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

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Message-ID: <CAGudoHGtvBArbAhpynYLd=FzshR+UM-qx=n_1wOq1BPqW9nTXA@mail.gmail.com>
Date: Wed, 9 Apr 2025 22:27:12 +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/

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 */

This diff should not be changing /proc/filesystems, that's for the
other patch to do.

So I think this patch is all you and my name needs to be dropped from
it, along with marking /proc/filesystems as permanent.

Given that you kept the name (proc_make_permanent), the
fs/filesystems.c side of things is identical so my first patch can be
just swapped for this one.

Alternatively I can resend the patchset with this (augmented as
described above).
-- 
Mateusz Guzik <mjguzik gmail.com>