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Fix bug: sshMaxArg main function was using only the first profile (#1363
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* Now all profiles with ssh mode are probed for the max ssh cmd length
* Code doc for config property "sshMaxArgLength" updated (used to generate man pages)
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@aryoda
aryoda authored Nov 17, 2022
1 parent e36d8dc commit 675c8b4
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Showing 2 changed files with 36 additions and 29 deletions.
5 changes: 3 additions & 2 deletions common/config.py
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Original file line number Diff line number Diff line change
Expand Up @@ -569,8 +569,9 @@ def setSshPrivateKeyFile(self, value, profile_id = None):
self.setProfileStrValue('snapshots.ssh.private_key_file', value, profile_id)

def sshMaxArgLength(self, profile_id = None):
#?Maximum argument length of commands run on remote host. This can be tested
#?with 'python3 /usr/share/backintime/common/sshMaxArg.py USER@HOST'.\n
#?Maximum command length of commands run on remote host. This can be tested
#?for all ssh profiles in the configuration
#?with 'python3 /usr/share/backintime/common/sshMaxArg.py [initial_ssh_cmd_length]'.\n
#?0 = unlimited;0, >700
value = self.profileIntValue('snapshots.ssh.max_arg_length', 0, profile_id)
if value and value < 700:
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60 changes: 33 additions & 27 deletions common/sshMaxArg.py
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Original file line number Diff line number Diff line change
Expand Up @@ -34,22 +34,23 @@
def probe_max_ssh_command_size(config,
ssh_command_size=_INITIAL_SSH_COMMAND_SIZE,
size_offset=_INITIAL_SSH_COMMAND_SIZE):
"""Determin the maximum length of an argument via SSH.
"""Determine the maximum length of SSH commands for the current config
Try a an SSH command with length ``ssh_command_size``. The command is
decreased by ``size_offset`` if it was to long or increased if it worked.
The function calls itself in a recursition until it finds the maximum
Try a SSH command with length ``ssh_command_size``. The command is
decreased by ``size_offset`` if it was too long or increased if it worked.
The function calls itself recursively until it finds the maximum
possible length. The offset ``size_offset`` is bisect in each try.
Args:
config (config.Config): Back In Time config instance including the
details about the current SSH snapshto profile.
details about the current SSH snapshot profile.
The current profile must use the SSH mode.
ssh_command_size (int): Initial length used for the test argument.
size_offset (int): Offset for increase or decrease
``ssh_command_size``.
Returns:
(int): The maximum possible length.
(int): The maximum possible SSH command length
Raises:
Exception: If there are unhandled cases or the recurse ends in an
Expand Down Expand Up @@ -86,78 +87,83 @@ def probe_max_ssh_command_size(config,
f'Python exception: "{err.strerror}". Decrease '
f'by {size_offset:,} and try again.')

# reducy by "r" and try again
# test again with new ssh_command_size
return probe_max_ssh_command_size(
config,
ssh_command_size - size_offset,
size_offset)

else:
# Successfull SSH command
# Successful SSH command
if out == command_string:

# no increases possible anymore
if size_offset == 0:
report_test(ssh_command_size,
'Found correct length. Adding '
f'length of "{ssh[-2]}" to it.')
'Found correct length. Adding '
f'length of "{ssh[-2]}" to it.')

# the final command size
return ssh_command_size + len(ssh[-2]) # length of "printf"

# there is room to increase the length
report_test(ssh_command_size,
f'Can be longer. Increase by {size_offset:,} '
'and try again.')
f'Can be longer. Increase by {size_offset:,} '
'and try again.')

# increae by "r" and try again
# increase by "size_offset" and try again
return probe_max_ssh_command_size(
config,
ssh_command_size + size_offset,
size_offset)

# command string was to long
# command string was too long
elif 'Argument list too long' in err:
report_test(ssh_command_size,
f'stderr: "{err.strip()}". Decrease '
f'by {size_offset:,} and try again.')
f'stderr: "{err.strip()}". Decrease '
f'by {size_offset:,} and try again.')

# reduce by "r" and try again
# reduce by "size_offset" and try again
return probe_max_ssh_command_size(
config,
ssh_command_size - size_offset,
size_offset)

raise Exception('Unhandled case.\n'
f'{ssh[:-1]}\nout="{out}"\nerr="{err}"\n'
f'mid={ssh_command_size:,}\nr={size_offset:,}')
f'ssh_command_size={ssh_command_size:,}\nsize_offset={size_offset:,}')


def report_test(ssh_command_size, msg):
print(f'Tried length {ssh_command_size:,}... {msg}')


def report_result(host, mid):
print(f'Maximum SSH argument length between "{socket.gethostname()}" '
f'and "{host}" is {mid:,}.')
def report_result(host, max_ssh_cmd_size):
print(f'Maximum SSH command length between "{socket.gethostname()}" '
f'and "{host}" is {max_ssh_cmd_size:,}.')


if __name__ == '__main__':
parser = argparse.ArgumentParser(
description='Check maximal argument length on SSH connection',
description='Check the maximal ssh command size for all ssh profiles in the configurations',
formatter_class=argparse.ArgumentDefaultsHelpFormatter)

parser.add_argument('SSH_COMMAND_SIZE',
type=int,
nargs='?',
default=_INITIAL_SSH_COMMAND_SIZE,
help='Start checking with SSH_COMMAND_SIZE arg length')
help='Start checking with SSH_COMMAND_SIZE as length')

args = parser.parse_args()

import config
cfg = config.Config()

ssh_command_size = probe_max_ssh_command_size(cfg, args.SSH_COMMAND_SIZE)

report_result(cfg.sshHost(), ssh_command_size)
profiles = cfg.profiles() # list of profile IDs

# loop over all profiles in the configuration
for profile_ID in profiles:
cfg.setCurrentProfile(profile_ID)
print(f"Profile {profile_ID} - {cfg.profileName()}: Mode = {cfg.snapshotsMode()}")
if cfg.snapshotsMode() == "ssh":
ssh_command_size = probe_max_ssh_command_size(cfg, args.SSH_COMMAND_SIZE)
report_result(cfg.sshHost(), ssh_command_size)

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