Re: [PATCH v5 3/3] fs: pipe2: Support O_SPECIFIC_FD

[Josh Triplett]

On Wed, Apr 22, 2020 at 05:44:38PM +0200, Florian Weimer wrote:
> * Josh Triplett:
> > This allows the caller of pipe2 to specify one or both file descriptors
> > rather than having them automatically use the lowest available file
> > descriptor. The caller can specify either file descriptor as -1 to
> > allow that file descriptor to use the lowest available.
> >
> > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > ---
> >  fs/pipe.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/pipe.c b/fs/pipe.c
> > index 16fb72e9abf7..4681a0d1d587 100644
> > --- a/fs/pipe.c
> > +++ b/fs/pipe.c
> > @@ -936,19 +936,19 @@ static int __do_pipe_flags(int *fd, struct file **files, int flags)
> >  	int error;
> >  	int fdw, fdr;
> >  
> > -	if (flags & ~(O_CLOEXEC | O_NONBLOCK | O_DIRECT))
> > +	if (flags & ~(O_CLOEXEC | O_NONBLOCK | O_DIRECT | O_SPECIFIC_FD))
> >  		return -EINVAL;
> >  
> >  	error = create_pipe_files(files, flags);
> >  	if (error)
> >  		return error;
> >  
> > -	error = get_unused_fd_flags(flags);
> > +	error = get_specific_unused_fd_flags(fd[0], flags);
> >  	if (error < 0)
> >  		goto err_read_pipe;
> >  	fdr = error;
> >  
> > -	error = get_unused_fd_flags(flags);
> > +	error = get_specific_unused_fd_flags(fd[1], flags);
> >  	if (error < 0)
> >  		goto err_fdr;
> >  	fdw = error;
> > @@ -969,7 +969,11 @@ static int __do_pipe_flags(int *fd, struct file **files, int flags)
> >  int do_pipe_flags(int *fd, int flags)
> >  {
> >  	struct file *files[2];
> > -	int error = __do_pipe_flags(fd, files, flags);
> > +	int error;
> > +
> > +	if (flags & O_SPECIFIC_FD)
> > +		return -EINVAL;
> > +	error = __do_pipe_flags(fd, files, flags);
> >  	if (!error) {
> >  		fd_install(fd[0], files[0]);
> >  		fd_install(fd[1], files[1]);
> > @@ -987,6 +991,10 @@ static int do_pipe2(int __user *fildes, int flags)
> >  	int fd[2];
> >  	int error;
> >  
> > +	if (flags & O_SPECIFIC_FD)
> > +		if (copy_from_user(fd, fildes, sizeof(fd)))
> > +			return -EFAULT;
> > +
> >  	error = __do_pipe_flags(fd, files, flags);
> >  	if (!error) {
> >  		if (unlikely(copy_to_user(fildes, fd, sizeof(fd)))) {
> 
> Mark, I think this will need (or at least benefit from) some valgrind
> changes.

Yes, this makes pipe2 read the memory of its first argument from
userspace, if and only if its second argument contains the O_SPECIFIC_FD
flag.

- Josh Triplett