From: Alexander Monakov <amonakov@ispras.ru>
To: musl@lists.openwall.com
Cc: Dominic Chen <d.c.ddcc@gmail.com>
Subject: Re: [musl] Issue with fread() and unaligned readv()
Date: Tue, 16 Mar 2021 01:09:16 +0300 (MSK) [thread overview]
Message-ID: <alpine.LNX.2.20.13.2103160104210.17743@monopod.intra.ispras.ru> (raw)
In-Reply-To: <20210315215120.GF32655@brightrain.aerifal.cx>
On Mon, 15 Mar 2021, Rich Felker wrote:
> On Mon, Mar 15, 2021 at 05:39:43PM -0400, Dominic Chen wrote:
> > Not sure this counts as a problem in musl or the application, but
> > I've been debugging a return error of EINVAL from `fread(&buf, 8,
> > 16, f)`, where `f = fopen("/proc/self/pagemap", "r")`. Internally,
> > musl converts this into a call to `readv(f->fd, iov, 2)`, where `iov
> > = {{iov_base = buf, iov_len = 127}, {iov_base = f->buf, iov_len =
> > 1024}}`. However, it turns out that the kernel VFS read
> > implementation inside `pagemap_read` checks that both the file
> > position and count are divisible by PM_ENTRY_BYTES (8 on x86_64),
> > otherwise it rejects the read with EINVAL. In comparison, glibc's
> > `_IO_file_xsgetn` does appear to try to maintain read alignment,
> > although I haven't looked at it in detail.
>
> You can't use stdio to read or write special files/devices that depend
> on the reads or writes happening in particular units, because the
> relationship between stdio operations and the underlying
> buffer-fill/flush operations on the underlying fd is unspecified. It's
> really unfortunate that the kernel lies that procfs files are regular
> files but doesn't give them regular-file semantics, but you really
> need to use direct operations on the fd in the units the interface
> requires, rather than stdio, to work with these files.
Where does iov_len = 127 for the first iov tuple come from, though?
From fread arguments I'd expect 8 * 16 = 128.
If musl always does such off-by-one, it is an efficiency issue (forces
a copy with mismatching source/dest alignment).
Alexander
next prev parent reply other threads:[~2021-03-15 22:09 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 21:39 Dominic Chen
2021-03-15 21:51 ` Rich Felker
2021-03-15 22:09 ` Alexander Monakov [this message]
2021-03-15 22:29 ` Rich Felker
2021-03-15 22:42 ` Alexander Monakov
2021-03-16 2:13 ` Rich Felker
2021-03-16 9:30 ` Alexander Monakov
2021-03-16 23:54 ` Rich Felker
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