Re: [Patch v2] Prevent a second round of irel application under static-pie

["H.J. Lu" <hjl dot tools at gmail dot com>]

On 3/22/19, Siva Chandra <sivachandra@google.com> wrote:
> I am sending this patch as v2 of the discussion I started on the v1
> patch: https://sourceware.org/ml/libc-alpha/2019-03/msg00421.html
>
> Firstly, I apologize for the fact that I miss-diagnosed the problem,
> but I think HJ Lu's response helped me do better with v2.
>
> So, for v1, I wrongly concluded that elf_irel should adjust for the
> load address. As HJ  Lu pointed out, that path is supposed to be taken
> only for non-pie static. The reason is, for static-pie,
> _dl_relocate_static_pie already applies the irel relocations.
>
> If I use ld as my linker, I do see what HJ Lu is pointing out and also
> that _dl_relocate_static_pie is indeed applying the irel relocations.
> But, notice that ARCH_SETUP_IREL is called in LIBC_START_MAIN after
> _dl_relocate_static_pie. So, it seems like irel relocations are being
> applied for a second time. And indeed, they would be applied for a
> second time if ld generates correct values for __rela_iplt_end and
> __rela_iplt_start. However, look at an example of what ld generates
> with -static-pie:
>
>    898: 00000000000080f0     0 NOTYPE  LOCAL  DEFAULT    7 __rela_iplt_end
>    899: 00000000000a8f20     0 NOTYPE  LOCAL  DEFAULT   20 __fini_array_end
>    900: 000000000007e0f0   542 FUNC    LOCAL  DEFAULT   11 __letf2
>    901: 00000000000080f0     0 NOTYPE  LOCAL  DEFAULT    7
> __rela_iplt_start
>
> Notice that __rela_iplt_end and __rela_iplt_start have the same value.
> Since ARCH_SETUP_IREL applies irel relocations iterating from
> __rela_iplt_start to __rela_iplt_end, for an ld generated static-pie
> binary, ARCH_SETUP_IREL ends up being a NOP. With any other linker,
> which generates correct __rela_iplt_end and __rela_iplt_start values,
> ARCH_SETUP_IREL tries to apply irel relocations for a second time,
> even though _dl_relocate_static_pie has already applied them.
>
> So, as opposed to my v1, this change is essentially protecting against
> reapplying irel relocations when the linker generates the correct
> values of __rela_iplt_end and __rela_iplt_start.
>
> I have seen HJ Lu's patch attempting the same but with a different
> approach: https://sourceware.org/ml/libc-alpha/2019-03/msg00445.html.
> However, I have a feeling it breaks for non-PIE static if glibc is
> configured with --enable-static-pie. So, I am putting my patch out
> there as well, just in case.
>
> ChangeLog:
>
>         * csu/libc-start.c (LIBC_START_MAIN): Call
> ARCH_SETUP_IREL/ARCH_APPLY_IREL
>         only if _dl_relocate_static_pie did not apply any relocations.
>         * csu/static-reloc.c (_dl_relocate_static_pie): Return 0.
>         * elf/dl-reloc-static-pie.c (_dl_relocate_static_pie): Return 1.
>         * sysdeps/generic/ldsodefs.h (_dl_relocate_static_pie): Add return
> type
>         int to the function. Define the macro to 0.
>

Can you fix lld instead?

-- 
H.J.