[Crash-utility] [PATCH v4 4/4] Add check if an syment element is installed one more time

[Tao Liu]

Hi Philipp,

Thanks  for reviewing the patch!

On Thu, Sep 23, 2021 at 8:19 PM Philipp Rudo <prudo at redhat.com> wrote:
>
> Hi Tao,
>
> On Sat, 18 Sep 2021 15:59:32 +0800
> Tao Liu <ltao at redhat.com> wrote:
>
> > symname_hash_install won't check if spn has been installed before. If
> > it does, the second install will corrupt the hash table as well as
> > spn->cnt counting. This patch adds the check to avoid such risks.
> >
> > Signed-off-by: Tao Liu <ltao at redhat.com>
> > ---
> >  symbols.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/symbols.c b/symbols.c
> > index f7157b1..6d12c55 100644
> > --- a/symbols.c
> > +++ b/symbols.c
> > @@ -1147,6 +1147,20 @@ mod_symtable_hash_remove_range(struct syment *from, struct syment *to)
> >               symname_hash_remove(st->mod_symname_hash, sp);
> >  }
> >
> > +static inline int
> > +syment_is_installed(struct syment *table[], struct syment *spn)
> > +{
> > +     struct syment *sp;
> > +     int index;
> > +
> > +     index = SYMNAME_HASH_INDEX(spn->name);
> > +     for (sp = table[index]; sp; sp = sp->name_hash_next) {
> > +             if (sp == spn)
> > +                     return TRUE;
> > +     }
> > +     return FALSE;
> > +}
> > +
> >  /*
> >   *  Install a single static kernel symbol into the symname_hash.
> >   */
> > @@ -1156,7 +1170,7 @@ symname_hash_install(struct syment *table[], struct syment *spn)
> >       struct syment *sp;
> >          int index;
> >
> > -     if (!spn)
> > +     if (!spn || syment_is_installed(table, spn))
> >               return;
> >
> >          index = SYMNAME_HASH_INDEX(spn->name);
>
> hmm... not sure if this is a little bit over the top. The idea I had
> was in your v3 simply replace
>
> assert(sp != spn);
>
> by
>
> if (sp == spn) {
>         error(WARNING, "Symbol %s already installed in symname_hash\n",
>               sp->name);
>         continue;
> }
>

It may not be easy to replace with "check if sp == spn and continue".
For example, if we already have
3 syments installed, which all have the same name, such as
(sp1{cnt == 3, name == "str"}, sp2{cnt == 3, name == "str"}, sp3{cnt
== 3, name == "str"})
in the hashtable, and we will install sp3 again(sp3 == spn) into the hashtable.

The cnt will get increased for sp1 and sp2 in the following code:
while (sp) {
        if (sp == spn) {
             error(....);
             continue;
        }
        if (STREQ(sp->name, spn->name)) {
                sp->cnt++;
                spn->cnt++;
        }
.....
}

However, when iteration reaches sp3, it will not increase cnt and not
install spn. Thus we will have
(sp1{cnt == 4}, sp2{cnt == 4}, sp3{cnt == 3}) in the hashtable, cnt
gets corrupted. In other words,
we need to revert all the previous cnt++ when we reach sp == spn. It
will require
more code to implement the 'revert' operation, and it is not as clear
as syment_is_installed
check.

> That's less code plus the warning makes it easier to detect that there
> is a problem (for me the case sp == spn is a sign for a bug in crash).
> What do you think?
>

>From my point,
1) assert() check: Use least code, simple and clear, but too strict.
2) syment_is_installed check: Use more code, clear, acceptable to me.
3) introduce 'cnt++ revert' operation: I haven't think of a better way, from
    the current inspection, it uses more code, and is not elegant.
What do you think?

Thanks,
Tao Liu

> Thanks
> Philipp
>