To enable building glibc with branch protection, assembly code
needs BTI landing pads and ELF object file markings in the form
of a GNU property note.
The landing pads are unconditionally added to all functions that
may be indirectly called. When the code segment is not mapped
with PROT_BTI these instructions are nops. They are kept in the
code when BTI is not supported so that the layout of performance
critical code is unchanged across configurations.
The GNU property notes are only added when there is support for
BTI in the toolchain, because old binutils does not handle the
notes right. (Does not know how to merge them nor to put them in
PT_GNU_PROPERTY segment instead of PT_NOTE, and some versions
of binutils emit warnings about the unknown GNU property. In
such cases the produced libc binaries would not have valid
ELF marking so BTI would not be enabled.)
Note: functions using ENTRY or ENTRY_ALIGN now start with an
additional BTI c, so alignment of the following code changes,
but ENTRY_ALIGN_AND_PAD was fixed so there is no change to the
existing code layout. Some string functions may need to be
tuned for optimal performance after this commit.
Co-authored-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
* sysdeps/aarch64/multiarch/memset_base64.S (DC_ZVA_THRESHOLD):
Disable DC ZVA code if this macro is defined as zero.
* sysdeps/aarch64/multiarch/memset_emag.S (DC_ZVA_THRESHOLD):
Change to zero to disable using DC ZVA.
This version uses general register based memory store instead of
vector register based, for the former is faster than the latter
in emag.
The fact that DC ZVA size in emag is 64-byte, is used by IFUNC
dispatch to select this memset, so that cost of runtime-check on
DC ZVA size can be saved.
* sysdeps/aarch64/multiarch/Makefile (sysdep_routines):
Add memset_emag.
* sysdeps/aarch64/multiarch/ifunc-impl-list.c
(__libc_ifunc_impl_list): Add __memset_emag to memset ifunc.
* sysdeps/aarch64/multiarch/memset.c (libc_ifunc):
Add IS_EMAG check for ifunc dispatch.
* sysdeps/aarch64/multiarch/memset_base64.S: New file.
* sysdeps/aarch64/multiarch/memset_emag.S: New file.
