linux-kernel - Re: [PATCH bpf v2] lib/buildid: use __kernel

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Message-ID: <aUTPl35UPcjc66l3@casper.infradead.org>
Date: Fri, 19 Dec 2025 04:07:51 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Shaurya Rane <ssrane_b23@...vjti.ac.in>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Meta kernel team <kernel-team@...a.com>, bpf@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	syzbot+09b7d050e4806540153d@...kaller.appspotmail.com,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH bpf v2] lib/buildid: use __kernel_read() for sleepable
 context

On Thu, Dec 18, 2025 at 04:16:40PM -0800, Shakeel Butt wrote:
> On Thu, Dec 18, 2025 at 11:55:39PM +0000, Matthew Wilcox wrote:
> > On Thu, Dec 18, 2025 at 12:55:05PM -0800, Shakeel Butt wrote:
> > > +	do {
> > > +		ret = __kernel_read(r->file, buf, sz, &pos);
> > > +		if (ret <= 0) {
> > > +			r->err = ret ?: -EIO;
> > > +			return NULL;
> > > +		}
> > > +		buf += ret;
> > > +		sz -= ret;
> > > +	} while (sz > 0);
> > 
> > Why are you doing a loop around __kernel_read()?  eg kernel_read() does
> > not do a read around __kernel_read().  The callers of kernel_read()
> > don't do a loop either.  So what makes you think it needs to have a loop
> > around it?
> 
> I am assuming that __kernel_read() can return less data than the
> requested. Is that assumption incorrect?

I think it can, but I don't think a second call will get any more data.
For example, it could hit EOF.  What led you to think that calling it in
a loop was the right approach?