linux-kernel - [RFC][PATCH 3/3] arm64: elf: Advertise relaxed ABI

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Date:   Mon, 10 Dec 2018 14:30:44 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kees Cook <keescook@...omium.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: [RFC][PATCH 3/3] arm64: elf: Advertise relaxed ABI

On arm64 the TCR_EL1.TBI0 bit has been set since Linux 3.x hence
the userspace (EL0) is allowed to set a non-zero value in the top
byte but the resulting pointers are not allowed at the user-kernel
syscall ABI boundary.

This patch sets ARM64_AT_FLAGS_SYSCALL_TBI (bit[0]) in the AT_FLAGS
to advertise the relaxation of the ABI to the userspace.

Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will.deacon@....com>
CC: Andrey Konovalov <andreyknvl@...gle.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
---
 arch/arm64/include/asm/atflags.h      | 7 +++++++
 arch/arm64/include/asm/elf.h          | 5 +++++
 arch/arm64/include/uapi/asm/atflags.h | 8 ++++++++
 3 files changed, 20 insertions(+)
 create mode 100644 arch/arm64/include/asm/atflags.h
 create mode 100644 arch/arm64/include/uapi/asm/atflags.h

diff --git a/arch/arm64/include/asm/atflags.h b/arch/arm64/include/asm/atflags.h
new file mode 100644
index 000000000000..b20093d61bf2
--- /dev/null
+++ b/arch/arm64/include/asm/atflags.h
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_ATFLAGS_H
+#define __ASM_ATFLAGS_H
+
+#include <uapi/asm/atflags.h>
+
+#endif
diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 433b9554c6a1..da5a6d310ff4 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -16,6 +16,7 @@
 #ifndef __ASM_ELF_H
 #define __ASM_ELF_H
 
+#include <asm/atflags.h>
 #include <asm/hwcap.h>
 
 /*
@@ -163,6 +164,10 @@ do {									\
 		NEW_AUX_ENT(AT_IGNORE, 0);				\
 } while (0)
 
+/* Platform specific AT_FLAGS */
+#define ELF_AT_FLAGS			ARM64_AT_FLAGS_SYSCALL_TBI
+#define COMPAT_ELF_AT_FLAGS		0
+
 #define ARCH_HAS_SETUP_ADDITIONAL_PAGES
 struct linux_binprm;
 extern int arch_setup_additional_pages(struct linux_binprm *bprm,
diff --git a/arch/arm64/include/uapi/asm/atflags.h b/arch/arm64/include/uapi/asm/atflags.h
new file mode 100644
index 000000000000..1cf25692ffd6
--- /dev/null
+++ b/arch/arm64/include/uapi/asm/atflags.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __UAPI_ASM_ATFLAGS_H
+#define __UAPI_ASM_ATFLAGS_H
+
+/* Platform specific AT_FLAGS */
+#define ARM64_AT_FLAGS_SYSCALL_TBI	(1 << 0)
+
+#endif
-- 
2.19.2