[Bug libc/29662] s390 glibc wrongly assumes GOT[0] is _DYNAMIC

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

https://sourceware.org/bugzilla/show_bug.cgi?id=29662

--- Comment #5 from Fangrui Song <i at maskray dot me> ---
(In reply to Rui Ueyama from comment #4)
> GOT[0] is indeed redundant, but removing dependencies to that entry is not
> probably productive at this point, because it is already part of the
> standard psABI of s390x. Even if we remove all dependencies to GOT[0] from
> glibc, there may be code that depends on it, and they are not technically
> wrong because GOT[0]'s value is guaranteed by the psABI, so we can't remove
> GOT[0] anyway.

It is not meaningful for a linker supporting this to remove the support (it
breaks linking glibc[1]) but it is meaningful for glibc. At this point glibc
can use the same implementation for many ports but s390x is one of the
stragglers. Moving away from it is a small code clean-up


[1] lld does not set _GLOBAL_OFFSET_TABLE_ for many architectures, and that
appears to work fine. I know glibc aarch64 used it, so I submitted a patch to
remove it, and that was the only instance I have seen. musl, FreeBSD rtld,
NetBSD rtld, etc use approaches that don't need _GLOBAL_OFFSET_TABLE_[0]. The
s390x psABI specifying _GLOBAL_OFFSET_TABLE_[0] really mixes psABI with glibc
internals.

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