Re: [RFC patch 01/15] entry: Provide generic syscall entry functionality

[Andy Lutomirski <luto@kernel.org>]

On Thu, Sep 19, 2019 at 8:09 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On syscall entry certain work needs to be done conditionally like tracing,
> seccomp etc. This code is duplicated in all architectures.
>
> Provide a generic version.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/Kconfig                 |    3 +
>  include/linux/entry-common.h |  122 +++++++++++++++++++++++++++++++++++++++++++
>  kernel/Makefile              |    1
>  kernel/entry/Makefile        |    3 +
>  kernel/entry/common.c        |   33 +++++++++++
>  5 files changed, 162 insertions(+)
>
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -27,6 +27,9 @@ config HAVE_IMA_KEXEC
>  config HOTPLUG_SMT
>         bool
>
> +config GENERIC_ENTRY
> +       bool
> +
>  config OPROFILE
>         tristate "OProfile system profiling"
>         depends on PROFILING
> --- /dev/null
> +++ b/include/linux/entry-common.h
> @@ -0,0 +1,122 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __LINUX_ENTRYCOMMON_H
> +#define __LINUX_ENTRYCOMMON_H
> +
> +#include <linux/tracehook.h>
> +#include <linux/syscalls.h>
> +#include <linux/seccomp.h>
> +#include <linux/sched.h>
> +#include <linux/audit.h>
> +
> +#include <asm/entry-common.h>
> +
> +/*
> + * Define dummy _TIF work flags if not defined by the architecture or for
> + * disabled functionality.
> + */
> +#ifndef _TIF_SYSCALL_TRACE
> +# define _TIF_SYSCALL_TRACE            (0)
> +#endif
> +
> +#ifndef _TIF_SYSCALL_EMU
> +# define _TIF_SYSCALL_EMU              (0)
> +#endif
> +
> +#ifndef _TIF_SYSCALL_TRACEPOINT
> +# define _TIF_SYSCALL_TRACEPOINT       (0)
> +#endif
> +
> +#ifndef _TIF_SECCOMP
> +# define _TIF_SECCOMP                  (0)
> +#endif
> +
> +#ifndef _TIF_AUDIT
> +# define _TIF_AUDIT                    (0)
> +#endif

I'm wondering if these should be __TIF (double-underscore) or
MAYBE_TIF_ or something to avoid errors where people do flags |=
TIF_WHATEVER and get surprised.

> +/**
> + * syscall_enter_from_usermode - Check and handle work before invoking
> + *                              a syscall
> + * @regs:      Pointer to currents pt_regs
> + * @syscall:   The syscall number
> + *
> + * Invoked from architecture specific syscall entry code with interrupts
> + * enabled.
> + *
> + * Returns: The original or a modified syscall number
> + */

Maybe document that it can return -1 to skip the syscall and that, if
this happens, it may use syscall_set_error() or
syscall_set_return_value() first.  If neither of those is called and
-1 is returned, then the syscall will fail with ENOSYS.