linux-kernel - Re: [PATCH v2] proc: alloc PATH_MAX bytes for /proc/${pid}/fd/ symlinks

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Message-ID: <CA+PpKPn3tVsJnijvATXBwfbd_SwcmRps-A-dCJTqi5WYx1Smfg@mail.gmail.com>
Date:   Sun, 23 Jan 2022 22:23:33 +0800
From:   Hao Lee <haolee.swjtu@...il.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        christian.brauner@...ntu.com, Kees Cook <keescook@...omium.org>,
        jamorris@...ux.microsoft.com, linux-fsdevel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] proc: alloc PATH_MAX bytes for /proc/${pid}/fd/ symlinks

On Sun, Jan 23, 2022 at 9:58 PM Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> From: Hao Lee <haolee.swjtu@...il.com>
>
> It's not a standard approach that use __get_free_page() to alloc path
> buffer directly. We'd better use kmalloc and PATH_MAX.
>
>         PAGE_SIZE is different on different archs.

This is indeed a worthy addition. Thanks.

Regards,
Hao Lee

> An unlinked file
>         with very long canonical pathname will readlink differently
>         because "(deleted)" eats into a buffer. --adobriyan
>
> Signed-off-by: Hao Lee <haolee.swjtu@...il.com>
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
>
>  fs/proc/base.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1764,25 +1764,25 @@ static const char *proc_pid_get_link(struct dentry *dentry,
>
>  static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
>  {
> -       char *tmp = (char *)__get_free_page(GFP_KERNEL);
> +       char *tmp = (char *)kmalloc(PATH_MAX, GFP_KERNEL);
>         char *pathname;
>         int len;
>
>         if (!tmp)
>                 return -ENOMEM;
>
> -       pathname = d_path(path, tmp, PAGE_SIZE);
> +       pathname = d_path(path, tmp, PATH_MAX);
>         len = PTR_ERR(pathname);
>         if (IS_ERR(pathname))
>                 goto out;
> -       len = tmp + PAGE_SIZE - 1 - pathname;
> +       len = tmp + PATH_MAX - 1 - pathname;
>
>         if (len > buflen)
>                 len = buflen;
>         if (copy_to_user(buffer, pathname, len))
>                 len = -EFAULT;
>   out:
> -       free_page((unsigned long)tmp);
> +       kfree(tmp);
>         return len;
>  }
>