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SubjectRe: [RFC v10][PATCH 08/13] Dump open file descriptors
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On Fri, 2008-11-28 at 10:19 +0000, Al Viro wrote:
> On Wed, Nov 26, 2008 at 08:04:39PM -0500, Oren Laadan wrote:
> > +int cr_scan_fds(struct files_struct *files, int **fdtable)
> > +{
> > + struct fdtable *fdt;
> > + int *fds;
> > + int i, n = 0;
> > + int tot = CR_DEFAULT_FDTABLE;
> > +
> > + fds = kmalloc(tot * sizeof(*fds), GFP_KERNEL);
> > + if (!fds)
> > + return -ENOMEM;
> > +
> > + /*
> > + * We assume that the target task is frozen (or that we checkpoint
> > + * ourselves), so we can safely proceed after krealloc() from where
> > + * we left off; in the worst cases restart will fail.
> > + */
> Task may be frozen, but it may share the table with any number of other
> tasks...

First of all, thanks for looking at this, Al.

I think Oren's assumption here is that all tasks possibly sharing the
table would be frozen. I don't think that's a good assumption,
either. :)

This would be a lot safer and bulletproof if we size the allocation
ahead of time, take all the locks, then retry if the size has changed.

I think that will just plain work of we do this:

> > + spin_lock(&files->file_lock);
> > + rcu_read_lock();
> > + fdt = files_fdtable(files);
> > + for (i = 0; i < fdt->max_fds; i++) {
> > + if (!fcheck_files(files, i))
> > + continue;
> > + if (n == tot) {
> > + /*
> > + * fcheck_files() is safe with drop/re-acquire
> > + * of the lock, because it tests: fd < max_fds
> > + */
> > + spin_unlock(&files->file_lock);
> > + rcu_read_unlock();
> > + tot *= 2; /* won't overflow: kmalloc will fail */

free(fds);
goto first_kmalloc_in_this_function;

> > + }
> > + fds[n++] = i;
> > + }
> > + rcu_read_unlock();
> > + spin_unlock(&files->file_lock);
> > +
> > + *fdtable = fds;
> > + return n;
> > +}

Right?

> > + switch (inode->i_mode & S_IFMT) {
> > + case S_IFREG:
> > + fd_type = CR_FD_FILE;
> > + break;
> > + case S_IFDIR:
> > + fd_type = CR_FD_DIR;
> > + break;
> > + case S_IFLNK:
> > + fd_type = CR_FD_LINK;
>
> Opened symlinks? May I have whatever you'd been smoking, please?

Ugh, that certainly doesn't have any place here. I do wonder if Oren
had some use for that in the fully put together code, but it can
certainly go for now.

I'll send patches for these shortly.

-- Dave



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