glibc:nsz/bug27072

Last commit made on 2021-01-20
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nsz/bug27072
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lp:glibc

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d7eb5cf... by Szabolcs Nagy <email address hidden> on 2021-01-06

Make libc symbols hidden in static PIE

Hidden visibility can avoid indirections and RELATIVE relocs in
static PIE libc.

The check should use IS_IN_LIB instead of IS_IN(libc) since all
symbols are defined locally in static PIE and the optimization is
useful in all libraries not just libc. However the test system
links objects from libcrypt.a into dynamic linked test binaries
where hidden visibility does not work. I think mixing static and
shared libc components in the same binary should not be supported
usage, but to be safe only use hidden in libc.a.

On some targets (i386) this optimization cannot be applied because
hidden visibility PIE ifunc functions don't work, so it is gated by
NO_HIDDEN_EXTERN_FUNC_IN_PIE.

From -static-pie linked 'int main(){}' this shaves off 71 relative
relocs on aarch64 and reduces code size by about 2k.

b5dbaa1... by Szabolcs Nagy <email address hidden> on 2021-01-19

csu: Move static pie self relocation later [BZ #27072]

IFUNC resolvers may depend on tunables and cpu feature setup so
move static pie self relocation after those.

It is hard to guarantee that the ealy startup code does not rely
on relocations so this is a bit fragile. It would be more robust
to handle RELATIVE relocs early and only IRELATIVE relocs later,
but the current relocation processing code cannot do that.

The early startup code up to relocation processing includes

  _dl_aux_init (auxvec);
  __libc_init_secure ();
  __tunables_init (__environ);
  ARCH_INIT_CPU_FEATURES ();
  _dl_relocate_static_pie ();

These are simple enough that RELATIVE relocs can be avoided.

The following steps include

  ARCH_SETUP_IREL ();
  ARCH_SETUP_TLS ();
  ARCH_APPLY_IREL ();

On some targets IRELATIVE processing relies on TLS setup on
others TLS setup relies on IRELATIVE relocs, so the right
position for _dl_relocate_static_pie is target dependent.
For now move self relocation as early as possible on targets
that support static PIE.

Fixes bug 27072.

54afd01... by Szabolcs Nagy <email address hidden> on 2021-01-15

Use hidden visibility for early static PIE code

Extern symbol access in position independent code usually involves GOT
indirection which needs RELATIVE reloc in a static linked PIE. (On
some targets this is avoided e.g. because the linker can relax a GOT
access to a pc-relative access, but this is not generally true.) Code
that runs before static PIE self relocation must avoid relying on
dynamic relocations which can be ensured by using hidden visibility.
However we cannot just make all symbols hidden:

On i386, all calls to IFUNC functions must go through PLT and calls to
hidden functions CANNOT go through PLT in PIE since EBX used in PIE PLT
may not be set up for local calls to hidden IFUNC functions.

This patch aims to make symbol references hidden in code that is used
before and by _dl_relocate_static_pie when building a static PIE libc.
Note: for an object that is used in the startup code, its references
and definition may not have consistent visibility: it is only forced
hidden in the startup code.

This is needed for fixing bug 27072.

Co-authored-by: H.J. Lu <email address hidden>

4da71cb... by Szabolcs Nagy <email address hidden> on 2021-01-06

csu: Avoid weak ref for __ehdr_start in static PIE

All linkers support __ehdr_start that support static PIE linking,
so there is no need to check for its presence via a weak reference.

This avoids a RELATIVE relocation in static PIE startup code on some
targets.

With non-PIE static linking the weak ref check is kept in case the
linker does not support __ehdr_start.

51119c6... by Szabolcs Nagy <email address hidden> on 2021-01-20

configure: Check for static PIE support

Add SUPPORT_STATIC_PIE that targets can define if they support
static PIE. This requires PI_STATIC_AND_HIDDEN support and various
linker features as described in

  commit 9d7a3741c9e59eba87fb3ca6b9f979befce07826
  Add --enable-static-pie configure option to build static PIE [BZ #19574]

Currently defined on x86_64, i386 and aarch64 where static PIE is
known to work.

1dcd94e... by Szabolcs Nagy <email address hidden> on 2021-01-12

elf: Avoid RELATIVE relocs in __tunables_init

With static pie linking pointers in the tunables list need
RELATIVE relocs since the absolute address is not known at link
time. We want to avoid relocations so the static pie self
relocation can be done after tunables are initialized.

This is a simple fix that embeds the tunable strings into the
tunable list instead of using pointers. It is possible to have
a more compact representation of tunables with some additional
complexity in the generator and tunable parser logic. Such
optimization will be useful if the list of tunables grows.

There is still an issue that tunables_strdup allocates and the
failure handling code path is sufficiently complex that it can
easily have RELATIVE relocations. It is possible to avoid the
early allocation and only change environment variables in a
setuid exe after relocations are processed. But that is a
bigger change and early failure is fatal anyway so it is not
as critical to fix right away. This is bug 27181.

Reviewed-by: Adhemerval Zanella <email address hidden>

e0ed9fd... by Szabolcs Nagy <email address hidden> on 2021-01-06

elf: Make the tunable struct definition internal only

The representation of the tunables including type information and
the tunable list structure are only used in the implementation not
in the tunables api that is exposed to usage within glibc.

This patch moves the representation related definitions into the
existing dl-tunable-types.h and uses that only for implementation.

The tunable callback and related types are moved to dl-tunables.h
because they are part of the tunables api.

This reduces the details exposed in the tunables api so the internals
are easier to change.

Reviewed-by: Adhemerval Zanella <email address hidden>

22b79ed... by "H.J. Lu" <email address hidden> on 2021-01-15

Use <startup.h> in __libc_init_secure

Since __libc_init_secure is called before ARCH_SETUP_TLS, it must use
"int $0x80" for system calls in i386 static PIE. Add startup_getuid,
startup_geteuid, startup_getgid and startup_getegid to <startup.h>.
Update __libc_init_secure to use them.

Reviewed-by: Adhemerval Zanella <email address hidden>

cabc8a2... by "H.J. Lu" <email address hidden> on 2021-01-18

elf: Avoid RELATIVE relocation for _dl_sysinfo

Set the default _dl_sysinfo in _dl_aux_init to avoid RELATIVE relocation
in static PIE.

This is needed for fixing bug 27072 on x86.

Reviewed-by: Adhemerval Zanella <email address hidden>

0ec583d... by "H.J. Lu" <email address hidden> on 2021-01-15

libmvec: Add extra-test-objs to test-extras

Add extra-test-objs to test-extras so that they are compiled with
-DMODULE_NAME=testsuite instead of -DMODULE_NAME=libc.

Reviewed-by: Adhemerval Zanella <email address hidden>