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

From: Sven Schnelle <svens@linux.ibm.com>
To: "Michal Suchánek" <msuchanek@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR
Date: Fri, 03 Jul 2026 13:39:18 +0200	[thread overview]
Message-ID: <yt9dtsqg480p.fsf@linux.ibm.com> (raw)
In-Reply-To: <akecJWAJP-e5CYP_@kunlun.suse.cz>

Michal Suchánek <msuchanek@suse.de> writes:

> The same could be asked of syscall_enter_from_user_mode. I find it very
> odd. Why does it conflate the syscall number with its return value?
>
> It never uses the syscall number passed in except when returning it
> unchanged. When it pokes the registers it reads the syscall number from
> them.
>
> If the caller of syscall_enter_from_user_mode only read the syscall
> number from the registers when syscall_enter_from_user_mode returns and
> indicates the syscall should be still executed this whole shenigan would

I agree. The fact that if (nr < NR_syscall) just works because -1 gets
casted to 0xffffffff and is therefore out of bounds is very odd.