linux-kernel - Re: [PATCH 7/8] autofs: convert autofs to use the new mount api

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Message-ID: <7121dcf7-23fe-07a5-9df4-9ea7af6f5964@themaw.net>
Date:   Fri, 22 Sep 2023 21:27:38 +0800
From:   Ian Kent <raven@...maw.net>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Al Viro <viro@...IV.linux.org.uk>,
        autofs mailing list <autofs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bill O'Donnell <billodo@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 7/8] autofs: convert autofs to use the new mount api

On 22/9/23 19:59, Christian Brauner wrote:
>> +	fsparam_fd	("fd",			Opt_fd),
>> +/*
>> + * Open the fd.  We do it here rather than in get_tree so that it's done in the
>> + * context of the system call that passed the data and not the one that
>> + * triggered the superblock creation, lest the fd gets reassigned.
>> + */
>> +static int autofs_parse_fd(struct fs_context *fc, int fd)
>>   {
>> +	struct autofs_sb_info *sbi = fc->s_fs_info;
>>   	struct file *pipe;
>>   	int ret;
>>   
>>   	pipe = fget(fd);
>>   	if (!pipe) {
>> -		pr_err("could not open pipe file descriptor\n");
>> +		errorf(fc, "could not open pipe file descriptor");
>>   		return -EBADF;
>>   	}
>>   
>>   	ret = autofs_check_pipe(pipe);
>>   	if (ret < 0) {
>> -		pr_err("Invalid/unusable pipe\n");
>> +		errorf(fc, "Invalid/unusable pipe");
>>   		fput(pipe);
>>   		return -EBADF;
>>   	}
>> +static int autofs_parse_param(struct fs_context *fc, struct fs_parameter *param)
>>   {
>> +		return autofs_parse_fd(fc, result.int_32);
> Mah, so there's a difference between the new and the old mount api that we
> should probably hide on the VFS level for fsparam_fd. Basically, if you're
> coming through the new mount api via fsconfig(FSCONFIG_SET_FD, fd) then the vfs
> will have done param->file = fget(fd) for you already so there's no need to
> call fget() again. We can just take ownership of the reference that the vfs
> took for us.
>
> But if we're coming in through the old mount api then we need to call fget.
> There's nothing wrong with your code but it doesn't take advantage of the new
> mount api which would be unfortunate. So I folded a small extension into this
> see [1].
>
> There's an unrelated bug in fs_param_is_fd() that I'm also fixing see [2].

Ok, that's interesting, I'll have a look at these developments tomorrow,

admittedly (obviously) I hadn't looked at the code for some time ...


Ian

>
> I've tested both changes with the old and new mount api.
>
> [1]:
> diff --git a/fs/autofs/inode.c b/fs/autofs/inode.c
> index 3f2dfed428f9..0477bce7d277 100644
> --- a/fs/autofs/inode.c
> +++ b/fs/autofs/inode.c
> @@ -150,13 +150,20 @@ struct autofs_fs_context {
>    * context of the system call that passed the data and not the one that
>    * triggered the superblock creation, lest the fd gets reassigned.
>    */
> -static int autofs_parse_fd(struct fs_context *fc, int fd)
> +static int autofs_parse_fd(struct fs_context *fc, struct autofs_sb_info *sbi,
> +                          struct fs_parameter *param,
> +                          struct fs_parse_result *result)
>   {
> -       struct autofs_sb_info *sbi = fc->s_fs_info;
>          struct file *pipe;
>          int ret;
>
> -       pipe = fget(fd);
> +       if (param->type == fs_value_is_file) {
> +               /* came through the new api */
> +               pipe = param->file;
> +               param->file = NULL;
> +       } else {
> +               pipe = fget(result->uint_32);
> +       }
>          if (!pipe) {
>                  errorf(fc, "could not open pipe file descriptor");
>                  return -EBADF;
> @@ -165,14 +172,15 @@ static int autofs_parse_fd(struct fs_context *fc, int fd)
>          ret = autofs_check_pipe(pipe);
>          if (ret < 0) {
>                  errorf(fc, "Invalid/unusable pipe");
> -               fput(pipe);
> +               if (param->type != fs_value_is_file)
> +                       fput(pipe);
>                  return -EBADF;
>          }
>
>          if (sbi->pipe)
>                  fput(sbi->pipe);
>
> -       sbi->pipefd = fd;
> +       sbi->pipefd = result->uint_32;
>          sbi->pipe = pipe;
>
>          return 0;
>
> [2]:
>  From 2f9171200505c82e744a235c85377e36ed190109 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@...nel.org>
> Date: Fri, 22 Sep 2023 13:49:05 +0200
> Subject: [PATCH] fsconfig: ensure that dirfd is set to aux
>
> The code in fs_param_is_fd() expects param->dirfd to be set to the fd
> that was used to set param->file to initialize result->uint_32. So make
> sure it's set so users like autofs using FSCONFIG_SET_FD with the new
> mount api can rely on this to be set to the correct value.
>
> Signed-off-by: Christian Brauner <brauner@...nel.org>
> ---
>   fs/fsopen.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/fs/fsopen.c b/fs/fsopen.c
> index ce03f6521c88..6593ae518115 100644
> --- a/fs/fsopen.c
> +++ b/fs/fsopen.c
> @@ -465,6 +465,7 @@ SYSCALL_DEFINE5(fsconfig,
>   		param.file = fget(aux);
>   		if (!param.file)
>   			goto out_key;
> +		param.dirfd = aux;
>   		break;
>   	default:
>   		break;