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

Florian Weimer
2020-11-03T10:38:34Z

* 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.

Thanks,
Florian
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