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

Szabolcs Nagy
2020-11-03T15:05:21Z

The 11/03/2020 04:36, H.J. Lu wrote:
> On Tue, Nov 3, 2020 at 2:38 AM Florian Weimer <fweimer@redhat.com> 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.

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

>
> 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.

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