[Bug dynamic-link/27164] New: csu/libc-start.c should not call ARCH_APPLY_IREL in static pie mode

["i at maskray dot me" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27164

            Bug ID: 27164
           Summary: csu/libc-start.c should not call ARCH_APPLY_IREL in
                    static pie mode
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

* In GNU ld, __rela_iplt_start is defined for -no-pie, but not for -pie.
* LLD defines __rela_iplt_start, regardless of -no-pie, -pie or -shared.

glibc csu/libc-start.c has undefined weak __rela_iplt_start/__rela_iplt_end and
currently requires the two to be equal in static pie mode.
Since there are unavoidable R_*_IRELATIVE relocations, this essentially
requires __rela_iplt_start and __rela_iplt_end to be 0, i.e.
the linker does not define the two symbols in -pie mode. However, this address
can be easily fixed on glibc side by differenciating static pie from static.

(Note GNU ld does not currently have -no-pie (its -no-pie is currently -n -o
"-pie"), but gold/LLD do have.)

binutils commit 3aa14d16c669ca75f9fa4e995a2e2d13069dff3f (2009-06-01) added
__rela_iplt_start to ld.
That was before glibc gained static pie support (2017;
http://sourceware.org/PR19574).
With static pie, I think the design of __rela_iplt_start should be revised.

diff -u =(ld.bfd --verbose) =(ld.bfd -pie --verbose) reveals two differences.
The image base (__executable_start) is an essential one but the other one
(whether __rela_iplt_start is defined) should not be their.


In
https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/google/grte/v5-2.27/master
,
I find "Make _dl_relocate_static_pie return an int indicating whether it
applied relocs." can address the exact glibc csu/libc-start.c issue.

I do see that GNU ld may not want a change (probably in a couple of years)
because it does not want to gratuitously break unpatched glibc `configure
--enable-static-pie`. But once glibc is fixed and the minimum required version
is bumped, the code can be removed from GNU ld. On glibc side, the explicit
guard before ARCH_APPLY_IREL also makes things more clear, not relying on
subtle zero address __rela_iplt_start.

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