Reputation: 5620
E.g. !ls
would execute ls
command in gdb itself, but how to do it on remote side?
It should be simple, but I can't figure it out. Per documentation something like target remote | ls
or target remote | !ls
ought to do the trick, but either it's wrong or I don't understand something: such command makes gdb to try to close current session, and start debugging ls
binary.
I also found some monitor cmd
mentioned, but monitor !ls
just triggers Unknown monitor command
message.
Upvotes: 4
Views: 5854
Reputation: 507
A workaround is to implement a custom gdb command that does the following:
There are several limitations to take into account:
call
command: we need to call libc functions by address, which requires loading addresses beforehand;call
command is not echoed in the gdb terminal: however, we can capture it in a remote temporary file, then read to memory and print.Example gdb session:
# Given remote terminal running `gdbserver :2345 ./remote_executable`, we connect to that server.
target extended-remote 192.168.1.4:2345
# Load our custom gdb command `rcmd`.
source ./remote-cmd.py
# Run until a point where libc has been loaded on the remote process, e.g. start of main().
b main
r
# Don't need the main() breakpoint anymore.
del 1
# Run the remote command, e.g. `ls`.
rcmd ls
remote-cmd.py:
#!/usr/bin/env python3
import gdb
import re
import traceback
import uuid
class RemoteCmd(gdb.Command):
def __init__(self):
self.addresses = {}
self.tmp_file = f'/tmp/{uuid.uuid4().hex}'
gdb.write(f"Using tmp output file: {self.tmp_file}.\n")
gdb.execute("set detach-on-fork off")
gdb.execute("set follow-fork-mode parent")
gdb.execute("set max-value-size unlimited")
gdb.execute("set pagination off")
gdb.execute("set print elements 0")
gdb.execute("set print repeats 0")
super(RemoteCmd, self).__init__("rcmd", gdb.COMMAND_USER)
def preload(self):
for symbol in [
"close",
"execl",
"fork",
"free",
"lseek",
"malloc",
"open",
"read",
]:
self.load(symbol)
def load(self, symbol):
if symbol not in self.addresses:
address_string = gdb.execute(f"info address {symbol}", to_string=True)
match = re.match(
f'Symbol "{symbol}" is at ([0-9a-fx]+) .*', address_string, re.IGNORECASE
)
if match and len(match.groups()) > 0:
self.addresses[symbol] = match.groups()[0]
else:
raise RuntimeError(f'Could not retrieve address for symbol "{symbol}".')
return self.addresses[symbol]
def output(self):
# From `fcntl-linux.h`
O_RDONLY = 0
gdb.execute(
f'set $fd = (int){self.load("open")}("{self.tmp_file}", {O_RDONLY})'
)
# From `stdio.h`
SEEK_SET = 0
SEEK_END = 2
gdb.execute(f'set $len = (int){self.load("lseek")}($fd, 0, {SEEK_END})')
gdb.execute(f'call (int){self.load("lseek")}($fd, 0, {SEEK_SET})')
if int(gdb.convenience_variable("len")) <= 0:
gdb.write("No output was captured.")
return
gdb.execute(f'set $mem = (void*){self.load("malloc")}($len)')
gdb.execute(f'call (int){self.load("read")}($fd, $mem, $len)')
gdb.execute('printf "%s\\n", (char*) $mem')
gdb.execute(f'call (int){self.load("close")}($fd)')
gdb.execute(f'call (int){self.load("free")}($mem)')
def invoke(self, arg, from_tty):
try:
self.preload()
is_auto_solib_add = gdb.parameter("auto-solib-add")
gdb.execute("set auto-solib-add off")
parent_inferior = gdb.selected_inferior()
gdb.execute(f'set $child_pid = (int){self.load("fork")}()')
child_pid = gdb.convenience_variable("child_pid")
child_inferior = list(
filter(lambda x: x.pid == child_pid, gdb.inferiors())
)[0]
gdb.execute(f"inferior {child_inferior.num}")
try:
gdb.execute(
f'call (int){self.load("execl")}("/bin/sh", "sh", "-c", "exec {arg} >{self.tmp_file} 2>&1", (char*)0)'
)
except gdb.error as e:
if (
"The program being debugged exited while in a function called from GDB"
in str(e)
):
pass
else:
raise e
finally:
gdb.execute(f"inferior {parent_inferior.num}")
gdb.execute(f"remove-inferiors {child_inferior.num}")
self.output()
except Exception as e:
gdb.write("".join(traceback.TracebackException.from_exception(e).format()))
raise e
finally:
gdb.execute(f'set auto-solib-add {"on" if is_auto_solib_add else "off"}')
RemoteCmd()
Upvotes: 2
Reputation: 744
! ls > /tmp/ls.txt
All the command that you try to execute using ! cmd
will works but you can't get the output because gdbserver is not able to redirect the stream correctly.
Just open a new remote connection to see the output of your gdb remote command redirect to a file like the example before.
Upvotes: 0