Re: [PATCH v3 00/11] sysctl: treewide: constify ctl_table argument of sysctl handlers

Joel Granados
2024-05-03T09:03:46Z

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Hey Thomas

Here is my feedback for your outstanding constification patches [1] and [2].

# You need to split the patch
The answer that you got from Jakub in the network subsystem is very clear and
baring a change of heart from the network folks, this will go in as but as a
split patchset. Please split it considering the following:
1. Create a different patchset for drivers/, fs/, kernel/, net, and a
miscellaneous that includes whatever does not fit into the others.
2. Consider that this might take several releases.
3. Consider the following sufix for the interim function name "_const". Like in
kfree_const. Please not "_new".
4. Please publish the final result somewhere. This is important so someone can
take over in case you need to stop.
5. Consistently mention the motivation in your cover letters. I specify more
further down in "#Motivation".
6. Also mention that this is part of a bigger effort (like you did in your
original cover letters). I would include [3,4,5,6]
7. Include a way to show what made it into .rodata. I specify more further down
in "#Show the move".

# Motivation
As I read it, the motivation for these constification efforts are:
1. It provides increased safety: Having things in .rodata section reduces the
attack surface. This is especially relevant for structures that have function
pointers (like ctl_table); having these in .rodata means that these pointers
always point to the "intended" function and cannot be changed.
2. Compiler optimizations: This was just a comment in the patchsets that I have
mentioned ([3,4,5]). Do you know what optimizations specifically? Does it
have to do with enhancing locality for the data in .rodata? Do you have other
specific optimizations in mind?
3. Readability: because it is easier to know up-front that data is not supposed
to change or its obvious that a function is re-entrant. Actually a lot of the
readability reasons is about knowing things "up-front".
As we move forward with the constification in sysctl, please include a more
detailed motivation in all your cover letters. This helps maintainers (that
don't have the context) understand what you are trying to do. It does not need
to be my three points, but it should be more than just "put things into
.rodata". Please tell me if I have missed anything in the motivation.

# Show the move
I created [8] because there is no easy way to validate which objects made it
into .rodata. I ran [8] for your Dec 2nd patcheset [7] and there are less in
.rodata than I expected (the results are in [9]) Why is that? Is it something
that has not been posted to the lists yet?

Best

[1] https://lore.kernel.org/all/20240423-sysctl-const-handler-v3-0-e0beccb836e2@weissschuh.net/
[2] https://lore.kernel.org/all/20240418-sysctl-const-table-arg-v2-1-4012abc31311@weissschuh.net
[3] [PATCH v2 00/14] ASoC: Constify local snd_sof_dsp_ops
https://lore.kernel.org/all/20240426-n-const-ops-var-v2-0-e553fe67ae82@kernel.org
[4] [PATCH v2 00/19] backlight: Constify lcd_ops
https://lore.kernel.org/all/20240424-video-backlight-lcd-ops-v2-0-1aaa82b07bc6@kernel.org
[5] [PATCH 1/4] iommu: constify pointer to bus_type
https://lore.kernel.org/all/20240216144027.185959-1-krzysztof.kozlowski@linaro.org
[6] [PATCH 00/29] const xattr tables
https://lore.kernel.org/all/20230930050033.41174-1-wedsonaf@gmail.com
[7] https://lore.kernel.org/all/20231204-const-sysctl-v2-0-7a5060b11447@weissschuh.net/

[8]
#!/usr/bin/python3

import subprocess
import re

def exec_cmd( cmd ):
try:
result = subprocess.run(cmd, shell=True, text=True, check=True, capture_output=True)
output_lines = result.stdout.splitlines()
return output_lines
except Exception as e:
print(f"An error occurred: {e}")
return []

def remove_tokens_re(lines, regex_patterns, uniq = True):
filtered_lines = []
seen_lines = set()
regexes = [re.compile(pattern) for pattern in regex_patterns]

for line in lines:
for regex in regexes:
line = regex.sub('', line) # Replace matches with empty string

if uniq:
if line not in seen_lines:
seen_lines.add(line)
filtered_lines.append(line)
else:
filtered_lines.append(line)

return filtered_lines

def filter_in_lines(lines, regex_patterns):
filtered_lines = []
regexes = [re.compile(pattern) for pattern in regex_patterns]

for line in lines:
if any(regex.search(line) for regex in regexes):
filtered_lines.append(line)

return filtered_lines

cmd = "git grep 'static \(const \)\?struct ctl_table '"
regex_patterns = ['[\}]*;$', ' = \{', '\[.*\]', '.*\.(c|h):[ \t]*static (const )?struct ctl_table ']
ctl_table_structs = remove_tokens_re(exec_cmd( cmd ), regex_patterns)

cmd = "readelf -X -Ws build/vmlinux"
regex_patterns = ['.*OBJECT.*']
output_lines = filter_in_lines(exec_cmd( cmd ), regex_patterns);

regex_patterns = ['^.*OBJECT', '[ \t]+[A-Z]+[ \t]+[A-Z]+.*\(.*\)[ \t]+']
obj_elems = remove_tokens_re( output_lines, regex_patterns, uniq=False)

regex_patterns = ['^.*\(', '\)[ ]+.*$']
sec_names = remove_tokens_re( output_lines, regex_patterns, uniq=False)

for i in range(len(sec_names)):
obj_name = obj_elems[i]
if obj_name in ctl_table_structs:
print ("section: {}\t\tobj_name : {}". format(sec_names[i], obj_name))

[9]
section: .rodata obj_name : kern_table
section: .rodata obj_name : sysctl_mount_point
section: .rodata obj_name : addrconf_sysctl
section: .rodata obj_name : ax25_param_table
section: .rodata obj_name : mpls_table
section: .rodata obj_name : mpls_dev_table
section: .data obj_name : sld_sysctls
section: .data obj_name : kern_panic_table
section: .data obj_name : kern_exit_table
section: .data obj_name : vm_table
section: .data obj_name : signal_debug_table
section: .data obj_name : usermodehelper_table
section: .data obj_name : kern_reboot_table
section: .data obj_name : user_table
section: .bss obj_name : sched_core_sysctls
section: .data obj_name : sched_fair_sysctls
section: .data obj_name : sched_rt_sysctls
section: .data obj_name : sched_dl_sysctls
section: .data obj_name : printk_sysctls
section: .data obj_name : pid_ns_ctl_table_vm
section: .data obj_name : seccomp_sysctl_table
section: .data obj_name : uts_kern_table
section: .data obj_name : vm_oom_kill_table
section: .data obj_name : vm_page_writeback_sysctls
section: .data obj_name : page_alloc_sysctl_table
section: .data obj_name : hugetlb_table
section: .data obj_name : fs_stat_sysctls
section: .data obj_name : fs_exec_sysctls
section: .data obj_name : fs_pipe_sysctls
section: .data obj_name : namei_sysctls
section: .data obj_name : fs_dcache_sysctls
section: .data obj_name : inodes_sysctls
section: .data obj_name : fs_namespace_sysctls
section: .data obj_name : dnotify_sysctls
section: .data obj_name : inotify_table
section: .data obj_name : epoll_table
section: .data obj_name : aio_sysctls
section: .data obj_name : locks_sysctls
section: .data obj_name : coredump_sysctls
section: .data obj_name : fs_shared_sysctls
section: .data obj_name : fs_dqstats_table
section: .data obj_name : root_table
section: .data obj_name : pty_table
section: .data obj_name : xfs_table
section: .data obj_name : ipc_sysctls
section: .data obj_name : key_sysctls
section: .data obj_name : kernel_io_uring_disabled_table
section: .data obj_name : tty_table
section: .data obj_name : random_table
section: .data obj_name : scsi_table
section: .data obj_name : iwcm_ctl_table
section: .data obj_name : net_core_table
section: .data obj_name : netns_core_table
section: .bss obj_name : nf_log_sysctl_table
section: .data obj_name : nf_log_sysctl_ftable
section: .data obj_name : vs_vars
section: .data obj_name : vs_vars_table
section: .data obj_name : ipv4_route_netns_table
section: .data obj_name : ipv4_route_table
section: .data obj_name : ip4_frags_ns_ctl_table
section: .data obj_name : ip4_frags_ctl_table
section: .data obj_name : ctl_forward_entry
section: .data obj_name : ipv4_table
section: .data obj_name : ipv4_net_table
section: .data obj_name : unix_table
section: .data obj_name : ipv6_route_table_template
section: .data obj_name : ipv6_icmp_table_template
section: .data obj_name : ip6_frags_ns_ctl_table
section: .data obj_name : ip6_frags_ctl_table
section: .data obj_name : ipv6_table_template
section: .data obj_name : ipv6_rotable
section: .data obj_name : sctp_net_table
section: .data obj_name : sctp_table
section: .data obj_name : smc_table
section: .data obj_name : lowpan_frags_ns_ctl_table
section: .data obj_name : lowpan_frags_ctl_table

--

Joel Granados

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