Messages in this thread |  | | Date | Sun, 26 Nov 2000 23:35:22 -0400 | Subject | access() says EROFS even for device files if /dev is mounted RO | From | Peter Cordes <> |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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