Re: access() says EROFS even for device files if /dev is mounted RO

[Andries Brouwer <aeb@veritas.com>]

On Sun, Nov 26, 2000 at 11:35:22PM -0400, Peter Cordes wrote:

>  While doing some hdparm hacking, after booting with init=/bin/sh, I noticed
> that open(1) doesn't work when / is mounted read only.

Already long ago open(1) was renamed to openvt(1), so it may be that
have a very old version. See a recent kbd or console-tools.

> access("/dev/tty2", R_OK|W_OK)          = -1 EROFS (Read-only file system)

>  However, this is wrong.  You _can_ write to device files on read-only
> filesystems.  (open shouldn't bother calling access(), but the kernel should
> definitely give the right answer!)

You misunderstand the function of access(). The standard says

[EROFS] write access shall fail if write access is requested
        for a file on a read-only file system

It does not look at whether you ask write access to a directory
or a special device file (and whether the filesystem was mounted nodev or not).

So, probably you found a flaw in openvt: the use of access is almost always
a bug - it doesnt tell you what you want to know. You may send me a patch
if you want to.

On the other hand, on recent kernels access() doesnt fail in this situation.
That is a kernel bug, I suppose. Will investigate later.

Andries - aeb@cwi.nl
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