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

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Date:   Fri, 20 May 2022 14:43:22 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Jens Axboe <axboe@...nel.dk>
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

Hi Jens,

On Thu, May 19, 2022 at 07:10:56PM -0600, Jens Axboe wrote:
> 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().

Indeed. Looks like that was your code from long long ago!

I posted
https://lore.kernel.org/lkml/20220520095747.123748-1-Jason@zx2c4.com/ to
fix it if you'd like to review.

Jason