[RFC v2 PATCH 0/6] x86/entry: cleanups and consistent syscall number handling

From: H. Peter Anvin
Date: Mon May 10 2021 - 14:53:59 EST

From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>

This patchset:

1. Cleans up some duplications between <entry/calling.h> and <asm/ptrace-abi.h>.

2. Swaps the arguments to do_syscall_64() for consistency *and* speed.

3. Adds the maximum number of flags to MSR_SYSCALL_MASK; the previous
is more of a minimum. The more flags that are masked, the less the
likelihood of a control leak into the kernel.

4. Consistently treat the system call number as a signed int. This is
what syscall_get_nr() already does, and therefore what all
architecture-independent code (e.g. seccomp) already expects.

5. As per the defined semantics of syscall_get_nr(), only the value -1
is defined as a non-system call, so comparing >= 0 is
incorrect. Change to != -1.

6. Call sys_ni_syscall() for system calls which are out of range
except for -1, which is used by ptrace and seccomp as a "skip
system call" marker) just as for system call numbers that
correspond to holes in the table.

7. In <entry/calling.h>, factor the PUSH_AND_CLEAR_REGS macro into
separate PUSH_REGS and CLEAR_REGS macros which can be used
separately if desired. This will be used by the FRED entry code at
a later date.

Changes from v1:

* Only -1 should be a non-system call per the cross-architectural
definition of sys_ni_syscall().
* Fix/improve patch descriptions.

arch/x86/entry/calling.h | 45 ++++++--------------------
arch/x86/entry/common.c | 71 ++++++++++++++++++++++++++++--------------
arch/x86/entry/entry_64.S | 4 +--
arch/x86/include/asm/syscall.h | 13 ++++----
arch/x86/kernel/cpu/common.c | 12 +++++--
arch/x86/kernel/head_64.S | 6 ++--
6 files changed, 77 insertions(+), 74 deletions(-)