linux-kernel - Re: [PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1

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Date:   Thu, 20 Jul 2023 11:08:37 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Celeste Liu <coelacanthushex@...il.com>,
        Andreas Schwab <schwab@...e.de>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Björn Töpel <bjorn@...osinc.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Felix Yan <felixonmars@...hlinux.org>,
        Ruizhe Pan <c141028@...il.com>,
        Shiqi Zhang <shiqi@...c.iscas.ac.cn>
Subject: Re: [PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1

Celeste Liu <coelacanthushex@...il.com> writes:

> On July 20, 2023 12:28:47 AM GMT+08:00, "Björn Töpel" <bjorn@...nel.org> wrote:
>>Andreas Schwab <schwab@...e.de> writes:
>>
>>> On Jul 19 2023, Celeste Liu wrote:
>>>
>>>> @@ -308,7 +312,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>>>>  
>>>>  		if (syscall < NR_syscalls)
>>>>  			syscall_handler(regs, syscall);
>>>> -		else
>>>> +		else if ((long)syscall != -1L)
>>>
>>> You can also use syscall != -1UL or even syscall != -1.
>>
>>The former is indeed better for the eyes! :-) The latter will get a
>>-Wsign-compare warning, no?
>>
>>
>>Björn
>
> Well, that's true. And I just found out that by C standards, converting
> ulong to long is implementation-defined behavior, unlike long to ulong
> which is well-defined. So it is really better than (long)syscall != -1L.

If you're respinning, I suggest you use David's suggestion:
 * Remove the comment I suggest you to add
 * Use (signed) long
 * Add syscall >= 0 &&
 * else if (syscall != -1)

Which is the least amount of surprises IMO.