linux-kernel - [PATCH 08/17] proc: merge proc_pid_readlink() into proc_pid_get

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Message-Id: <1473708559-12714-9-git-send-email-mszeredi@redhat.com>
Date:   Mon, 12 Sep 2016 21:29:10 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: [PATCH 08/17] proc: merge proc_pid_readlink() into proc_pid_get_link()

This will allow us to move the generic readlink logic into the VFS and get
rid of the readlink method.

And it's a cleanup, removing more lines than it adds, since the two
functions have a lot in common.

/proc/$$/map_files/A allowed reading the symlink with the normal proc
permission checks, but following only allowed for CAP_SYS_ADMIN capable
tasks.  So in proc_map_files_get_link() check for is_following_link()
before bailing out if not CAP_SYS_ADMIN capable.

Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
---
 fs/proc/base.c | 68 ++++++++++++++++++----------------------------------------
 1 file changed, 21 insertions(+), 47 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ac0df4dde823..84769c763afe 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1578,6 +1578,7 @@ static const char *proc_pid_get_link(struct dentry *dentry,
 {
 	struct path path;
 	int error = -EACCES;
+	char *res;
 
 	if (!dentry)
 		return ERR_PTR(-ECHILD);
@@ -1590,58 +1591,31 @@ static const char *proc_pid_get_link(struct dentry *dentry,
 	if (error)
 		goto out;
 
-	nd_jump_link(&path);
-	return NULL;
-out:
-	return ERR_PTR(error);
-}
-
-static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
-{
-	char *tmp = (char*)__get_free_page(GFP_TEMPORARY);
-	char *pathname;
-	int len;
-
-	if (!tmp)
-		return -ENOMEM;
-
-	pathname = d_path(path, tmp, PAGE_SIZE);
-	len = PTR_ERR(pathname);
-	if (IS_ERR(pathname))
-		goto out;
-	len = tmp + PAGE_SIZE - 1 - pathname;
-
-	if (len > buflen)
-		len = buflen;
-	if (copy_to_user(buffer, pathname, len))
-		len = -EFAULT;
- out:
-	free_page((unsigned long)tmp);
-	return len;
-}
-
-static int proc_pid_readlink(struct dentry * dentry, char __user * buffer, int buflen)
-{
-	int error = -EACCES;
-	struct inode *inode = d_inode(dentry);
-	struct path path;
+	if (is_following_link()) {
+		nd_jump_link(&path);
+		res = NULL;
+	} else {
+		char *buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
 
-	/* Are we allowed to snoop on the tasks file descriptors? */
-	if (!proc_fd_access_allowed(inode))
-		goto out;
+		error = -ENOMEM;
+		if (!buf)
+			goto out;
 
-	error = PROC_I(inode)->op.proc_get_link(dentry, &path);
-	if (error)
-		goto out;
+		res = d_path(&path, buf, PAGE_SIZE);
+		if (IS_ERR(res))
+			kfree(buf);
+		else
+			set_delayed_call(done, kfree_link, buf);
+	}
+	return res;
 
-	error = do_proc_readlink(&path, buffer, buflen);
-	path_put(&path);
 out:
-	return error;
+	return ERR_PTR(error);
 }
 
+
 const struct inode_operations proc_pid_link_inode_operations = {
-	.readlink	= proc_pid_readlink,
+	.readlink	= generic_readlink,
 	.get_link	= proc_pid_get_link,
 	.setattr	= proc_setattr,
 };
@@ -1966,7 +1940,7 @@ proc_map_files_get_link(struct dentry *dentry,
 			struct inode *inode,
 		        struct delayed_call *done)
 {
-	if (!capable(CAP_SYS_ADMIN))
+	if (is_following_link() && !capable(CAP_SYS_ADMIN))
 		return ERR_PTR(-EPERM);
 
 	return proc_pid_get_link(dentry, inode, done);
@@ -1976,7 +1950,7 @@ proc_map_files_get_link(struct dentry *dentry,
  * Identical to proc_pid_link_inode_operations except for get_link()
  */
 static const struct inode_operations proc_map_files_link_inode_operations = {
-	.readlink	= proc_pid_readlink,
+	.readlink	= generic_readlink,
 	.get_link	= proc_map_files_get_link,
 	.setattr	= proc_setattr,
 };
-- 
2.5.5