Re: [musl] [PATCH] stdio: Fix fdopen bug

[zhangtianci <zhangtianci1@huawei.com>]

> On Wed, Feb 19, 2020 at 10:37:29AM +0800, Zhang Tianci wrote:
> > Currently, in musl the fdopen doesn't check the consistence between
> > fd's mode and corresponding file's mode.
> >
> > For example,
> >
> > int fd = open("file1", O_RDONLY);
> > FILE *f = fdopen(fd, "W")
> >
> > In musl, above code will be Okay.
> > While according to POSIX, above code (fdopen) will return EINVAL.
> >
> > Signed-off-by: Zhang Tianci <zhangtianci1@huawei.com>
> > ---
> >  src/stdio/__fdopen.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/src/stdio/__fdopen.c b/src/stdio/__fdopen.c index
> > 116e78e..23c4ffd 100644
> > --- a/src/stdio/__fdopen.c
> > +++ b/src/stdio/__fdopen.c
> > @@ -26,6 +26,16 @@ FILE *__fdopen(int fd, const char *mode)
> >  	/* Impose mode restrictions */
> >  	if (!strchr(mode, '+')) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
> >
> > +	int fd_flag = __syscall(SYS_fcntl, fd, F_GETFL);
> > +
> > +	if (fd_flag == -1) return 0;
> > +
> > +	if (((fd_flag & O_ACCMODE) == O_RDONLY && !(f->flags & F_NORD))
> ||
> > +	    ((fd_flag & O_ACCMODE) == O_WRONLY && !(f->flags &
> F_NOWR))) {
> > +		errno = EINVAL;
> > +		return 0;
> > +	}
> > +
> >  	/* Apply close-on-exec flag */
> >  	if (strchr(mode, 'e')) __syscall(SYS_fcntl, fd, F_SETFD,
> > FD_CLOEXEC);
> >
> > --
> > 2.17.1
> 
> Per POSIX this is a "may fail" not a "shall fail". Testing for this is more costly
> (see added code/syscalls in the patch) and serves no purpose, which is why
> it's not done.
> 
> Rich

POSIX's require on fdopen:

     The application shall ensure that the mode of the stream as expressed by the
     mode argument is allowed by the file access mode of the open file description 
     to which fildes refers.

So I think the example above should return EINVAL.