linux-kernel - Re: [PATCH] proc: allow to mark /proc files permanent outside of fs/proc/

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Message-ID: <CAGudoHGs8LNQ9qT8k0dibgMLVoyjYM2FrNwgNMA-AzV5yz72GA@mail.gmail.com>
Date: Wed, 17 Sep 2025 05:44:45 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] proc: allow to mark /proc files permanent outside of fs/proc/

Hi. This fell through the cracks.

Can you please resubmit your patch with my name dropped from it? It's
basically all you.

Alternatively perhaps Christian will be happy to do the edits?

After this is sorted out I'll resend my thing altering how the content
of the file is generated.

On Wed, Apr 9, 2025 at 9:20 PM Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> From 06e2ff406942fef65b9c397a7f44478dd4b61451 Mon Sep 17 00:00:00 2001
> From: Alexey Dobriyan <adobriyan@...il.com>
> Date: Sat, 5 Apr 2025 14:50:10 +0300
> Subject: [PATCH 1/1] proc: allow to mark /proc files permanent outside of
>  fs/proc/
>
> From: Mateusz Guzik <mjguzik@...il.com>
>
> Add proc_make_permanent() function to mark PDE as permanent to speed up
> open/read/close (one alloc/free and lock/unlock less).
>
> Enable it for built-in code and for compiled-in modules.
> This function becomes nop magically in modular code.
>
> Use it on /proc/filesystems to add a user.
>
>                 Note, note, note!
>
> If built-in code creates and deletes PDEs dynamically (not in init
> hook), then proc_make_permanent() must not be used.
>
> It is intended for simple code:
>
>         static int __init xxx_module_init(void)
>         {
>                 g_pde = proc_create_single();
>                 proc_make_permanent(g_pde);
>                 return 0;
>         }
>         static void __exit xxx_module_exit(void)
>         {
>                 remove_proc_entry(g_pde);
>         }
>
> If module is built-in then exit hook never executed and PDE is
> permanent so it is OK to mark it as such.
>
> If module is module then rmmod will yank PDE, but proc_make_permanent()
> is nop and core /proc code will do everything right.
>
> [adobriyan@...il.com: unexport function (usual exporting is a bug)]
> [adobriyan@...il.com: rewrite changelog]
>
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
>  fs/filesystems.c        |  4 +++-
>  fs/proc/generic.c       | 12 ++++++++++++
>  fs/proc/internal.h      |  3 +++
>  include/linux/proc_fs.h | 10 ++++++++++
>  4 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/fs/filesystems.c b/fs/filesystems.c
> index 58b9067b2391..81dcd0ddadb6 100644
> --- a/fs/filesystems.c
> +++ b/fs/filesystems.c
> @@ -252,7 +252,9 @@ static int filesystems_proc_show(struct seq_file *m, void *v)
>
>  static int __init proc_filesystems_init(void)
>  {
> -       proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
> +       struct proc_dir_entry *pde =
> +               proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
> +       proc_make_permanent(pde);
>         return 0;
>  }
>  module_init(proc_filesystems_init);
> diff --git a/fs/proc/generic.c b/fs/proc/generic.c
> index a3e22803cddf..0342600c0172 100644
> --- a/fs/proc/generic.c
> +++ b/fs/proc/generic.c
> @@ -826,3 +826,15 @@ ssize_t proc_simple_write(struct file *f, const char __user *ubuf, size_t size,
>         kfree(buf);
>         return ret == 0 ? size : ret;
>  }
> +
> +/*
> + * Not exported to modules:
> + * modules' /proc files aren't permanent because modules aren't permanent.
> + */
> +void impl_proc_make_permanent(struct proc_dir_entry *pde);
> +void impl_proc_make_permanent(struct proc_dir_entry *pde)
> +{
> +       if (pde) {
> +               pde_make_permanent(pde);
> +       }
> +}
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index 96122e91c645..885b1cd38020 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -80,8 +80,11 @@ static inline bool pde_is_permanent(const struct proc_dir_entry *pde)
>         return pde->flags & PROC_ENTRY_PERMANENT;
>  }
>
> +/* This is for builtin code, not even for modules which are compiled in. */
>  static inline void pde_make_permanent(struct proc_dir_entry *pde)
>  {
> +       /* Ensure magic flag does something. */
> +       static_assert(PROC_ENTRY_PERMANENT != 0);
>         pde->flags |= PROC_ENTRY_PERMANENT;
>  }
>
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index ea62201c74c4..2d59f29b49eb 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -247,4 +247,14 @@ static inline struct pid_namespace *proc_pid_ns(struct super_block *sb)
>
>  bool proc_ns_file(const struct file *file);
>
> +static inline void proc_make_permanent(struct proc_dir_entry *pde)
> +{
> +       /* Don't give matches to modules. */
> +#if defined CONFIG_PROC_FS && !defined MODULE
> +       /* This mess is created by defining "struct proc_dir_entry" elsewhere. */
> +       void impl_proc_make_permanent(struct proc_dir_entry *pde);
> +       impl_proc_make_permanent(pde);
> +#endif
> +}
> +
>  #endif /* _LINUX_PROC_FS_H */
> --
> 2.47.0
>