[PATCH] Add new syscall \`inotify_add_watch_fd'.

[Giuseppe Scrivano]

It makes possible to register a file to be watched by inotify using
its file descriptor instead of its path.
---
 arch/x86/kernel/syscall_table_32.S |    1 +
 fs/notify/inotify/inotify_user.c   |   54 ++++++++++++++++++++++++++++++++++--
 include/linux/syscalls.h           |    3 ++
 3 files changed, 55 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S
index d51321d..46eb4ac 100644
--- a/arch/x86/kernel/syscall_table_32.S
+++ b/arch/x86/kernel/syscall_table_32.S
@@ -336,3 +336,4 @@ ENTRY(sys_call_table)
 	.long sys_pwritev
 	.long sys_rt_tgsigqueueinfo	/* 335 */
 	.long sys_perf_counter_open
+	.long sys_inotify_add_watch_fd
diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index dcd2040..53f3c41 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -747,16 +747,64 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname,
 
 	/* create/update an inode mark */
 	ret = inotify_update_watch(group, inode, mask);
-	if (unlikely(ret))
-		goto path_put_and_out;
 
-path_put_and_out:
 	path_put(&path);
 fput_and_out:
 	fput_light(filp, fput_needed);
 	return ret;
 }
 
+SYSCALL_DEFINE3(inotify_add_watch_fd, int, wd, int, fd,	u32, mask)
+{
+	struct fsnotify_group *group;
+	struct inode *inode;
+	struct path path;
+	struct file *filp;
+	struct file *filp2;
+	int ret, fput_needed, fput2_needed;
+	unsigned flags = 0;
+
+	filp = fget_light(wd, &fput_needed);
+	if (unlikely(!filp))
+		return -EBADF;
+
+	/* verify that this is indeed an inotify instance */
+	if (unlikely(filp->f_op != &inotify_fops)) {
+		ret = -EINVAL;
+		goto fput_and_out;
+	}
+
+	if (!(mask & IN_DONT_FOLLOW))
+		flags |= LOOKUP_FOLLOW;
+	if (mask & IN_ONLYDIR)
+		flags |= LOOKUP_DIRECTORY;
+
+	filp2 = fget_light(fd, &fput2_needed);
+
+	if (unlikely(!filp2)){
+		ret = -EBADF;
+		goto fput_and_out;
+	}
+
+	/* you can only watch an inode if you have read permissions on it */
+	ret = inode_permission(filp2->f_path.dentry->d_inode, MAY_READ);
+	if (ret)
+		goto fput2_and_out;
+
+	/* inode held in place by reference to path; group by fget on fd */
+	inode = filp2->f_path.dentry->d_inode;
+	group = filp->private_data;
+
+	/* create/update an inode mark */
+  ret = inotify_update_watch(group, inode, mask);
+
+fput2_and_out:
+	fput_light(filp2, fput2_needed);
+fput_and_out:
+	fput_light(filp, fput_needed);
+	return ret;
+}
+
 SYSCALL_DEFINE2(inotify_rm_watch, int, fd, __s32, wd)
 {
 	struct fsnotify_group *group;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 80de700..0833b1d 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -761,4 +761,7 @@ int kernel_execve(const char *filename, char *const argv[], char *const envp[]);
 asmlinkage long sys_perf_counter_open(
 		struct perf_counter_attr __user *attr_uptr,
 		pid_t pid, int cpu, int group_fd, unsigned long flags);
+
+
+asmlinkage long sys_inotify_add_watch_fd(int wd, int fd, u32 mask);
 #endif
-- 
1.6.3.3
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