[PATCH 1/2] powerpc: properly negate error in syscall_set_return_value() in sc case

[Christophe Leroy]

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