Date:   Mon, 22 Nov 2021 16:07:59 -0800
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     Alexey Dobriyan <adobriyan@...il.com>, viro@...iv.linux.org.uk,
        ebiederm@...ssion.com
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] proc: "mount -o lookup=..." support

Hi Alexey,

On 11/22/21 11:23, Alexey Dobriyan wrote:
> Docker implements MaskedPaths configuration option
> 
> 	https://github.com/estesp/docker/blob/9c15e82f19b0ad3c5fe8617a8ec2dddc6639f40a/oci/defaults.go#L97
> 
> to disable certain /proc files. It does overmount with /dev/null per
> masked file.
> 
> Give them proper mount option which selectively disables lookup/readdir
> so that MaskedPaths doesn't need to be updated as time goes on.
> 
> Syntax is
> 
> 	mount -t proc proc -o lookup=cpuinfo/uptime /proc
> 
> 	# ls /proc
> 				...
> 	dr-xr-xr-x   8 root       root          0 Nov 22 21:12 995
> 	-r--r--r--   1 root       root          0 Nov 22 21:12 cpuinfo
> 	lrwxrwxrwx   1 root       root          0 Nov 22 21:12 self -> 1163
> 	lrwxrwxrwx   1 root       root          0 Nov 22 21:12 thread-self -> 1163/task/1163
> 	-r--r--r--   1 root       root          0 Nov 22 21:12 uptime
> 
> Works at top level only (1 lookup list per superblock)
> Trailing slash is optional but saves 1 allocation.
> 
> TODO:
> 	think what to do with dcache entries across "mount -o remount,lookup=".
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
> 
>   fs/proc/generic.c       |   19 +++++--
>   fs/proc/internal.h      |   23 +++++++++
>   fs/proc/proc_net.c      |    2
>   fs/proc/root.c          |  115 ++++++++++++++++++++++++++++++++++++++++++++++++
>   include/linux/proc_fs.h |    2
>   5 files changed, 152 insertions(+), 9 deletions(-)
> 
> --- a/fs/proc/generic.c
> +++ b/fs/proc/generic.c
> @@ -282,7 +282,7 @@ struct dentry *proc_lookup(struct inode *dir, struct dentry *dentry,
>    * for success..
>    */
>   int proc_readdir_de(struct file *file, struct dir_context *ctx,
> -		    struct proc_dir_entry *de)
> +		    struct proc_dir_entry *de, const struct proc_lookup_list *ll)
>   {
>   	int i;
>   
> @@ -305,14 +305,18 @@ int proc_readdir_de(struct file *file, struct dir_context *ctx,
>   
>   	do {
>   		struct proc_dir_entry *next;
> +
>   		pde_get(de);
>   		read_unlock(&proc_subdir_lock);
> -		if (!dir_emit(ctx, de->name, de->namelen,
> -			    de->low_ino, de->mode >> 12)) {
> -			pde_put(de);
> -			return 0;
> +
> +		if (ll ? in_lookup_list(ll, de->name, de->namelen) : true) {

This looks a bit odd, what about the following?

     if (!ll || in_lookup_list(ll, de->name, de->namelen))

But this is maybe just a personal preference.

> +			if (!dir_emit(ctx, de->name, de->namelen, de->low_ino, de->mode >> 12)) {
> +				pde_put(de);
> +				return 0;
> +			}
> +			ctx->pos++;
>   		}
> -		ctx->pos++;
> +
>   		read_lock(&proc_subdir_lock);
>   		next = pde_subdir_next(de);
>   		pde_put(de);
> @@ -330,7 +334,8 @@ int proc_readdir(struct file *file, struct dir_context *ctx)
>   	if (fs_info->pidonly == PROC_PIDONLY_ON)
>   		return 1;
>   
> -	return proc_readdir_de(file, ctx, PDE(inode));
> +	return proc_readdir_de(file, ctx, PDE(inode),
> +				PDE(inode) == &proc_root ? fs_info->lookup_list : NULL);
>   }
>   
>   /*
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -190,7 +190,7 @@ struct proc_dir_entry *proc_register(struct proc_dir_entry *dir,
>   extern struct dentry *proc_lookup(struct inode *, struct dentry *, unsigned int);
>   struct dentry *proc_lookup_de(struct inode *, struct dentry *, struct proc_dir_entry *);
>   extern int proc_readdir(struct file *, struct dir_context *);
> -int proc_readdir_de(struct file *, struct dir_context *, struct proc_dir_entry *);
> +int proc_readdir_de(struct file *, struct dir_context *, struct proc_dir_entry *, const struct proc_lookup_list *);
>   
>   static inline void pde_get(struct proc_dir_entry *pde)
>   {
> @@ -318,3 +318,24 @@ static inline void pde_force_lookup(struct proc_dir_entry *pde)
>   	/* /proc/net/ entries can be changed under us by setns(CLONE_NEWNET) */
>   	pde->proc_dops = &proc_net_dentry_ops;
>   }
> +
> +/*
> + * "cpuinfo", "uptime" is represented as
> + *
> + *	(u8[]){
> + *		7, 'c', 'p', 'u', 'i', 'n', 'f', 'o',
> + *		6, 'u', 'p', 't', 'i', 'm', 'e',
> + *		0
> + *	}
> + */
> +struct proc_lookup_list {
> +	u8 len;
> +	char str[];
> +};
> +
> +static inline struct proc_lookup_list *lookup_list_next(const struct proc_lookup_list *ll)
> +{
> +	return (struct proc_lookup_list *)((void *)ll + 1 + ll->len);
> +}
> +
> +bool in_lookup_list(const struct proc_lookup_list *ll, const char *str, unsigned int len);
> --- a/fs/proc/proc_net.c
> +++ b/fs/proc/proc_net.c
> @@ -321,7 +321,7 @@ static int proc_tgid_net_readdir(struct file *file, struct dir_context *ctx)
>   	ret = -EINVAL;
>   	net = get_proc_task_net(file_inode(file));
>   	if (net != NULL) {
> -		ret = proc_readdir_de(file, ctx, net->proc_net);
> +		ret = proc_readdir_de(file, ctx, net->proc_net, NULL);
>   		put_net(net);
>   	}
>   	return ret;
> --- a/fs/proc/root.c
> +++ b/fs/proc/root.c
> @@ -35,18 +35,22 @@ struct proc_fs_context {
>   	enum proc_hidepid	hidepid;
>   	int			gid;
>   	enum proc_pidonly	pidonly;
> +	struct proc_lookup_list	*lookup_list;
> +	unsigned int		lookup_list_len;
>   };
>   
>   enum proc_param {
>   	Opt_gid,
>   	Opt_hidepid,
>   	Opt_subset,
> +	Opt_lookup,
>   };
>   
>   static const struct fs_parameter_spec proc_fs_parameters[] = {
>   	fsparam_u32("gid",	Opt_gid),
>   	fsparam_string("hidepid",	Opt_hidepid),
>   	fsparam_string("subset",	Opt_subset),
> +	fsparam_string("lookup",	Opt_lookup),
>   	{}
>   };
>   
> @@ -112,6 +116,65 @@ static int proc_parse_subset_param(struct fs_context *fc, char *value)
>   	return 0;
>   }
>   
> +static int proc_parse_lookup_param(struct fs_context *fc, char *str0)
> +{
> +	struct proc_fs_context *ctx = fc->fs_private;
> +	struct proc_lookup_list *ll;
> +	char *str;
> +	const char *slash;
> +	const char *src;
> +	unsigned int len;
> +	int rv;
> +
> +	/* Force trailing slash, simplify loops below. */
> +	len = strlen(str0);
> +	if (len > 0 && str0[len - 1] == '/') {
> +		str = str0;
> +	} else {
> +		str = kmalloc(len + 2, GFP_KERNEL);
> +		if (!str) {
> +			rv = -ENOMEM;
> +			goto out;
> +		}
> +		memcpy(str, str0, len);
> +		str[len] = '/';
> +		str[len + 1] = '\0';
> +	}
> +
> +	len = 0;
> +	for (src = str; (slash = strchr(src, '/')); src = slash + 1) {
> +		if (slash - src >= 256) {
> +			rv = -EINVAL;
> +			goto out_free_str;
> +		}
> +		len += 1 + (slash - src);
> +	}
> +	len += 1;
> +
> +	ctx->lookup_list = ll = kmalloc(len, GFP_KERNEL);
> +	ctx->lookup_list_len = len;
> +	if (!ll) {
> +		rv = -ENOMEM;
> +		goto out_free_str;
> +	}
> +
> +	for (src = str; (slash = strchr(src, '/')); src = slash + 1) {
> +		ll->len = slash - src;
> +		memcpy(ll->str, src, ll->len);
> +		ll = lookup_list_next(ll);
> +	}
> +	ll->len = 0;
> +
> +	rv = 0;
> +
> +out_free_str:
> +	if (str != str0) {
> +		kfree(str);
> +	}
> +out:
> +	return rv;
> +}
> +
>   static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
>   {
>   	struct proc_fs_context *ctx = fc->fs_private;
> @@ -137,6 +200,11 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
>   			return -EINVAL;
>   		break;
>   
> +	case Opt_lookup:
> +		if (proc_parse_lookup_param(fc, param->string) < 0)
> +			return -EINVAL;
> +		break;
> +
>   	default:
>   		return -EINVAL;
>   	}
> @@ -157,6 +225,10 @@ static void proc_apply_options(struct proc_fs_info *fs_info,
>   		fs_info->hide_pid = ctx->hidepid;
>   	if (ctx->mask & (1 << Opt_subset))
>   		fs_info->pidonly = ctx->pidonly;
> +	if (ctx->mask & (1 << Opt_lookup)) {
> +		fs_info->lookup_list = ctx->lookup_list;
> +		ctx->lookup_list = NULL;
> +	}
>   }
>   
>   static int proc_fill_super(struct super_block *s, struct fs_context *fc)
> @@ -234,11 +306,34 @@ static void proc_fs_context_free(struct fs_context *fc)
>   	struct proc_fs_context *ctx = fc->fs_private;
>   
>   	put_pid_ns(ctx->pid_ns);
> +	kfree(ctx->lookup_list);
>   	kfree(ctx);
>   }
>   
> +static int proc_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
> +{
> +	struct proc_fs_context *src = fc->fs_private;
> +	struct proc_fs_context *dst;
> +
> +	dst = kmemdup(src, sizeof(struct proc_fs_context), GFP_KERNEL);
> +	if (!dst) {
> +		return -ENOMEM;
> +	}
> +
> +	get_pid_ns(dst->pid_ns);
> +	dst->lookup_list = kmemdup(dst->lookup_list, dst->lookup_list_len, GFP_KERNEL);

If dst->lookup_list is NULL and dst->lookup_list_len is 0 (which seems 
to be the default state if lookup= is not passed), then kmemdup will end 
up calling __kmalloc_track_caller() with size 0. Each of the sl[auo]b 
implementations returns ZERO_SIZE_POINTER in this case, which is just 
((void *)16).

This is safe to pass to kfree et al. I was worried that if 
ZERO_SIZE_POINTER gets copied into the proc_fs_info, then it could get 
dereferenced and cause a nasty crash. But that can only happen if 
Opt_lookup is in ctx->mask, at which point we're guaranteed that 
lookup_list is not ZERO_SIZE_POINTER.

Just wanted to document why this is safe, as it caught my eye at first.

> +	if (!dst->lookup_list) {
> +		kfree(dst);
> +		return -ENOMEM;
> +	}
> +
> +	fc->fs_private = dst;
> +	return 0;
> +}
> +
>   static const struct fs_context_operations proc_fs_context_ops = {
>   	.free		= proc_fs_context_free,
> +	.dup		= proc_fs_context_dup,

 From my reading of vfs_dup_fs_context, it seems like if the dup() 
operation doesn't exist, the operation is simply bailed out and not 
supported.

339  struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc)
340  {
341  	struct fs_context *fc;
342  	int ret;
343
344  	if (!src_fc->ops->dup)
345  		return ERR_PTR(-EOPNOTSUPP);

So this patch is also adding support for the dup() operation where there 
wasn't before. Is that significant to call out in the changelog or split 
out as a separate patch?

Thanks,
Stephen

>   	.parse_param	= proc_parse_param,
>   	.get_tree	= proc_get_tree,
>   	.reconfigure	= proc_reconfigure,
> @@ -274,6 +369,7 @@ static void proc_kill_sb(struct super_block *sb)
>   
>   	kill_anon_super(sb);
>   	put_pid_ns(fs_info->pid_ns);
> +	kfree(fs_info->lookup_list);
>   	kfree(fs_info);
>   }
>   
> @@ -317,11 +413,30 @@ static int proc_root_getattr(struct user_namespace *mnt_userns,
>   	return 0;
>   }
>   
> +bool in_lookup_list(const struct proc_lookup_list *ll, const char *str, unsigned int len)
> +{
> +	while (ll->len > 0) {
> +		if (ll->len == len && strncmp(ll->str, str, len) == 0) {
> +			return true;
> +		}
> +		ll = lookup_list_next(ll);
> +	}
> +	return false;
> +}
> +
>   static struct dentry *proc_root_lookup(struct inode * dir, struct dentry * dentry, unsigned int flags)
>   {
> +	struct proc_fs_info *proc_sb = proc_sb_info(dir->i_sb);
> +
>   	if (!proc_pid_lookup(dentry, flags))
>   		return NULL;
>   
> +	/* Top level only for now */
> +	if (proc_sb->lookup_list &&
> +	    !in_lookup_list(proc_sb->lookup_list, dentry->d_name.name, dentry->d_name.len)) {
> +		    return NULL;
> +	}
> +
>   	return proc_lookup(dir, dentry, flags);
>   }
>   
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -10,6 +10,7 @@
>   #include <linux/fs.h>
>   
>   struct proc_dir_entry;
> +struct proc_lookup_list;
>   struct seq_file;
>   struct seq_operations;
>   
> @@ -65,6 +66,7 @@ struct proc_fs_info {
>   	kgid_t pid_gid;
>   	enum proc_hidepid hide_pid;
>   	enum proc_pidonly pidonly;
> +	const struct proc_lookup_list *lookup_list;
>   };
>   
>   static inline struct proc_fs_info *proc_sb_info(struct super_block *sb)
> 

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