linux-kernel - Re: [PATCHSET 0/2] Fix splice from random/urandom

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Message-ID: <435cc499-7564-13e2-c4ef-a71119379cf0@kernel.dk>
Date:   Thu, 19 May 2022 19:10:56 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Theodore Ts'o <tytso@....edu>, Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHSET 0/2] Fix splice from random/urandom

On 5/19/22 7:00 PM, Jason A. Donenfeld wrote:
> Hi Jens,
> 
> On Thu, May 19, 2022 at 06:56:12PM -0600, Jens Axboe wrote:
>> On 5/19/22 6:48 PM, Jason A. Donenfeld wrote:
>>> sendfile() returns -EINVAL even with your patches. Only splicing to pipes
>>> seems to work.
>>
>> Huh, that really should work. Are you trying to sendfile() to random? If
>> so, you need that last write_iter patch too, and add the splice_write as
>> I mentioned.
>  
> No, I've only tried the read side so far. I made a little program:
> 
> #include <sys/sendfile.h>
> #include <stdio.h>
> 
> int main(int argc, char *argv[])
> {
>         ssize_t s = sendfile(1, 0, NULL, 0xffff);
>         fprintf(stderr, "ret: %zd\n", s);
>         return 0;
> }
> 
> Then I ran `./a.out < /dev/urandom > /dev/null`. Fails. OTOH, if I
> replace /dev/urandom with an ordinary file, it succeeds.

Here's why, it's limited to regular files or block devices:

if (unlikely(!S_ISREG(i_mode) && !S_ISBLK(i_mode)))
	return -EINVAL;

in splice_direct_to_actor().


-- 
Jens Axboe