Re: [PATCH v2 1/7] powerpc: properly negate error in syscall_set_return_value()

Alexey Gladkov
2025-01-14T14:53:26Z

On Tue, Jan 14, 2025 at 03:48:44PM +0200, Dmitry V. Levin wrote:
> 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@strace.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).

Yeah. Now I see it.

if (trap_is_scv(regs)) {
regs->result = -EINTR;
regs->gpr[3] = -EINTR;
} else {
regs->result = -EINTR;
regs->gpr[3] = EINTR;
regs->ccr |= 0x10000000;
}

Two different APIs imply gpr[3] with a different sign.

You can add:

Reviewed-by: Alexey Gladkov <legion@kernel.org>

> > 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

--
Rgrds, legion