Re: [PATCH RFC v3 00/13] sysctl: add module aliases

[Joel Granados]

On Wed, Aug 19, 2026 at 03:16:13PM -0300, Mauricio Faria de Oliveira wrote:
> This series adds 'sysctl:' aliases to modules that register sysctl tables; e.g.:
> 
>         $ modinfo ./mpls_router.ko  | grep sysctl:
>         alias:          sysctl:*/net/mpls/conf/*/input
>         alias:          sysctl:*/net/mpls/default_ttl
>         alias:          sysctl:*/net/mpls/ip_ttl_propagate
>         alias:          sysctl:*/net/mpls/platform_labels
> 
> It provides a trivial way to map /proc/sys files to modules (not trivial today),
> and for userspace to handle nonexistent /proc/sys files (e.g., procps's sysctl
> and systemd-sysctl applying tunables) with "modprobe sysctl:<...>" and a retry.
> 
> This is done almost automatically with register_sysctl(), register_net_sysctl()
> and friends as wrappers of MODULE_SYSCTL_TABLE (similar to MODULE_DEVICE_TABLE),
> which emits symbols for file2alias/modpost to find and parse the sysctl tables.
> 
> The big exception to 'almost' are sysctl tables and paths allocated or defined
> at runtime (e.g., per-namespace or per-device), as all information is required
> at build-time. Fortunately, such tables and paths are often based on 'templates'
> which are static and can be used.
> 
> This is done by plumbing the template table/path as optional arguments (macros
> with default values as default_gfp()), so not to create functions for all cases:
> 
>         register_sysctl(path, table [, table_tmpl [, path_tmpl]]);
>         register_net_sysctl(net, path, table [, table_tmpl[, path_tmpl]]);
>         register_net_sysctl_sz(net, path, table, size [, table_tmpl[, path_tmpl]]);

The "what" is described but I'm missing more clarity on the "why". Why
does this need to be trivial? Where is it that you will know the sysctl
file path of a module and not the module name or alias?

Additionally, sysctls are not module specific; they are a way to
read/write kernel variables. Putting a module specific aspect in the
function arguments (that needs to be ignored in non-module cases) seems
wrong. Why not use the module subsys to add sysctl alias instead of the
sysctl subsys?

> 
> In this series:
> - Patch 1 prevents a build error later.
> - Patch 2 adds CONFIG_SYSCTL_MODULE_ALIASES.
> - Patch 3 adds MODULE_SYSCTL_TABLE().
> - Patches 4-5 add register_sysctl() wrapper and update some callers.
> - Patches 6-10 add register_net_sysctl[_sz]() wrappers and update some callers.
> - Patches 11-13 add file2alias support.
> 
> Not all maintainers/reviewers (specially for the many changes in net/) are
> in To/Cc in respect of their time, as this RFC probably needs more general
> and earlier feedback before settling on specific changes for their review.
> All lists are included for visibility, though.
> 
> Example
> =======
> 
> To put it all together, 'mpls_router.ko' (used above) from 'net/mpls/af_mpls.c':
>         
> - Tables:
> 
>         static const struct ctl_table mpls_table[] = {
>                 {
>                         .procname       = "platform_labels",
>                 ...
>                         .procname       = "ip_ttl_propagate",
>                 ...
>                         .procname       = "default_ttl",
>                 },
>         };
<... snip ...>
> 
> - Per-file:
> 
>   Use '#define SYSCTL_MODULE_ALIASES_DISABLE'.
> 
> - System-wide:
> 
>   Maybe something along these lines:
> 
>         # cat /etc/modprobe.d/no-sysctl.conf
>         alias sysctl:* no-sysctl
>         install no-sysctl /bin/false
> 
>         # modinfo -F name sysctl:/proc/net/mpls/default_ttl
>         mpls_router
> 
>         # modprobe sysctl:/proc/net/mpls/default_ttl
>         modprobe: ERROR: Error running install command '/bin/false' for module no_sysctl: retcode 1
>         modprobe: ERROR: could not insert 'no_sysctl': Invalid argument
> 
> P.S.
> ====
> 
> I wrote proof-of-concept patches for procps sysctl and systemd-sysctl some time
> ago, which worked as expected, i.e., successfully set sysctl tunables which did
> not exist in /proc/sys, by running 'modprobe sysctl:/proc/sys/...' and retrying.

Do you have links to this? To the old one, not necessarily a new
version.

Best
> 
> Should this series eventually be merged, the patches will be submitted upstream
> as well, for userspace to start consuming it.
> 
> Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
> 
> Changes in v3:
> - All issues were reported by sashiko-bot@xxxxxxxxxx.
> - Tested allmodconfig with option enabled on x86_64.
> - Patch 2:
>   - Fix typo ('s/options/option/').
> - Patch 3:
>   - Fix usage of __UNIQUE_ID().
>   - Add __must_be_array() in MODULE_SYSCTL_TABLE().
> - Patch 4:
>   - Remove trailing backslash in register_sysctl().
> - Patch 6:
>   - Fix missing __register_net_sysctl_sz() in !CONFIG_SYSCTL.
> - Patch 13:
>   - Fix potential out-of-bounds read in do_sysctl_entry().
>   - Add check for entry size of zero.
> - Link to v2: https://lore.kernel.org/r/20260818-sysctl-module-aliases-v2-0-d5a69dae5798@xxxxxxxxxx
> 
> Changes in v2:
> - This is based on the series submitted 4 years ago,
>   with Originally-by: tags added in related patches.
> - Link to v1: https://lore.kernel.org/linux-fsdevel/20220722022416.137548-1-mfo@xxxxxxxxxxxxx/
> 
> ---
> Mauricio Faria de Oliveira (13):
>       keys, pidns, fs/verity, riscv/vector: reorder '#include <linux/sysctl.h>'
>       proc: add config option SYSCTL_MODULE_ALIASES
>       sysctl, mod_devicetable: add macro MODULE_SYSCTL_TABLE
>       sysctl: add register_sysctl() wrapper for MODULE_SYSCTL_TABLE
>       sysctl, parport: update register_sysctl() callers with template arguments
>       sysctl, net: add register_net_sysctl{_sz}() wrappers for MODULE_SYSCTL_TABLE
>       sysctl, net: update register_net_sysctl{_sz}() callers with template arguments
>       sysctl, net: update register_net_sysctl_sz(ARRAY_SIZE(table_tmpl)) with template arguments
>       sysctl, ipv6: update register_net_sysctl{_sz}() callers with template arguments
>       sysctl, net: update register_net_sysctl_sz() edge case
>       sysctl: unrandomize struct ctl_table.procname
>       modpost: move addend_*_rel() calls into addend_rel()
>       modpost: handle MODULE_SYSCTL_TABLE symbols
> 
>  arch/riscv/kernel/vector.c              |   1 +
>  drivers/net/vrf.c                       |   3 +-
>  drivers/parport/procfs.c                |  26 +++-
>  fs/proc/Kconfig                         |  13 ++
>  fs/verity/init.c                        |   1 +
>  include/linux/key.h                     |   1 -
>  include/linux/mod_devicetable.h         |   7 +
>  include/linux/pid_namespace.h           |   1 +
>  include/linux/sysctl.h                  | 110 ++++++++++++++-
>  include/net/ipv6.h                      |   6 +-
>  include/net/net_namespace.h             |  47 ++++++-
>  net/bridge/br_netfilter_hooks.c         |   3 +-
>  net/core/neighbour.c                    |  11 +-
>  net/core/sysctl_net_core.c              |   3 +-
>  net/ieee802154/6lowpan/reassembly.c     |   2 +-
>  net/ipv4/devinet.c                      |  12 +-
>  net/ipv4/ip_fragment.c                  |   3 +-
>  net/ipv4/route.c                        |   3 +-
>  net/ipv4/sysctl_net_ipv4.c              |   2 +-
>  net/ipv4/xfrm4_policy.c                 |   3 +-
>  net/ipv6/addrconf.c                     |   7 +-
>  net/ipv6/icmp.c                         |   6 +-
>  net/ipv6/netfilter/nf_conntrack_reasm.c |   4 +-
>  net/ipv6/reassembly.c                   |   3 +-
>  net/ipv6/route.c                        |  10 +-
>  net/ipv6/sysctl_net_ipv6.c              |  13 +-
>  net/ipv6/xfrm6_policy.c                 |   3 +-
>  net/mpls/af_mpls.c                      |   9 +-
>  net/mptcp/ctrl.c                        |   3 +-
>  net/netfilter/ipvs/ip_vs_ctl.c          |   4 +-
>  net/netfilter/ipvs/ip_vs_lblc.c         |   3 +-
>  net/netfilter/ipvs/ip_vs_lblcr.c        |   3 +-
>  net/netfilter/nf_conntrack_standalone.c |   5 +-
>  net/netfilter/nf_log.c                  |   9 +-
>  net/rds/tcp.c                           |   4 +-
>  net/sctp/sysctl.c                       |   3 +-
>  net/smc/smc_sysctl.c                    |   2 +-
>  net/sysctl_net.c                        |  10 +-
>  net/unix/sysctl_net_unix.c              |   3 +-
>  net/vmw_vsock/af_vsock.c                |   4 +-
>  net/xfrm/xfrm_sysctl.c                  |   2 +-
>  scripts/mod/devicetable-offsets.c       |   6 +
>  scripts/mod/file2alias.c                | 237 ++++++++++++++++++++++++++++++++
>  scripts/mod/modpost.c                   |  46 ++++---
>  scripts/mod/modpost.h                   |  25 ++++
>  45 files changed, 575 insertions(+), 107 deletions(-)
> ---
> base-commit: 2697ef8943c9985c14708a6429e21812693857b2
> change-id: 20260818-sysctl-module-aliases-2f5801b1eb71
> 
> Best regards,
> -- 
> Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
>

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