Re: [PATCH] linux-user: mprotect() should returns 0 when len is 0.

From: Richard Henderson <richard.henderson@linaro.org>
To: Peter Maydell <peter.maydell@linaro.org>,
	Soichiro Isshiki <sisshiki@isshiki-clinic.com>
Cc: qemu-devel@nongnu.org, qemu-trivial@nongnu.org,
	sisshiki1969 <sisshiki@mac.com>
Subject: Re: [PATCH] linux-user: mprotect() should returns 0 when len is 0.
Date: Thu, 6 Oct 2022 11:31:21 -0700	[thread overview]
Message-ID: <15c94fb7-3861-7933-91e9-49a6983a185d@linaro.org> (raw)
In-Reply-To: <CAFEAcA-fXq47T2kNUcjbpZ9fQbLdc=jzkT_phMqJ5wyKrdXNmg@mail.gmail.com>

On 10/6/22 11:13, Peter Maydell wrote:
> On Thu, 6 Oct 2022 at 19:05, Soichiro Isshiki
> <sisshiki@isshiki-clinic.com> wrote:
>>
>> From: sisshiki1969 <sisshiki@mac.com>
>>
>> For now, qemu-x86_64 returns ENOMEM when mprotect() was called with an argument
>> len is 0 from a guest process.
>> This behavior is incompatible with the current Linux implementation,
>> which mprotect() with len = 0 does nothing and returns 0,
>> although it does not appear to be explicitly described in man.
>>
>> This is due to the following function which always returns false if len = 0.
>>
>> ```C
>> static inline bool guest_range_valid_untagged(abi_ulong start, abi_ulong len)
>> {
>>      return len - 1 <= GUEST_ADDR_MAX && start <= GUEST_ADDR_MAX - len + 1;
>> }
>>
...
> Cc'ing Richard -- is this the right fix, or would it be better instead
> to make guest_range_valid_untagged() correctly handle a zero-length
> range ?

I think fixing the range check might be best.

r~