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Subjectaccess() says EROFS even for device files if /dev is mounted RO
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While doing some hdparm hacking, after booting with init=/bin/sh, I noticed
that open(1) doesn't work when / is mounted read only. It complains that it
"Cannot open /dev/tty2 read/write"...

I straced it:

execve("/usr/bin/open", ["open"], [/* 13 vars */]) = 0
...
brk(0x804c000) = 0x804c000
open("/dev/tty", O_RDWR) = -1 ENXIO (No such device or address)
open("/dev/tty0", O_RDWR) = 4
ioctl(4, KDGKBTYPE, 0xbffffcdb) = 0
ioctl(4, VT_GETSTATE, 0xbffffda4) = 0
ioctl(4, VT_OPENQRY, 0xbffffd90) = 0
open("/dev/tty2", O_RDWR) = 5
close(5) = 0


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


write(2, "Cannot open /dev/tty2 read/write"..., 57) = 57
_exit(5) = ?


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!) Running
(bash < /dev/tty2 &>/dev/tty2 &)
will work (but for reasons unknown to me it won't do job control or handle
^C, etc.)

I'm pretty sure the problem is in linux/fs/open.c, in sys_access():
...
int res = -EINVAL;
...
dentry = namei(filename);
res = PTR_ERR(dentry);
if (!IS_ERR(dentry)) {
res = permission(dentry->d_inode, mode);
/* SuS v2 requires we report a read only fs too */

if(!res && (mode & S_IWOTH) && IS_RDONLY(dentry->d_inode))

res = -EROFS;
dput(dentry);
}
...
return res;

I think the if( !res ... ) line is the problem. I think the fix is to
add a check that the file is not a device file, socket, named pipe, or a
symlink to a file on a non-readonly FS (unless permission already follow
links? There's probably some file type I didn't think of that needs to get
checked, too.).

I'm don't know what macro to use, since I don't have much kernel hacking
experience (yet ;), so I'll leave the fix for someone who knows what
they're doing :->

BTW, this is in a 2.2.17 kernel on an IA32 machine.

Please CC me on any replies, since I'm not subscribed to the list.

--
#define X(x,y) x##y
Peter Cordes ; e-mail: X(peter@llama.nslug. , ns.ca)

"The gods confound the man who first found out how to distinguish the hours!
Confound him, too, who in this place set up a sundial, to cut and hack
my day so wretchedly into small pieces!" -- Plautus, 200 BCE
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