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

[Siva Chandra <sivachandra at google dot com>]

On Thu, Mar 21, 2019 at 2:32 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> 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?

It wouldn't really be "fixing" lld, but introducing a bug in it to
incorrectly generate equal __rela_iplt_end and __rela_iplt_start for
static-pie.

But, do you see any issues with the v2 patch I posted?

>
> --
> H.J.