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Subject: Re: [PATCH v2] vfs: add RWF_NOAPPEND flag for pwritev2
To:     Jann Horn <jannh@google.com>, Rich Felker <dalias@libc.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Pavel Begunkov <asml.silence@gmail.com>
References: <20200831153207.GO3265@brightrain.aerifal.cx>
 <CAG48ez39WNuoxYO=RaW5OeVGSOy=uEAZ+xW_++TP7yjkUKGqkg@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <a9d26744-ba7a-2223-7284-c0d1a5ddab8a@kernel.dk>
Date:   Mon, 31 Aug 2020 11:05:34 -0600
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On 8/31/20 9:46 AM, Jann Horn wrote:
> On Mon, Aug 31, 2020 at 5:32 PM Rich Felker <dalias@libc.org> wrote:
>> The pwrite function, originally defined by POSIX (thus the "p"), is
>> defined to ignore O_APPEND and write at the offset passed as its
>> argument. However, historically Linux honored O_APPEND if set and
>> ignored the offset. This cannot be changed due to stability policy,
>> but is documented in the man page as a bug.
>>
>> Now that there's a pwritev2 syscall providing a superset of the pwrite
>> functionality that has a flags argument, the conforming behavior can
>> be offered to userspace via a new flag. Since pwritev2 checks flag
>> validity (in kiocb_set_rw_flags) and reports unknown ones with
>> EOPNOTSUPP, callers will not get wrong behavior on old kernels that
>> don't support the new flag; the error is reported and the caller can
>> decide how to handle it.
>>
>> Signed-off-by: Rich Felker <dalias@libc.org>
> 
> Reviewed-by: Jann Horn <jannh@google.com>
> 
> Note that if this lands, Michael Kerrisk will probably be happy if you
> send a corresponding patch for the manpage man2/readv.2.
> 
> Btw, I'm not really sure whose tree this should go through - VFS is
> normally Al Viro's turf, but it looks like the most recent
> modifications to this function have gone through Jens Axboe's tree?

Should probably go through Al's tree, I've only carried them when
they've been associated with io_uring in some shape or form.

-- 
Jens Axboe