Re: git: 40a42785dbba - main - fcntl(F_SETFL): only allow one thread to perform F_SETFL

On Mon, Sep 22, 2025 at 09:50:09AM +0100, John Baldwin wrote:
> On 9/22/25 04:41, John Baldwin wrote:
> > On 9/19/25 10:19, Konstantin Belousov wrote:
> > > The branch main has been updated by kib:
> > > 
> > > URL: https://cgit.FreeBSD.org/src/commit/?id=40a42785dbba93cc5196178fc49d340c1a89cabe
> > > 
> > > commit 40a42785dbba93cc5196178fc49d340c1a89cabe
> > > Author:     Konstantin Belousov <kib@FreeBSD.org>
> > > AuthorDate: 2025-09-11 10:05:04 +0000
> > > Commit:     Konstantin Belousov <kib@FreeBSD.org>
> > > CommitDate: 2025-09-19 14:19:13 +0000
> > > 
> > >       fcntl(F_SETFL): only allow one thread to perform F_SETFL
> > >       Use f_vflags file locking for this.
> > >       Allowing more than one thread handling F_SETFL might cause de-sync
> > >       between real driver state and flags.
> > >       Reviewed by:    markj
> > >       Tested by:      pho
> > >       Sponsored by:   The FreeBSD Foundation
> > >       MFC after:      2 weeks
> > >       Differential revision:  https://reviews.freebsd.org/D52487
> > 
> > Thanks for fixing this.  I still slightly worry that "home-grown" locks
> > aren't visible to WITNESS and it's checking.
> > 
> > I was also expecting this to require more changes, but apparently if a
> > process directly invokes FIONBIO on a file descriptor, f_flags isn't
> > updated currently.  I wonder if that is a bug.  (Similarly for FIOASYNC.)
> > 
> > Oh, we do handle that, but poorly.  We don't revert on errors, and this
> > should be updated to use fsetfl_lock now I think:
> > 
> > kern_ioctl(...)
> > {
> > 	...
> > 	switch (com) {
> > 	...
> > 	case FIONBIO:
> > 		if ((tmp = *(int *)data))
> > 			atomic_set_int(&fp->f_flag, FNONBLOCK);
> > 		else
> > 			atomic_clear_int(&fp->f_flag, FNONBLOCK);
> > 		data = (void *)&tmp;
> > 		break;
> > 	case FIOASYNC:
> > 		if ((tmp = *(int *)data))
> > 			atomic_set_int(&fp->f_flag, FASYNC);
> > 		else
> > 			atomic_clear_int(&fp->f_flag, FASYNC);
> > 		data = (void *)&tmp;
> > 		break;
> > 	}
> > 
> > 	error = fo_ioctl(fp, com, data, td->td_ucred, td);
> > out:
> > 
> > I think instead we want something like:
> > 
> > 	int f_flag;
> > 
> > 	switch (com) {
> > 	...
> > 	case FIONBIO:
> > 	case FIOASYNC:
> > 		fsetfl_lock(fp);
> > 		tmp = *(int *)data;
> > 		f_flag = com == FIONBIO ? FNONBLOCK : FASYNC;
> > 		if ((fp->f_flag & f_flag) != 0) {
> 
> This is wrong, should be:
> 
> 	if (((fp->f_flag & f_flag) != 0) == (tmp != 0))
> 
> > 			fsetfl_unlock(fp);
> > 			goto out;
> > 		}
> > 		data = (void *)&tmp;
> > 		break;
> > 	}
> > 
> > 	error = fo_ioctl(fp, com, data, td->td_ucred, td);
> > 	switch (com) {
> > 	...
> > 	case FIONBIO:
> > 	case FIOASYNC:
> > 		if (error == 0) {
> > 			if (tmp)
> 
> Probably 'if (tmp != 0)'
> 
> > 				atomic_set_int(&fp->f_flag, f_flag);
> > 			else
> > 				atomic_clear_int(&fp->f_flag, f_flag);
> > 		}
> > 		fsetfl_unlock(fp);
> > 		break;
> > 	}
> > 
> > out:
> > 
> > This only updates the flag if the underlying ioctl succeeds, and it also
> > avoids invoking the underlying ioctl if the flag is already in the correct\
> > state.

So will you handle this?