linux-kernel - Re: [PATCH v2 1/7] powerpc: properly negate error in syscall_set_return

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Message-ID: <20250114134844.GA10630@strace.io>
Date: Tue, 14 Jan 2025 15:48:44 +0200
From: "Dmitry V. Levin" <ldv@...ace.io>
To: Alexey Gladkov <legion@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Michael Ellerman <mpe@...erman.id.au>,
	Eugene Syromyatnikov <evgsyr@...il.com>,
	Mike Frysinger <vapier@...too.org>,
	Renzo Davoli <renzo@...unibo.it>,
	Davide Berardi <berardi.dav@...il.com>,
	strace-devel@...ts.strace.io,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Naveen N Rao <naveen@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/7] powerpc: properly negate error in
 syscall_set_return_value()

On Tue, Jan 14, 2025 at 02:00:16PM +0100, Alexey Gladkov wrote:
> On Mon, Jan 13, 2025 at 07:10:54PM +0200, Dmitry V. Levin wrote:
> > Bring syscall_set_return_value() in sync with syscall_get_error(),
> > and let upcoming ptrace/set_syscall_info selftest pass on powerpc.
> > 
> > This reverts commit 1b1a3702a65c ("powerpc: Don't negate error in
> > syscall_set_return_value()").
> > 
> > Signed-off-by: Dmitry V. Levin <ldv@...ace.io>
> > ---
> >  arch/powerpc/include/asm/syscall.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/include/asm/syscall.h b/arch/powerpc/include/asm/syscall.h
> > index 3dd36c5e334a..422d7735ace6 100644
> > --- a/arch/powerpc/include/asm/syscall.h
> > +++ b/arch/powerpc/include/asm/syscall.h
> > @@ -82,7 +82,11 @@ static inline void syscall_set_return_value(struct task_struct *task,
> >  		 */
> >  		if (error) {
> >  			regs->ccr |= 0x10000000L;
> > -			regs->gpr[3] = error;
> > +			/*
> > +			 * In case of an error regs->gpr[3] contains
> > +			 * a positive ERRORCODE.
> > +			 */
> > +			regs->gpr[3] = -error;
> 
> After this change the syscall_get_error() will return positive value if
> the system call failed. Since syscall_get_error() still believes
> regs->gpr[3] is still positive in case !trap_is_scv().
> 
> Or am I missing something?

syscall_get_error() does the following in case of !trap_is_scv():

                /*
                 * If the system call failed,
                 * regs->gpr[3] contains a positive ERRORCODE.
                 */
                return (regs->ccr & 0x10000000UL) ? -regs->gpr[3] : 0;

That is, in !trap_is_scv() case it assumes that regs->gpr[3] is positive
and is going to return a negative value (-ERRORCODE).

> It looks like the selftest you mentioned in the commit message doesn't
> check the !trap_is_scv() branch.

The selftest is architecture-agnostic, it just executes syscalls and
checks whether the data returned by PTRACE_GET_SYSCALL_INFO meets
expectations.  Do you mean that syscall() is not good enough for syscall
invocation from coverage perspective on powerpc?

See also commit d72500f99284 ("powerpc/64s/syscall: Fix ptrace syscall
info with scv syscalls").


-- 
ldv