On Mar 6, 2020, at 5:32 AM, Vincenzo Frascino [off-list ref] wrote:

Back in July last year we started having a problem in building compat
vDSOs on arm64 [1] [2] that was not present when the arm64 porting to
the Unified vDSO was done. In particular when the compat vDSO on such
architecture is built with gcc it generates the warning below:

In file included from ./arch/arm64/include/asm/thread_info.h:17:0,
                from ./include/linux/thread_info.h:38,
                from ./arch/arm64/include/asm/preempt.h:5,
                from ./include/linux/preempt.h:78,
                from ./include/linux/spinlock.h:51,
                from ./include/linux/seqlock.h:36,
                from ./include/linux/time.h:6,
                from ./lib/vdso/gettimeofday.c:7,
                from <command-line>:0:
./arch/arm64/include/asm/memory.h: In function ‘__tag_set’:
./arch/arm64/include/asm/memory.h:233:15: warning: cast from pointer
               to integer of different size [-Wpointer-to-int-cast]
 u64 __addr = (u64)addr & ~__tag_shifted(0xff);
              ^
In file included from ./arch/arm64/include/asm/pgtable-hwdef.h:8:0,
                from ./arch/arm64/include/asm/processor.h:34,
                from ./arch/arm64/include/asm/elf.h:118,
                from ./include/linux/elf.h:5,
                from ./include/linux/elfnote.h:62,
                from arch/arm64/kernel/vdso32/note.c:11:
./arch/arm64/include/asm/memory.h: In function ‘__tag_set’:
./arch/arm64/include/asm/memory.h:233:15: warning: cast from pointer
               to integer of different size [-Wpointer-to-int-cast]
 u64 __addr = (u64)addr & ~__tag_shifted(0xff);

The same porting does not build at all when the selected compiler is
clang.

I started an investigation to try to understand better the problem and
after various discussions at Plumbers and Recipes last year the
conclusion was that the vDSO library as it stands it is including more
headers that it needs. In particular, being a user-space library, it
should require only the UAPI and a minimal vDSO kernel interface instead
of all the kernel-related inline functions which are not directly used
and in some cases can have side effects.

To solve the problem, I decided to use the approach below:
 * Extract from include/linux/ the vDSO required kernel interface
   and place it in include/common/

I really like the approach, but I’m wondering if “common” is the right name. This directory is headers that aren’t stable ABI like uapi but are shared between the kernel and the vDSO. Regular user code should *not* include these, right?

Would “vdso” or perhaps “private-abi” be clearer?

 * Make sure that where meaningful the kernel includes "common"
 * Limit the vDSO library to include headers coming only from UAPI
   and "common" (with 2 exceptions compiler.h for barriers and
   param.h for HZ).
 * Adapt all the architectures that support the unified vDSO library
   to use "common" headers.

According to me this approach allows up to exercise a better control on
what the vDSO library can include and to prevent potential issues in
future.

This patch series contains the implementation of the described approach.

The "common" headers have been verified on all the architectures that support
unified vDSO using the vdsotest [3] testsuite for what concerns the vDSO part
and randconfig to verify that they are included in the correct places.

To simplify the testing, a copy of the patchset on top of a recent linux
tree can be found at [4].

[1] https://github.com/ClangBuiltLinux/linux/issues/595
[2] https://lore.kernel.org/lkml/20190926151704.GH9689@arrakis.emea.arm.com (local)
[3] https://github.com/nathanlynch/vdsotest
[4] git://linux-arm.org/linux-vf.git common-headers/v2

Changes:
--------
v2:
 - Addressed review comments for clang support.
 - Rebased on 5.6-rc4.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Paul Burton <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Mark Salyzyn <redacted>
Cc: Kees Cook <redacted>
Cc: Peter Collingbourne <redacted>
Cc: Dmitry Safonov <redacted>
Cc: Andrei Vagin <redacted>
Cc: Nick Desaulniers <redacted>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

Vincenzo Frascino (20):
 linux/const.h: Extract common header for vDSO
 linux/bits.h: Extract common header for vDSO
 linux/limits.h: Extract common header for vDSO
 linux/math64.h: Extract common header for vDSO
 linux/time.h: Extract common header for vDSO
 linux/time32.h: Extract common header for vDSO
 linux/time64.h: Extract common header for vDSO
 linux/jiffies.h: Extract common header for vDSO
 linux/ktime.h: Extract common header for vDSO
 common: Introduce processor.h
 linux/elfnote.h: Replace elf.h with UAPI equivalent
 arm64: Introduce asm/common/processor.h
 arm64: vdso: Include common headers in the vdso library
 arm64: vdso32: Include common headers in the vdso library
 arm64: Introduce asm/common/arch_timer.h
 mips: vdso: Enable mips to use common headers
 x86: vdso: Enable x86 to use common headers
 arm: vdso: Enable arm to use common headers
 lib: vdso: Enable common headers
 arm64: vdso32: Enable Clang Compilation

arch/arm/include/asm/common/cp15.h            | 38 +++++++++++++++++++
arch/arm/include/asm/common/processor.h       | 22 +++++++++++
arch/arm/include/asm/cp15.h                   | 20 +---------
arch/arm/include/asm/processor.h              | 11 +-----
arch/arm/include/asm/vdso/gettimeofday.h      |  4 +-
arch/arm64/include/asm/arch_timer.h           | 29 +++-----------
arch/arm64/include/asm/common/arch_timer.h    | 33 ++++++++++++++++
arch/arm64/include/asm/common/processor.h     | 31 +++++++++++++++
arch/arm64/include/asm/processor.h            | 16 +-------
.../include/asm/vdso/compat_gettimeofday.h    |  2 +-
arch/arm64/include/asm/vdso/gettimeofday.h    |  8 ++--
arch/arm64/kernel/vdso/vgettimeofday.c        |  2 -
arch/arm64/kernel/vdso32/Makefile             | 13 ++++++-
arch/arm64/kernel/vdso32/vgettimeofday.c      |  3 --
arch/mips/include/asm/common/processor.h      | 27 +++++++++++++
arch/mips/include/asm/processor.h             | 16 +-------
arch/mips/include/asm/vdso/gettimeofday.h     |  4 --
arch/x86/include/asm/common/processor.h       | 23 +++++++++++
arch/x86/include/asm/processor.h              | 12 +-----
include/common/bits.h                         |  9 +++++
include/common/const.h                        | 10 +++++
include/common/jiffies.h                      | 11 ++++++
include/common/ktime.h                        | 16 ++++++++
include/common/limits.h                       | 18 +++++++++
include/common/math64.h                       | 24 ++++++++++++
include/common/processor.h                    | 14 +++++++
include/common/time.h                         | 12 ++++++
include/common/time32.h                       | 17 +++++++++
include/common/time64.h                       | 14 +++++++
include/linux/bits.h                          |  2 +-
include/linux/const.h                         |  5 +--
include/linux/elfnote.h                       |  2 +-
include/linux/jiffies.h                       |  4 +-
include/linux/ktime.h                         |  9 +----
include/linux/limits.h                        | 13 +------
include/linux/math64.h                        | 20 +---------
include/linux/time.h                          |  5 +--
include/linux/time32.h                        | 13 +------
include/linux/time64.h                        | 10 +----
include/vdso/datapage.h                       | 32 ++++++++++++++--
lib/vdso/gettimeofday.c                       | 21 ----------
41 files changed, 388 insertions(+), 207 deletions(-)
create mode 100644 arch/arm/include/asm/common/cp15.h
create mode 100644 arch/arm/include/asm/common/processor.h
create mode 100644 arch/arm64/include/asm/common/arch_timer.h
create mode 100644 arch/arm64/include/asm/common/processor.h
create mode 100644 arch/mips/include/asm/common/processor.h
create mode 100644 arch/x86/include/asm/common/processor.h
create mode 100644 include/common/bits.h
create mode 100644 include/common/const.h
create mode 100644 include/common/jiffies.h
create mode 100644 include/common/ktime.h
create mode 100644 include/common/limits.h
create mode 100644 include/common/math64.h
create mode 100644 include/common/processor.h
create mode 100644 include/common/time.h
create mode 100644 include/common/time32.h
create mode 100644 include/common/time64.h

-- 
2.25.1

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