Date: Wed, 21 Jul 1999 10:19:47 +1000 From: Peter Jeremy <jeremyp@gsmx07.alcatel.com.au> To: hackers@FreeBSD.ORG Subject: Proposal for new syscall to close files Message-ID: <99Jul21.100128est.40397@border.alcanet.com.au>
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It's fairly common, when spawning new processes, to want to make sure
all unwanted FDs are closed. Currently, the options for doing this are:
1) Use fcntl(fd, F_SETFD, FD_CLOEXEC) to set the close-on-exec flag
when the file is opened/cloned. This may not be practical if the
FD must remain open across some exec's, but not others. It is not
possible to ensure that FDs implicitly opened within library
functions have the close-on-exec flag set. It may be inefficient if
there are lots of opens and few execs.
2) Explicitly close unwanted FDs in the child, before the exec().
This suffers from the usual resource tracking problems (ie it's
easy to forget to close one - especially in a maze of pipe()/
dup()/dup2() calls designed to join the child's stdin/out/err
to the parent). It also requires that the FD's be visible to the
function - which may be difficult (the FD may be hidden within
another function somewhere).
3) Close all FDs except the ones you explicitly want to keep. This
is normally something like:
for (i = getdtablesize(); --i > 2; )
close(i);
The advantage is that you are sure you don't miss any. The
disadvantage is that it requires a system call for each potentially
open FD - >600 on my system - whereas maybe only 4 or 5 are
actually open.
In the case of option 3, you only really need to attempt to close file
descriptors less then curproc->p_fd->fd_lastfile or even
curproc->p_fd->fd_nfiles, but these values aren't readily accessible
from userland. (And this still suffers the overhead of a userland
to kernel transition for each FD).
I'd therefore like to propose a new syscall that closes _all_ file
descriptors associated with a process, except for those passed as
1 bits in an fd_set. The proposed API is:
int cleanup_files(int nfds, const fd_set *leavefds);
where nfds specifies the number of fds in *leavefds to potentially
keep open (ie all fds >= nfds will be closed).
The function would return the number of FDs closed.
The implementation would be along the lines of:
struct cleanup_files_args {
int nd;
fd_set *leave;
};
int
cleanup_files(p, uap)
register struct proc *p;
register struct cleanup_files_args *uap;
{
fd_set s_fdset;
fd_set *lset;
int error, nclosed = 0;
u_int i, ncpbytes, nfdbits;
struct filedesc *fdp = p->p_fd;
struct file **fpp;
char *fdfp;
if (uap->nd < 0 || uap->leave = NULL)
return (EINVAL);
/* some daemons mught not have any file descriptors */
if (fdp == NULL) {
p->p_retval[0] = 0;
return (0);
}
if (uap->nd > fdp->fd_lastfile)
uap->nd = fdp->fd_lastfile + 1;
/*
* Allocate just enough bits for the passed fd_set. Use the
* preallocated auto buffer if possible.
*/
nfdbits = roundup(uap->nd, NFDBITS);
ncpbytes = nfdbits / NBBY;
if (ncpbytes <= sizeof s_fdset)
lset = &s_fdset;
else
lset = malloc(ncpbytes, M_SELECT, M_WAITOK);
error = copyin(uap->leave, lbits, ncpbytes);
if (error != 0)
goto done;
fpp = fdp->fd_ofiles;
fdfp = fdp->fd_ofileflags;
for (i = 0; i < uap->nd; i++, fpp++, fdfp++) {
if (!FD_ISSET(i, lset) && *fpp != NULL) {
if (*fdfp & UF_MAPPED)
(void) munmapfd(p, i);
error = closef(*fpp, p);
if (error != 0)
goto done;
nclosed++;
*fpp = NULL;
*fdfp = 0;
if (i < fdp->fd_freefile)
fdp->fd_freefile = i;
}
}
for (; i <= fdp->fd_lastfile; i++, fpp++, fdfp++)
if (*fpp != NULL) {
if (*fdfp & UF_MAPPED)
(void) munmapfd(p, i);
error = closef(*fpp, p);
if (error != 0)
goto done;
nclosed++;
*fpp = NULL;
*fdfp = 0;
if (i < fdp->fd_freefile)
fdp->fd_freefile = i;
}
}
while (fdp->fd_lastfile > 0 && fdp->fd_ofiles[fdp->fd_lastfile] == NULL)
fdp->fd_lastfile--;
done:
if (lset != &s_fdset)
free(lset, M_SELECT);
p->p_retval[0] = nclosed;
return (error);
}
Peter
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