[PATCH 1/3] readfile: implement readfile syscall

Greg Kroah-Hartman
2020-07-04T14:03:02Z

It's a tiny syscall, meant to allow a user to do a single "open this
file, read into this buffer, and close the file" all in a single shot.

Should be good for reading "tiny" files like sysfs, procfs, and other
"small" files.

There is no restarting the syscall, this is a "simple" syscall, with the
attempt to make reading "simple" files easier with less syscall
overhead.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/open.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 50 insertions(+)

diff --git a/fs/open.c b/fs/open.c
index 6cd48a61cda3..4469faa9379c 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1370,3 +1370,53 @@ int stream_open(struct inode *inode, struct file *filp)
}

EXPORT_SYMBOL(stream_open);
+
+static struct file *readfile_open(int dfd, const char __user *filename,
+ struct open_flags *op)
+{
+ struct filename *tmp;
+ struct file *f;
+
+ tmp = getname(filename);
+ if (IS_ERR(tmp))
+ return (struct file *)tmp;
+
+ f = do_filp_open(dfd, tmp, op);
+ if (!IS_ERR(f))
+ fsnotify_open(f);
+
+ putname(tmp);
+ return f;
+}
+
+SYSCALL_DEFINE5(readfile, int, dfd, const char __user *, filename,
+ char __user *, buffer, size_t, bufsize, int, flags)
+{
+ struct open_flags op;
+ struct open_how how;
+ struct file *file;
+ loff_t pos = 0;
+ int retval;
+
+ /* only accept a small subset of O_ flags that make sense */
+ if ((flags & (O_NOFOLLOW | O_NOATIME)) != flags)
+ return -EINVAL;
+
+ /* add some needed flags to be able to open the file properly */
+ flags |= O_RDONLY | O_LARGEFILE;
+
+ how = build_open_how(flags, 0000);
+ retval = build_open_flags(&how, &op);
+ if (retval)
+ return retval;
+
+ file = readfile_open(dfd, filename, &op);
+ if (IS_ERR(file))
+ return PTR_ERR(file);
+
+ retval = vfs_read(file, buffer, bufsize, &pos);
+
+ filp_close(file, NULL);
+
+ return retval;
+}
--
2.27.0