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The current 64-bit system call entry code treats out-of-range system calls differently than system calls that map to a hole in the system call table. This is visible to the user if system calls are intercepted via ptrace or seccomp and the return value (regs->ax) is modified: in the former case, the return value is preserved, and in the latter case, sys_ni_syscall() is called and the return value is forced to -ENOSYS. The API spec in <asm-generic/syscalls.h> is very clear that only (int)-1 is the non-system-call sentinel value, so make the system call behavior consistent by calling sys_ni_syscall() for all invalid system call numbers except for -1. Although currently sys_ni_syscall() simply returns -ENOSYS, calling it explicitly is friendly for tracing and future possible extensions, and as this is an error path there is no reason to optimize it. Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20210518191303.4135296-6-hpa@zytor.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.