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	Fri, 19 Dec 2025 04:07:51 +0000
Date: Fri, 19 Dec 2025 04:07:51 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Shaurya Rane <ssrane_b23@ee.vjti.ac.in>,
	"Darrick J . Wong" <djwong@kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Meta kernel team <kernel-team@meta.com>, bpf@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	syzbot+09b7d050e4806540153d@syzkaller.appspotmail.com,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH bpf v2] lib/buildid: use __kernel_read() for sleepable
 context
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References: <20251218205505.2415840-1-shakeel.butt@linux.dev>
 <aUSUe9jHnYJ577Gh@casper.infradead.org>
 <3lf3ed3xn2oaenvlqjmypuewtm6gakzbecc7kgqsadggyvdtkr@uyw4boj6igqu>
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In-Reply-To: <3lf3ed3xn2oaenvlqjmypuewtm6gakzbecc7kgqsadggyvdtkr@uyw4boj6igqu>

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?