On Mon, Jun 23, 2014 at 10:05:01AM +0200, Mateusz Guzik wrote:
> On Mon, Jun 23, 2014 at 10:25:19AM +0300, Konstantin Belousov wrote:
> > On Mon, Jun 23, 2014 at 09:06:52AM +0200, Mateusz Guzik wrote:
> > > The table is modified in these functions and is reachable from the rest
> > > of the kernel (can be found by e.g. sysctl_kern_proc_filedesc), thus
> > > XLOCK is needed to ensure consistency for readers. It can also be
> > > altered by mountcheckdirs, although not in a way which disrupts any of
> > > these functions.
> > I would think that such cases should be avoided by testing for P_INEXEC,
> > but I do not insist on this.
> >
>
> proc lock has to be dropped before filedesc lock is taken and I don't
> see any way to prevent the proc from transitioning to P_INEXEC afterwards.
> Since sysctl_kern_proc_filedesc et al can take a long time it does not
> seem feasible to pursue this.
>
> This also makes me realize I screwed up r265247 "Request a non-exiting
> process in sysctl_kern_proc_{o,}filedesc". As it is the code has to hold
> the process and release it later to really close the race and not only
> affect the window. I'll poke around this later, maybe it is better to
> nullify some stuff in exit1 with PROC_LOCK held and access it in a
> similar manner.
>
> > > @@ -2097,7 +2102,6 @@ setugidsafety(struct thread *td)
> > > fdfree(fdp, i);
> > > FILEDESC_XUNLOCK(fdp);
> > > (void) closef(fp, td);
> > > - FILEDESC_XLOCK(fdp);
> > > }
> > > }
> > > FILEDESC_XUNLOCK(fdp);
> > This unlock should be removed ?
> >
>
> Ugh, yes.
>
> > > + /* See the comment in setugidsafety */
> > > + FILEDESC_XLOCK(fdp);
> > > fdfree(fdp, i);
> > > (void) closefp(fdp, i, fp, td, 0);
> > > /* closefp() drops the FILEDESC lock. */
> > > - FILEDESC_XLOCK(fdp);
> > This should be XUNLOCK ?
>
> This one is fine as closefp drops the lock.
>
> > > }
> > > }
> > > FILEDESC_XUNLOCK(fdp);
> > And this unlock removed ?
>
> Yes.
>
> I tested this patch and it works fine:
>
> diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
> index 25c3a1e..a900464 100644
> --- a/sys/kern/kern_descrip.c
> +++ b/sys/kern/kern_descrip.c
> @@ -2082,13 +2082,18 @@ setugidsafety(struct thread *td)
> int i;
>
> fdp = td->td_proc->p_fd;
> + /*
> + * While no other thread can alter filedescriptors in this table,
> + * there may be code trying to read it, thus the lock is required
> + * to provide consistent view if we are going to change it.
> + */
> KASSERT(fdp->fd_refcnt == 1, ("the fdtable should not be shared"));
> - FILEDESC_XLOCK(fdp);
> for (i = 0; i <= fdp->fd_lastfile; i++) {
> if (i > 2)
> break;
> fp = fdp->fd_ofiles[i].fde_file;
> if (fp != NULL && is_unsafe(fp)) {
> + FILEDESC_XLOCK(fdp);
> knote_fdclose(td, i);
> /*
> * NULL-out descriptor prior to close to avoid
> @@ -2097,10 +2102,8 @@ setugidsafety(struct thread *td)
> fdfree(fdp, i);
> FILEDESC_XUNLOCK(fdp);
> (void) closef(fp, td);
> - FILEDESC_XLOCK(fdp);
> }
> }
> - FILEDESC_XUNLOCK(fdp);
> }
>
> /*
> @@ -2136,19 +2139,18 @@ fdcloseexec(struct thread *td)
>
> fdp = td->td_proc->p_fd;
> KASSERT(fdp->fd_refcnt == 1, ("the fdtable should not be shared"));
> - FILEDESC_XLOCK(fdp);
> for (i = 0; i <= fdp->fd_lastfile; i++) {
> fde = &fdp->fd_ofiles[i];
> fp = fde->fde_file;
> if (fp != NULL && (fp->f_type == DTYPE_MQUEUE ||
> (fde->fde_flags & UF_EXCLOSE))) {
> + /* See the comment in setugidsafety */
> + FILEDESC_XLOCK(fdp);
> fdfree(fdp, i);
> (void) closefp(fdp, i, fp, td, 0);
> /* closefp() drops the FILEDESC lock. */
> - FILEDESC_XLOCK(fdp);
> }
> }
> - FILEDESC_XUNLOCK(fdp);
> }
>
> /*This looks fine, but please e.g. add a reference to the comment in etugidsafety(), to fdcloseexec(). Or duplicate it.
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