[AArch64] Add v8.5 Branch Target Identification support.

[danielkiss]

Summary

The .note.gnu.property must be in the assembly file to indicate the
support for BTI otherwise BTI will be disabled for the whole library.
__unw_getcontext and libunwind::Registers_arm64::jumpto() may be called
indirectly therefore they should start with a landing pad.

[danielkiss]

Please help me how to add a test for it, in the current state this patch won't do anything because the standard build for libunwind does not add -mbranch-protection=standard (or bti).

[chill]

libunwind/src/assembly.h

47

This will be emitted for all functions, but we need only one per object file, isn't it ?

Also, could you, please, rename PPC64OPD{1,2} to TARGET_OPD{1,2}, or {ARCH ...}, something like that.

[miyuki]

In D77786#1971519, @danielkiss wrote:

Please help me how to add a test for it, in the current state this patch won't do anything because the standard build for libunwind does not add -mbranch-protection=standard (or bti).

I suggest adding a CMake variable to configure the branch protection option used when building the C++ libraries

[MaskRay]

libunwind/src/assembly.h

46

Just use defined(__aarch64__). I deleted __arm64__ in D77829

48

I am worried that we may want to change the section type for .note.gnu.property

https://sourceware.org/pipermail/binutils/2020-April/000372.html

[danielkiss]

In D77786#1971668, @miyuki wrote:

In D77786#1971519, @danielkiss wrote:

Please help me how to add a test for it, in the current state this patch won't do anything because the standard build for libunwind does not add -mbranch-protection=standard (or bti).

I suggest adding a CMake variable to configure the branch protection option used when building the C++ libraries

Thanks, it will be a new patch.

libunwind/src/assembly.h

47

Yes, only one function per file today, but let's add the section per include.

Rename is done.

48

Thanks for the heads-up, I'm not on that mailing list...

[MaskRay]

Mostly good apart from the renaming issue and a nit about comments. I have some patches in the BTI/PAC area, so I am confident.

libunwind/src/assembly.h

47

@chill
Please don't rename OPD{1,2}. They are really specific to PowerPC ELFv1 function descriptors. No other architecture needs them.

53

The comments can be added right after .long

[MaskRay]

libunwind/src/assembly.h

62

This really needs to use a new macro.

I expect a __ARM_FEATURE_BTI_DEFAULT toolchain capable of assembling bti c

[chill]

libunwind/src/assembly.h

163

All right, of these are to stay named PPC64_OPD, then just add a new line with, say, AARCH64_BTI, e..g

#define DEFINE_LIBUNWIND_FUNCTION(name)                                        \
.globl SYMBOL_NAME(name) SEPARATOR                                           \
HIDDEN_SYMBOL(SYMBOL_NAME(name)) SEPARATOR                                   \
SYMBOL_IS_FUNC(SYMBOL_NAME(name)) SEPARATOR                                  \
PPC64_OPD1                                                                    \ 
SYMBOL_NAME(name):                                                           \
PPC64_OPD2                                                                   \
AARCH64_BTI

[danielkiss]

libunwind/src/assembly.h

62

Expectation is right, D71658 broke it, I will take a look.

[danielkiss]

libunwind/src/assembly.h

53

Missed this comment in the previous version.

[danielkiss]

D81257 is merged, so now BTI C mnemonic can be used instead of the hint.

[danielkiss]

Ping.

[compnerd]

libunwind/src/assembly.h

51

I'm not sure if the defined(__aarch64__) check is absolutely required, but it doesn't give the wrong result.

[danielkiss]

fix review comment.

[danielkiss]

libunwind/src/assembly.h

51

Thanks, fixed it.

[danielkiss]

libunwind/src/assembly.h

51–63

I would drop this because it could be now replaced with a compiler flag -mmark-bti-property (D81930)

[danielkiss]

libunwind/src/assembly.h

51–63

adding the flag to only the assembly files would be huge change in the cmake files, so I revoke my idea to drop the note from here.

[tamas.petz]

LGTM

libunwind/src/assembly.h

 #define PPC64_OPD1 .section .opd,"aw",@progbits SEPARATOR
 #define PPC64_OPD2 SEPARATOR \
   .p2align 3 SEPARATOR \
   .quad .Lfunc_begin0 SEPARATOR \
   .quad .TOC.@tocbase SEPARATOR \
   .quad 0 SEPARATOR \
   .text SEPARATOR \
 .Lfunc_begin0:
 #else
 #define PPC64_OPD1
 #define PPC64_OPD2
 #endif
 
+#if defined(__ARM_FEATURE_BTI_DEFAULT)
+  .pushsection ".note.gnu.property", "a" SEPARATOR                             \
+  .balign 8 SEPARATOR                                                          \
+  .long 4 SEPARATOR                                                            \
+  .long 0x10 SEPARATOR                                                         \
+  .long 0x5 SEPARATOR                                                          \
+  .asciz "GNU" SEPARATOR                                                       \
+  .long 0xc0000000 SEPARATOR /* GNU_PROPERTY_AARCH64_FEATURE_1_AND */          \
+  .long 4 SEPARATOR                                                            \
+  .long 3 SEPARATOR /* GNU_PROPERTY_AARCH64_FEATURE_1_BTI AND */               \
+                    /* GNU_PROPERTY_AARCH64_FEATURE_1_PAC */                   \
+  .long 0 SEPARATOR                                                            \
+  .popsection SEPARATOR
+#define AARCH64_BTI  bti c
+#else
+#define AARCH64_BTI
+#endif
+
 #define GLUE2(a, b) a ## b
 #define GLUE(a, b) GLUE2(a, b)
 #define SYMBOL_NAME(name) GLUE(__USER_LABEL_PREFIX__, name)
 
 #if defined(__APPLE__)
 
 #define SYMBOL_IS_FUNC(name)
 #define EXPORT_SYMBOL(name)

 #error Unsupported target
 
 #endif
 
 #define DEFINE_LIBUNWIND_FUNCTION(name)                                        \
   .globl SYMBOL_NAME(name) SEPARATOR                                           \
   HIDDEN_SYMBOL(SYMBOL_NAME(name)) SEPARATOR                                   \
   SYMBOL_IS_FUNC(SYMBOL_NAME(name)) SEPARATOR                                  \
   PPC64_OPD1                                                                   \
   SYMBOL_NAME(name):                                                           \
-  PPC64_OPD2
+  PPC64_OPD2                                                                   \
+  AARCH64_BTI
 
 #if defined(__arm__)
 #if !defined(__ARM_ARCH)
 #define __ARM_ARCH 4
 #endif
 
 #if defined(__ARM_ARCH_4T__) || __ARM_ARCH >= 5
 #define ARM_HAS_BX