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

From: Peter Zijlstra <peterz@infradead.org>
To: Sven Schnelle <svens@linux.ibm.com>
Cc: "Thomas Gleixner" <tglx@kernel.org>,
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Subject: Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR
Date: Fri, 3 Jul 2026 11:25:44 +0200	[thread overview]
Message-ID: <20260703092544.GB651302@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <yt9dwlvca8rn.fsf@linux.ibm.com>

On Fri, Jul 03, 2026 at 08:26:36AM +0200, Sven Schnelle wrote:

> The difference between x86 and s390 is that on s390, regs->gprs[2] is
> used for both the syscall number and the syscall return value.
> That was a design mistake early in the begin about 25 years ago, but
> it's ABI now, so it cannot be changed.
> 
> When seccomp decides to skip a syscall, it write a return value into
> regs->gprs[2]. When syscall_enter_from_user_mode_work() returns, it
> returns this number. If it's negative all is good - the 'if (likely(nr <
> NR_syscalls))' conditiion would just catch it and skip the syscall.

You do have regs->orig_gpr2; I didn't fully track its usage, but can't
you treat that as the syscall nr and always consider regs->gprs[2] as the
return value?