[Bug libc/31042] New: [s390x] .init and .fini padding

["rui314 at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31042

            Bug ID: 31042
           Summary: [s390x] .init and .fini padding
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: rui314 at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

I noticed this issue while developing the mold linker for the s390x
architecture.

The CRT files for glibc/s390x include .init and .fini sections, which contain
functions for initializing and finalizing a process, respectively. The linker
simply concatenates .init or .fini input sections into a single output section
for each. Upon process startup, the runtime branches to the start of .init as
though it were a single function. The final input .init section contains a ret
instruction to return to the caller.

In this setup, there should be no gaps between the input .init/.fini sections.
However, the .init/.fini sections in glibc/s390x may include trailing padding;
each input section is aligned to a 4-byte boundary, but they do not always end
on these boundaries. As a result, padding may occur between sections after the
linker concatenates them.

The linker can fill this padding with any value. GNU ld appears to use NOPs to
fill the gaps, which happens to work. However, I believe relying on it is an
incorrect approach, or at least a very fragile one.

To fix this issue, you could simply add an `.align 4` directive at the end of
each .init/.fini input section. This would prompt GAS to insert NOPs, ensuring
that each function ends at a 4-byte boundary.

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/s390/s390-64/crti.S;h=11ab75e8d95f86ff225f84673715ff457e8b63cd;hb=HEAD

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/s390/s390-64/crtn.S;h=0eabcb346caa843a4657c3fb23739bd7bf166091;hb=HEAD

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