svn commit: r267760 - head/sys/kern

[Konstantin Belousov]

On Mon, Jun 23, 2014 at 09:06:52AM +0200, Mateusz Guzik wrote:
> On Mon, Jun 23, 2014 at 09:40:44AM +0300, Konstantin Belousov wrote:
> > On Mon, Jun 23, 2014 at 01:28:18AM +0000, Mateusz Guzik wrote:
> > > +	KASSERT(fdp->fd_refcnt == 1, ("the fdtable should not be shared"));
> > >  	FILEDESC_XLOCK(fdp);
> > This  is at least weird.  Not incorrect, but the code now looks strange.
> > The fd_refcnt == 1 assert just states the circumstances of the code which
> > currently calls the functions.  Would the functions become incorrect or
> > destructive if there are other references to the filedescriptor table ?
> > 
> > In case you argument is that refcnt == 1 must hold to prevent the parallel
> > modifications of the descriptor table, which would invalidate the checks
> > and actions of the functions, then XLOCK is not needed (similar to your
> > earlier commit).
> > 
> > Note that kern_execve() is executed with the process single-threaded,
> > which, together with statement fd_refcnt == 1 must prevent the parallel
> > modifications.
> 
> 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.

> 
> For now I do agree that both the assertion and XLOCK look weird as it is
> and as such deserve a comment.
> 
> Actually we can take the lock only if we are going to modify something.
> 
> How about the following then (untested):
See below, but I would be completely satisfied even by only
comment addition.

> 
> diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
> index 25c3a1e..31f4881 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
> @@ -2097,7 +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 ?

> @@ -2136,16 +2140,16 @@ 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);
This should be XUNLOCK ?
>                 }
>         }
>         FILEDESC_XUNLOCK(fdp);
And this unlock removed ?
> 
> 
> -- 
> Mateusz Guzik <mjguzik gmail.com>
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