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SubjectRe: [PATCH] proc: sysctl: fix double drop_sysctl_table()
On Sat, Dec 22, 2018 at 11:17 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Sat, Dec 22, 2018 at 10:12:21AM +0800, Yafang Shao wrote:
> > The callers of insert_header() will drop sysctl table whatever
> > insert_header() successes or fails.
> > So we can't call drop_sysctl_table() in insert_header().
> >
> > The call sites of insert_header() are all in fs/proc/proc_sysctl.c.
>
> Except that at least insert_links() does this:
>
> ...
> core_parent->header.nreg++;
> ...
> err = insert_header(core_parent, links);
> if (err)
> kfree(links);
> out:
> drop_sysctl_table(&core_parent->header);
> with that drop clearly paired with explicit increment of nreg. So your
> patch appears to break at least that one.
>
> Looking at the rest of callers... __register_sysctl_table() demonstrates
> the same pattern - explicit increment of nreg, then insert_header(),
> then *unconditional* drop undoing that increment.
>
> The third and last caller (get_subdir()) appears to be different.
> We do insert_header(), if it succeeds we bump nreg on new and
> unconditionally drop a reference to dir, as well as new.
>
> Let's look at the callers of that sucker.
>
> /* Reference moved down the diretory tree get_subdir */
> dir->header.nreg++;
> spin_unlock(&sysctl_lock);
>
> /* Find the directory for the ctl_table */
> for (name = path; name; name = nextname) {
> int namelen;
> nextname = strchr(name, '/');
> if (nextname) {
> namelen = nextname - name;
> nextname++;
> } else {
> namelen = strlen(name);
> }
> if (namelen == 0)
> continue;
>
> dir = get_subdir(dir, name, namelen);
> if (IS_ERR(dir))
> goto fail;
> }
>
> spin_lock(&sysctl_lock);
> if (insert_header(dir, header))
> goto fail_put_dir_locked;
>
> Aha... So we are traversing a tree and it smells like get_subdir()
> expects dir to be pinned by the caller, drops that reference and
> either fails or returns the next tree node pinned.
>
> _IF_ that matches the behaviour of get_subdir(), the code above
> makes sense -
> grab dir
> for each non-empty pathname component
> next = get_subdir(dir, component)
> // dir got dropped
> if get_subdir has failed
> goto fail
> // next got grabbed
> dir = next
> insert_header(dir, header)
> drop dir
> if insert_header was successful
> return header
> fail:
> // all references grabbed by the above are dropped by now
>
> So let's look at get_subdir() from refcounting POV (ignoring the locking
> for now):
> subdir = find_subdir(dir, name, namelen);
> if (!IS_ERR(subdir))
> goto found;
> if (PTR_ERR(subdir) != -ENOENT)
> goto failed;
> yeccchhhhhhh.... What's wrong with if (subdir == ERR_PTR(-ENOENT))?
> Anyway, find_subdir() appears to be refcounting-neutral.
> new = new_dir(set, name, namelen);
> OK, looks like we are creating a new object here. What's the value of .nreg
> in it? new_dir() itself doesn't seem to set it, but the thing it calls at
> the end (init_header()) does set it to 1. OK, so we'd got a reference to
> new object, our counter being 1. On failure it appears to return NULL.
> OK...
> subdir = ERR_PTR(-ENOMEM);
> if (!new)
> goto failed;
> right, so if allocation in new_dir() has failed, we are buggering off to the
> same 'failed' label as on other exits. In failure case new_dir() is
> refcounting-neutral, so we are in the same environment.
> /* Was the subdir added while we dropped the lock? */
> subdir = find_subdir(dir, name, namelen);
> if (!IS_ERR(subdir))
> goto found;
> if (PTR_ERR(subdir) != -ENOENT)
> goto failed;
> Interesting... So we can get to 'failed' in some cases when new_dir()
> has not failed.
> /* Nope. Use the our freshly made directory entry. */
> err = insert_header(dir, &new->header);
> subdir = ERR_PTR(err);
> if (err)
> goto failed;
> Looks like it expects insert_header() to be refcounting-neutral in failure
> case...
> subdir = new;
> found:
> subdir->header.nreg++;
>
> OK, we can get here from 3 places:
> * subdir found by lookup before we even tried to allocate new.
> new is NULL, subdir has just gotten a reference grabbed.
> * subdir found by re-lookup after new had been allocated.
> new is non-NULL and we are holding one reference to it, subdir has
> just gotten a reference grabbed and it's not equal to new.
> * new got successfully inserted into dir; subdir is equal
> to new and we'd just grabbed the second reference to it.
>
> Looks like in all those cases we have a reference grabbed on subdir
> *and* a reference grabbed on new (if non-NULL). Covers all 3 cases.,,
>
> failed:
> ... and now we rejoin the failure paths (a lousy label name, that - we
> get here on success as well).
> if (IS_ERR(subdir))
> whine
> drop reference to dir (the one that caller had grabbed for us)
> if new is not NULL
> drop reference to new.
> return subdir.
>
> OK, in success case we have ended up with the total effect
> drop dir
> grab subdir
> Holds in all 3 cases above. On failure, we have dropped the reference
> grabbed by new_dir (if any) and dropped dir. That matches the behaviour
> guessed by the look of the caller, and it definitely expects
> insert_header() to be refcounting-neutral on failures.
>
> And AFAICS, insert_header() is that. Before your patch, that is...
>
> The bottom line is, proposed behaviour change appears to be broken for all
> callers.
>
> NAK.
>
> PS: I was going to add that get_subdir() was in bad need of comments, until
> I actually looked around. And right in front of that function we have this:
> /**
> * get_subdir - find or create a subdir with the specified name.
> * @dir: Directory to create the subdirectory in
> * @name: The name of the subdirectory to find or create
> * @namelen: The length of name
> *
> * Takes a directory with an elevated reference count so we know that
> * if we drop the lock the directory will not go away. Upon success
> * the reference is moved from @dir to the returned subdirectory.
> * Upon error an error code is returned and the reference on @dir is
> * simply dropped.
> */
>
> So it *IS* documented, description does match the behaviour and
> I should've bloody well checked if it's there first. And verified
> that it does match the code, of course, but that's generally
> simpler than deriving the behaviour from code and trying to come
> up with sane description of such.

Many thanks for your detailed explanation!
I undanstand it.

Thanks
Yafang

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