linux-kernel - Re: [PATCH 1/2] powerpc: properly negate error in syscall_set_return

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Message-ID: <b12039ce-99da-41e9-ba2f-dc0e0c62f140@csgroup.eu>
Date: Tue, 28 Jan 2025 17:00:31 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Dmitry V. Levin" <ldv@...ace.io>
Cc: Michael Ellerman <mpe@...erman.id.au>, Alexey Gladkov
 <legion@...nel.org>, Eugene Syromyatnikov <evgsyr@...il.com>,
 Oleg Nesterov <oleg@...hat.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Nicholas Piggin <npiggin@...il.com>, Naveen N Rao <naveen@...nel.org>,
 linuxppc-dev@...ts.ozlabs.org, strace-devel@...ts.strace.io,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] powerpc: properly negate error in
 syscall_set_return_value() in sc case

Le 28/01/2025 à 16:52, Dmitry V. Levin a écrit :
> On Tue, Jan 28, 2025 at 03:59:29PM +0100, Christophe Leroy wrote:
>> Le 27/01/2025 à 19:13, Dmitry V. Levin a écrit :
>>> According to the Power Architecture Linux system call ABI documented in
>>> [1], when the syscall is made with the sc instruction, both a value and an
>>> error condition are returned, where r3 register contains the return value,
>>> and cr0.SO bit specifies the error condition.  When cr0.SO is clear, the
>>> syscall succeeded and r3 is the return value.  When cr0.SO is set, the
>>> syscall failed and r3 is the error value.  This syscall return semantics
>>> was implemented from the very beginning of Power Architecture on Linux,
>>> and syscall tracers and debuggers like strace that read or modify syscall
>>> return information also rely on this ABI.
>>
>> I see a quite similar ABI on microblaze, mips, nios2 and sparc. Do they
>> behave all the same ?
> 
> Yes, also on alpha.  I don't think microblaze should be in this list,
> though.

Microblaze has

static inline void syscall_set_return_value(struct task_struct *task,
					    struct pt_regs *regs,
					    int error, long val)
{
	if (error)
		regs->r3 = -error;
	else
		regs->r3 = val;
}

So it has a positive error setting allthough it has no flag to tell it 
is an error. Wondering how it works at the end.

Alpha I'm not sure, I see nothing obvious in include/asm/ptrace.h or 
include/asm/syscall.h