Last commit made on 2020-06-12
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20e7c9d... by Szabolcs Nagy <email address hidden> on 2020-06-11

aarch64: add NEWS entry about branch protection support

This is a new security feature that relies on architecture
extensions and needs glibc to be built with a gcc configured
with branch protection.

a8aad40... by Szabolcs Nagy <email address hidden> on 2020-04-15

aarch64: redefine RETURN_ADDRESS to strip PAC

RETURN_ADDRESS is used at several places in glibc to mean a valid
code address of the call site, but with pac-ret it may contain a
pointer authentication code (PAC), so its definition is adjusted.

This is gcc PR target/95891: __builtin_return_address should not
expose signed pointers to user code where it can cause ABI issues.
In glibc RETURN_ADDRESS is only changed if it is built with pac-ret.
There is no detection for the specific gcc issue because it is
hard to test and the additional xpac does not cause problems.

f70dbaf... by Szabolcs Nagy <email address hidden> on 2020-05-13

aarch64: fix pac-ret support in _mcount

Currently gcc -pg -mbranch-protection=pac-ret passes signed return
address to _mcount, so _mcount now has to always strip pac from the
frompc since that's from user code that may be built with pac-ret.

This is gcc PR target/94791: signed pointers should not escape and get
passed across extern call boundaries, since that's an ABI break, but
because existing gcc has this issue we work it around in glibc until
that is resolved. This is compatible with a fixed gcc and it is a nop
on systems without PAuth support. The bug was introduced in gcc-7 with
-msign-return-address=non-leaf|all support which in gcc-9 got renamed
to -mbranch-protection=pac-ret|pac-ret+leaf|standard.

strip_pac uses inline asm instead of __builtin_aarch64_xpaclri since
that is not a documented api and not available in all supported gccs.

a7833d6... by Szabolcs Nagy <email address hidden> on 2020-04-29

aarch64: Add pac-ret support to assembly files

Use return address signing in assembly files for functions that save
LR when pac-ret is enabled in the compiler.

The GNU property note for PAC-RET is not meaningful to the dynamic
linker so it is not strictly required, but it may be used to track
the security property of binaries. (The PAC-RET property is only set
if BTI is set too because BTI implies working GNU property support.)

Reviewed-by: Adhemerval Zanella <email address hidden>

f69c4d0... by Szabolcs Nagy <email address hidden> on 2020-05-07

aarch64: configure check for pac-ret code generation

Return address signing requires unwinder support, which is
present in libgcc since >=gcc-7, however due to bugs the
support may be broken in <gcc-10 (and similarly there may
be issues in custom unwinders), so pac-ret is not always
safe to use. So in assembly code glibc should only use
pac-ret if the compiler uses it too. Unfortunately there
is no predefined feature macro for it set by the compiler
so pac-ret is inferred from the code generation.

Reviewed-by: Adhemerval Zanella <email address hidden>

3d0cab6... by Sudakshina Das on 2020-03-17

aarch64: enable BTI at runtime

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). A new _dl_process_pt_gnu_property
hook is introduced in dl-prop.h and to keep it maintainable the
rtld and dlopen code paths use the same function (if the main
map needs special treatment, that should be inferred by the hook
from the link map). Unlike the _dt_process_pt_note hook this one
is called after segments are mapped to avoid unbounded allocation
and additional read syscall. Otherwise the AArch64 logic follows
the x86 logic for handling GNU properties (but the code is not
shared because x86 needs to manage internal CET state and look
out for multiple property notes).

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

This patch relies on Linux userspace API that is scheduled to be
merged in Linux 5.8 and now it is in the for-next/bti-user branch
of git://

Co-authored-by: Szabolcs Nagy <email address hidden>
Reviewed-by: Adhemerval Zanella <email address hidden>

479a62f... by Szabolcs Nagy <email address hidden> on 2020-03-31

aarch64: fix RTLD_START for BTI

Tailcalls must use x16 or x17 for the indirect branch instruction
to be compatible with code that uses BTI c at function entries.
(Other forms of indirect branches can only land on BTI j.)

Also added a BTI c at the ELF entry point of rtld, this is not
strictly necessary since the kernel does not use indirect branch
to get there, but it seems safest once building glibc itself with
BTI is supported.

Reviewed-by: Adhemerval Zanella <email address hidden>

72d607d... by Szabolcs Nagy <email address hidden> on 2020-04-01

aarch64: fix swapcontext for BTI

setcontext returns to the specified context via an indirect jump,
so there should be a BTI j.

In case of getcontext (and all other returns_twice functions) the
compiler adds BTI j at the call site, but swapcontext is a normal
c call that is currently not handled specially by the compiler.

So we change swapcontext such that the saved context returns to a
local address that has BTI j and then swapcontext returns to the
caller via a normal RET. For this we save the original return
address in the slot for x1 of the context because x1 need not be
preserved by swapcontext but it is restored when the context saved
by swapcontext is resumed.

The alternative fix (which is done on x86) would make swapcontext
special in the compiler so BTI j is emitted at call sites, on
x86 there is an indirect_return attribute for this, on AArch64
we would have to use returns_twice. It was decided against because
such fix may need user code updates: the attribute has to be added
when swapcontext is called via a function pointer and it breaks
always_inline functions with swapcontext.

Reviewed-by: Adhemerval Zanella <email address hidden>

26245e2... by Szabolcs Nagy <email address hidden> on 2020-04-01

aarch64: Rename place holder .S files to .c

The compiler can add required elf markings based on CFLAGS
but the assembler cannot, so using C code for empty files
creates less of a maintenance problem.

Reviewed-by: Adhemerval Zanella <email address hidden>

f440d0a... by Sudakshina Das on 2020-03-17

aarch64: Add BTI support to assembly files

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 <email address hidden>
Reviewed-by: Adhemerval Zanella <email address hidden>