bits ELF only).

readelf

readelf

It uses the section headers to generate and run an add-symbol-file command with all the sections correctly relocated. The usage is pretty simple, you give it the elf file and the base address of the file. And since the remove-symbol-file wasn't working properly by just giving it the filename, I made a remove-symbol-file-all that generate and run the right remove-symbol-file -a address command.

add-symbol-file

remove-symbol-file

remove-symbol-file-all

remove-symbol-file -a address

(gdb) add-symbol-file-all bios/com32/elflink/ldlinux/ldlinux.elf 0x7fab000
add symbol table from file "bios/com32/elflink/ldlinux/ldlinux.elf" at
.text_addr = 0x7fafc50
.gnu.hash_addr = 0x7fab094
.dynsym_addr = 0x7fab874
.dynstr_addr = 0x7face34
.rel.dyn_addr = 0x7fadf28
.rel.plt_addr = 0x7faec08
.plt_addr = 0x7faf170
.rodata_addr = 0x7fc34e0
.ctors_addr = 0x7fc7af0
.dtors_addr = 0x7fc7b00
.data.rel.ro_addr = 0x7fc7b20
.dynamic_addr = 0x7fc8658
.got_addr = 0x7fc86f0
.got.plt_addr = 0x7fc87bc
.data_addr = 0x7fc8a80
.bss_addr = 0x7fc8b60
(gdb) remove-symbol-file-all bios/com32/elflink/ldlinux/ldlinux.elf 0x7fab000


Here is the code to be added in the .gdbinit file.

.gdbinit

python
import subprocess
import re

def relocatesections(filename, addr):
p = subprocess.Popen(["readelf", "-S", filename], stdout = subprocess.PIPE)

sections =
textaddr = '0'
for line in p.stdout.readlines():
line = line.decode("utf-8").strip()
if not line.startswith('[') or line.startswith('[Nr]'):
continue

line = re.sub(r' +', ' ', line)
line = re.sub(r'[ *(d+)]', 'g<1>', line)
fieldsvalue = line.split(' ')
fieldsname = ['number', 'name', 'type', 'addr', 'offset', 'size', 'entsize', 'flags', 'link', 'info', 'addralign']
sec = dict(zip(fieldsname, fieldsvalue))

if sec['number'] == '0':
continue

sections.append(sec)

if sec['name'] == '.text':
textaddr = sec['addr']

return (textaddr, sections)


class AddSymbolFileAll(gdb.Command):
"""The right version for add-symbol-file"""

def __init__(self):
super(AddSymbolFileAll, self).__init__("add-symbol-file-all", gdb.COMMAND_USER)
self.dont_repeat()

def invoke(self, arg, from_tty):
argv = gdb.string_to_argv(arg)
filename = argv[0]

if len(argv) > 1:
offset = int(str(gdb.parse_and_eval(argv[1])), 0)
else:
offset = 0

(textaddr, sections) = relocatesections(filename, offset)

cmd = "add-symbol-file %s 0x%08x" % (filename, int(textaddr, 16) + offset)

for s in sections:
addr = int(s['addr'], 16)
if s['name'] == '.text' or addr == 0:
continue

cmd += " -s %s 0x%08x" % (s['name'], addr + offset)

gdb.execute(cmd)

class RemoveSymbolFileAll(gdb.Command):
"""The right version for remove-symbol-file"""

def __init__(self):
super(RemoveSymbolFileAll, self).__init__("remove-symbol-file-all", gdb.COMMAND_USER)
self.dont_repeat()

def invoke(self, arg, from_tty):
argv = gdb.string_to_argv(arg)
filename = argv[0]

if len(argv) > 1:
offset = int(str(gdb.parse_and_eval(argv[1])), 0)
else:
offset = 0

(textaddr, _) = relocatesections(filename, offset)

cmd = "remove-symbol-file -a 0x%08x" % (int(textaddr, 16) + offset)
gdb.execute(cmd)


AddSymbolFileAll()
RemoveSymbolFileAll()
end


Can I tell gdb to load all the symbols of all the sections provided that I specify the base address of the file in memory?

Yes, but you need to provide the address of .text section, i.e. 0x7fab000+0x00004c60 here. I agree: it's quite annoying to have to fish out address of .text, and I wanted to fix it many times, so that e.g.

.text

0x7fab000+0x00004c60

.text

(gdb) add-symbol-file foo.so @0x7abc0000


just works. Feel free to file a feature request in GDB bugzilla.

Does the behavior of gdb make sens?

I am guessing that this is rooted in how GDB was used to debug embedded ROMs, where each section can be at arbitrary memory address.

.data

.rodata

.bss

foo.so

.text

.text

.data

.data

A syntax error in expression, near '0x7abc000'.

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