linux-kernel - Re: [RFC v2 PATCH 7/7] x86/entry: use int for syscall number; handle all invalid syscall nrs

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Date:   Wed, 12 May 2021 10:51:20 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 PATCH 7/7] x86/entry: use int for syscall number; handle
 all invalid syscall nrs


* H. Peter Anvin <hpa@...or.com> wrote:

> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
> 
> Redefine the system call number consistently to be "int". The value -1
> is a non-system call (which can be poked in by ptrace/seccomp to
> indicate that no further processing should be done and that the return
> value should be the current value in regs->ax, default to -ENOSYS; any
> other value which does not correspond to a valid system call
> unconditionally calls sys_ni_syscall() and returns -ENOSYS just like
> any system call that corresponds to a hole in the system call table.
> 
> This is the defined semantics of syscall_get_nr(), so that is what all
> the architecture-independent code already expects.  As documented in
> <asm-generic/syscall.h> (which is simply the documentation file for
> <asm/syscall.h>):
> 
> /**
>  * syscall_get_nr - find what system call a task is executing
>  * @task:       task of interest, must be blocked
>  * @regs:       task_pt_regs() of @task
>  *
>  * If @task is executing a system call or is at system call
>  * tracing about to attempt one, returns the system call number.
>  * If @task is not executing a system call, i.e. it's blocked
>  * inside the kernel for a fault or signal, returns -1.
>  *
>  * Note this returns int even on 64-bit machines.  Only 32 bits of
>  * system call number can be meaningful.  If the actual arch value
>  * is 64 bits, this truncates to 32 bits so 0xffffffff means -1.
>  *
>  * It's only valid to call this when @task is known to be blocked.
>  */
> int syscall_get_nr(struct task_struct *task, struct pt_regs *regs);

I've applied patches 1-6, thanks Peter!

Wrt. patch #7 - the changelog is hedging things a bit and the changes are 
non-trivial. Does this patch (intend to) change any actual observable 
behavior in the system call interface, and if yes, in which areas?

Or is this a pure cleanup with no observable changes expected?

Thanks,

	Ingo