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

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Message-ID: <20250128161005.GC11869@strace.io>
Date: Tue, 28 Jan 2025 18:10:05 +0200
From: "Dmitry V. Levin" <ldv@...ace.io>
To: Christophe Leroy <christophe.leroy@...roup.eu>
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

On Tue, Jan 28, 2025 at 05:00:31PM +0100, Christophe Leroy wrote:
> 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.

It's a bug, but given that microblaze doesn't enable
CONFIG_HAVE_ARCH_TRACEHOOK, most likely this function is unused there.

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

Alpha doesn't enable CONFIG_HAVE_ARCH_TRACEHOOK, it just lacks the
necessary interfaces, but it uses a3 register for this purpose, see
arch/alpha/kernel/entry.S for details.


-- 
ldv