Binaries can opt-in to using BTI via an ELF object file marking.
The dynamic linker has to then mprotect the executable segments
with PROT_BTI. In case of static linked executables or in case
of the dynamic linker itself, PROT_BTI protection is done by the
operating system.
On AArch64 glibc uses PT_GNU_PROPERTY instead of PT_NOTE to check
the properties of a binary because PT_NOTE can be unreliable with
old linkers (old linkers just append the notes of input objects
together and add them to the output without checking them for
consistency which means multiple incompatible GNU property notes
can be present in PT_NOTE).
BTI property is handled in the loader even if glibc is not built
with BTI support, so in theory user code can be BTI protected
independently of glibc. In practice though user binaries are not
marked with the BTI property if glibc has no support because the
static linked libc objects (crt files, libc_nonshared.a) are
unmarked.
This patch relies on Linux userspace API that is not yet in a
linux release but in v5.8-rc1 so scheduled to be in Linux 5.8.
Co-authored-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Remove unnecessary variant_pcs field: the dynamic tag can be checked
directly.
* sysdeps/aarch64/dl-machine.h (elf_machine_runtime_setup): Remove the
DT_AARCH64_VARIANT_PCS check.
(elf_machine_lazy_rel): Use l_info[DT_AARCH64 (VARIANT_PCS)].
* sysdeps/aarch64/linkmap.h (struct link_map_machine): Remove
variant_pcs.
Avoid lazy binding of symbols that may follow a variant PCS with different
register usage convention from the base PCS.
Currently the lazy binding entry code does not preserve all the registers
required for AdvSIMD and SVE vector calls. Saving and restoring all
registers unconditionally may break existing binaries, even if they never
use vector calls, because of the larger stack requirement for lazy
resolution, which can be significant on an SVE system.
The solution is to mark all symbols in the symbol table that may follow
a variant PCS so the dynamic linker can handle them specially. In this
patch such symbols are always resolved at load time, not lazily.
So currently LD_AUDIT for variant PCS symbols are not supported, for that
the _dl_runtime_profile entry needs to be changed e.g. to unconditionally
save/restore all registers (but pass down arg and retval registers to
pltentry/exit callbacks according to the base PCS).
This patch also removes a __builtin_expect from the modified code because
the branch prediction hint did not seem useful.
* sysdeps/aarch64/dl-dtprocnum.h: New file.
* sysdeps/aarch64/dl-machine.h (DT_AARCH64): Define.
(elf_machine_runtime_setup): Handle DT_AARCH64_VARIANT_PCS.
(elf_machine_lazy_rel): Check STO_AARCH64_VARIANT_PCS and bind such
symbols at load time.
* sysdeps/aarch64/linkmap.h (struct link_map_machine): Add variant_pcs.
It was noted in
<https://sourceware.org/ml/libc-alpha/2012-09/msg00305.html> that the
bits/*.h naming scheme should only be used for installed headers.
This patch renames bits/linkmap.h to plain linkmap.h to follow that
convention.
Tested for x86_64 (testsuite, and that installed stripped shared
libraries are unchanged by the patch).
[BZ #14912]
* bits/linkmap.h: Move to ...
* sysdeps/generic/linkmap.h: ...here.
* sysdeps/aarch64/bits/linkmap.h: Move to ...
* sysdeps/aarch64/linkmap.h: ...here.
* sysdeps/arm/bits/linkmap.h: Move to ...
* sysdeps/arm/linkmap.h: ...here.
* sysdeps/hppa/bits/linkmap.h: Move to ...
* sysdeps/hppa/linkmap.h: ...here.
* sysdeps/ia64/bits/linkmap.h: Move to ...
* sysdeps/ia64/linkmap.h: ...here.
* sysdeps/mips/bits/linkmap.h: Move to ...
* sysdeps/mips/linkmap.h: ...here.
* sysdeps/s390/bits/linkmap.h: Move to ...
* sysdeps/s390/linkmap.h: ...here.
* sysdeps/sh/bits/linkmap.h: Move to ...
* sysdeps/sh/linkmap.h: ...here.
* sysdeps/x86/bits/linkmap.h: Move to ...
* sysdeps/x86/linkmap.h: ...here.
* include/link.h: Include <linkmap.h> instead of <bits/linkmap.h>.
