Re: [PATCH 1/3] readfile: implement readfile syscall

On Sat, Jul 4, 2020 at 4:03 PM Greg Kroah-Hartman gregkh@linuxfoundation.org wrote:

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;
  

Manpage says: "doing the sequence of open() and then read() and then close()", which is exactly what it does.

But then it goes on to say: "If the file is larger than the value provided in count then only count number of bytes will be copied into buf", which is only half true, it should be: "If the file is larger than the value provided in count then at most count number of bytes will be copied into buf", which is not a lot of information.

And "If the size of file is smaller than the value provided in count then the whole file will be copied into buf", which is simply a lie; for example seq_file will happily return a smaller-than-PAGE_SIZE chunk if at least one record fits in there. You'll have a very hard time explaining that in the man page. So I think there are two possible ways forward:

1) just leave the first explanation (it's an open + read + close equivalent) and leave out the rest

2) add a loop around the vfs_read() in the code.

I'd strongly prefer #2 because with the non-looping read it's impossible to detect whether the file was completely read or not, and that's just going to lead to surprises and bugs in userspace code.

Thanks, Miklos