Re: [PATCH 2/4] elf: Move note processing after l_phdr is updated [BZ #26831]

[H.J. Lu]

On Tue, Nov 3, 2020 at 7:04 AM Szabolcs Nagy <szabolcs.nagy@xxxxxxx> wrote:
>
> The 11/03/2020 04:36, H.J. Lu wrote:
> > On Tue, Nov 3, 2020 at 2:38 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote:
> > > * Szabolcs Nagy:
> > >
> > > > Program headers are processed in two pass: after the first pass
> > > > load segments are mmapped so in the second pass target specific
> > > > note processing logic can access the notes.
> > > >
> > > > The second pass is moved later so various link_map fields are
> > > > set up that may be useful for note processing such as l_phdr.
> > > > ---
> > > >  elf/dl-load.c | 30 +++++++++++++++---------------
> > > >  1 file changed, 15 insertions(+), 15 deletions(-)
> > > >
> > > > diff --git a/elf/dl-load.c b/elf/dl-load.c
> > > > index ceaab7f18e..673cf960a0 100644
> > > > --- a/elf/dl-load.c
> > > > +++ b/elf/dl-load.c
> > > > @@ -1259,21 +1259,6 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
> > > >                                 maplength, has_holes, loader);
> > > >      if (__glibc_unlikely (errstring != NULL))
> > > >        goto call_lose;
> > > > -
> > > > -    /* Process program headers again after load segments are mapped in
> > > > -       case processing requires accessing those segments.  Scan program
> > > > -       headers backward so that PT_NOTE can be skipped if PT_GNU_PROPERTY
> > > > -       exits.  */
> > > > -    for (ph = &phdr[l->l_phnum]; ph != phdr; --ph)
> > > > -      switch (ph[-1].p_type)
> > > > -     {
> > > > -     case PT_NOTE:
> > > > -       _dl_process_pt_note (l, fd, &ph[-1]);
> > > > -       break;
> > > > -     case PT_GNU_PROPERTY:
> > > > -       _dl_process_pt_gnu_property (l, fd, &ph[-1]);
> > > > -       break;
> > > > -     }
> > > >    }
> > > >
> > > >    if (l->l_ld == 0)
> > > > @@ -1481,6 +1466,21 @@ cannot enable executable stack as shared object requires");
> > > >      /* Assign the next available module ID.  */
> > > >      l->l_tls_modid = _dl_next_tls_modid ();
> > > >
> > > > +  /* Process program headers again after load segments are mapped in
> > > > +     case processing requires accessing those segments.  Scan program
> > > > +     headers backward so that PT_NOTE can be skipped if PT_GNU_PROPERTY
> > > > +     exits.  */
> > > > +  for (ph = &l->l_phdr[l->l_phnum]; ph != l->l_phdr; --ph)
> > > > +    switch (ph[-1].p_type)
> > > > +      {
> > > > +      case PT_NOTE:
> > > > +     _dl_process_pt_note (l, fd, &ph[-1]);
> > > > +     break;
> > > > +      case PT_GNU_PROPERTY:
> > > > +     _dl_process_pt_gnu_property (l, fd, &ph[-1]);
> > > > +     break;
> > > > +      }
> > > > +
> > > >  #ifdef DL_AFTER_LOAD
> > > >    DL_AFTER_LOAD (l);
> > > >  #endif
> > >
> > > Is this still compatible with the CET requirements?
> > >
> > > I hope it is because the CET magic happens in _dl_open_check, so after
> > > the the code in elf/dl-load.c has run.
>
> i believe the note processing and later cet magic
> are not affected by this code move.
>
> but i did not test this with cet.
>
> >
> > _dl_process_pt_note and _dl_process_pt_gnu_property may call
> > _dl_signal_error.  Are we prepared to clean more things up when it
> > happens?  I am investigating:
>
> yeah, this is difficult to reason about.
>
> it seems to me that after _dl_map_object returns there
> may be _dl_map_object_deps which can fail in a way that
> all of dlopen has to be rolled back, so if i move things
> around in _dl_map_object that should not introduce new
> issues.

I haven't investigated it in detail.  But there are

1314   if (l->l_phdr == NULL)
1315     {
1316       /* The program header is not contained in any of the segments.
1317          We have to allocate memory ourself and copy it over from out
1318          temporary place.  */
1319       ElfW(Phdr) *newp = (ElfW(Phdr) *) malloc (header->e_phnum
1320                                                 * sizeof (ElfW(Phdr)));
1321       if (newp == NULL)
1322         {
1323           errstring = N_("cannot allocate memory for program header");
1324           goto call_lose_errno;
1325         }
1326
1327       l->l_phdr = memcpy (newp, phdr,
1328                           (header->e_phnum * sizeof (ElfW(Phdr))));
1329       l->l_phdr_allocated = 1;
1330     }

When _dl_process_pt_gnu_property is moved after it, will l_phdr be
free on _dl_signal_error?

> but it is not clear to me how robust the dlopen code is
> against arbitrary failure in dl_open_worker.

I think we are mostly OK, except for some corner cases.   Delay
_dl_process_pt_gnu_property may introduce more corner cases.

> >
> > https://sourceware.org/bugzilla/show_bug.cgi?id=26825
> >
> > I don't think cleanup of _dl_process_pt_gnu_property failure is done
> > properly.
> >
> > --
> > H.J.
>
> --



-- 
H.J.