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Date: Wed, 19 Feb 2020 09:16:36 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] stdio: Fix fdopen bug
On Wed, Feb 19, 2020 at 06:47:53AM +0000, zhangtianci wrote:
> > 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@...wei.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.
The text you're quoting is placing a requirement on the application,
not the implementation. If the application fails to meet a "shall", it
has undefined behavior and there are no obligations whatsoever on the
implementation.
The error is clearly a "may fail" if you read the ERRORS section of
the specification.
Rich
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