Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

From: Thomas Gleixner <tglx@kernel.org>
To: "Michal Suchánek" <msuchanek@suse.de>,
	"Peter Zijlstra" <peterz@infradead.org>
Cc: "Jonathan Corbet" <corbet@lwn.net>,
	"Shuah Khan" <skhan@linuxfoundation.org>,
	"Huacai Chen" <chenhuacai@kernel.org>,
	"WANG Xuerui" <kernel@xen0n.name>,
	"Madhavan Srinivasan" <maddy@linux.ibm.com>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	"Paul Walmsley" <pjw@kernel.org>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Heiko Carstens" <hca@linux.ibm.com>,
	"Vasily Gorbik" <gor@linux.ibm.com>,
	"Alexander Gordeev" <agordeev@linux.ibm.com>,
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	"Andrew Donnellan" <andrew+kernel@donnellan.id.au>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Michal Suchánek" <msuchanek@suse.de>,
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Subject: Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR
Date: Mon, 06 Jul 2026 10:16:57 +0200	[thread overview]
Message-ID: <87ik6sjzwm.ffs@fw13> (raw)
In-Reply-To: <akVRcPsD_R_CE1qW@kunlun.suse.cz>

On Wed, Jul 01 2026 at 19:42, Michal Suchánek wrote:
>  
>  	if (unlikely(work & SYSCALL_WORK_SYSCALL_TRACEPOINT))
> -		syscall = trace_syscall_enter(regs, syscall);
> +		*syscall = trace_syscall_enter(regs, *syscall);
>  
> -	syscall_enter_audit(regs, syscall);
> +	syscall_enter_audit(regs, *syscall);
>  
> -	return ret ? : syscall;
> +	return 0;

That breaks trace_syscall_enter() because the probes/BPF muck attached
ot that can:

     1) set the return code
     2) set syscall to -1L

That works correctly today, but with your sloppy hackery the low level
x86 code falls into sys_ni_syscall() which sets the return code to
-ENOSYS.