From: "H. Peter Anvin" <hpa@zytor.com>
To: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] new system call mknod64
Date: 21 Apr 2003 12:59:13 -0700 [thread overview]
Message-ID: <b81iih$5f3$1@cesium.transmeta.com> (raw)
In-Reply-To: Pine.LNX.4.44.0304211153270.9109-100000@home.transmeta.com
Followup to: <Pine.LNX.4.44.0304211153270.9109-100000@home.transmeta.com>
By author: Linus Torvalds <torvalds@transmeta.com>
In newsgroup: linux.dev.kernel
>
> But that _will_ force aliasing, unless you start doing some really funky
> things (make the dev_t look more like a UTF-8 unicode-like extension,
> which is obviously possible). In other words, there will be OTHER values
> for "dev_t" that will _also_ look like the tuple <3,1>.
>
> - other values of dev_t that also look like <3,1> had better act
> _identically_ to the legacy values. It _has_ to work this way, since
> otherwise you'd have a total maintenance nightmare, with "ls -l"
> showing two device files as being identical, yet having different
> behaviour.
>
Actually, the lessons learned from many things including UTF-8 (which
unfortunately does have aliasing) seems to indicate that the only
right answer is that noncanonical aliases are *illegal.* If we do
mapping on the syscall boundary, then the kernel will always report
canonical form, and we should just throw -EINVAL on receiving a
noncanonical device number if such a thing can exist at all.
FWIW, here is a completely alias-free encoding of dev_t which is also
backwards compatible and hole-free:
dev_t := major<31:8> . minor<31:8> . major<7:0> . minor<7:0>
where . is bitwise concatenation. One of the major advantages, other
that being alias-free, is that the resulting code is free from
conditionals.
typedef __u64 dev_t;
static inline __u32 MAJOR(dev_t __d)
{
return (__u32)(__d >> 32) & 0xffffff00 |
(__u32)(__d >> 8) & 0x000000ff;
}
static inline __u32 MINOR(dev_t __d)
{
return (__u32)(__d >> 8) & 0xffffff00 |
(__u32)__d & 0x000000ff;
}
static inline dev_t MKDEV(__u32 __ma, __u32 __mi)
{
return ((dev_t)(__ma & 0xffffff00) << 32) |
((dev_t)(__ma & 0x000000ff) << 8) |
((dev_t)(__mi & 0xffffff00) << 8) |
((dev_t)__mi & 0x000000ff);
}
In i386 assembly language, using regcall(%eax,%edx,%ecx):
MAJOR:
movb %ah,%dl
movl %edx,%eax
ret
MINOR:
movb %al,%ah
movb %dl,%al
rorl $8,%eax
ret
MKDEV:
movl %eax,%ecx
shll $8,%eax
movb %dl,%ah
movb %cl,%al
shrl $24,%ecx
movb %cl,%dl
ret
--
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
Architectures needed: ia64 m68k mips64 ppc ppc64 s390 s390x sh v850 x86-64
next prev parent reply other threads:[~2003-04-21 19:48 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-04-20 21:26 [PATCH] new system call mknod64 Andries.Brouwer
2003-04-20 21:43 ` David S. Miller
2003-04-20 21:56 ` Linus Torvalds
2003-04-21 11:59 ` Roman Zippel
2003-04-21 18:01 ` Linus Torvalds
2003-04-21 18:10 ` Christoph Hellwig
2003-04-21 18:21 ` viro
2003-04-21 18:22 ` Linus Torvalds
2003-04-21 18:27 ` viro
2003-04-21 18:35 ` Linus Torvalds
2003-04-21 18:54 ` viro
2003-04-21 19:16 ` Jörn Engel
2003-04-21 18:35 ` Christoph Hellwig
2003-04-21 18:42 ` H. Peter Anvin
2003-04-21 18:44 ` Linus Torvalds
2003-04-21 18:47 ` Christoph Hellwig
2003-04-21 18:58 ` viro
2003-04-21 19:05 ` Linus Torvalds
2003-04-21 19:35 ` viro
2003-04-21 20:02 ` Linus Torvalds
2003-04-21 19:04 ` Linus Torvalds
2003-04-21 19:59 ` H. Peter Anvin [this message]
2003-04-21 18:51 ` H. Peter Anvin
2003-04-21 23:49 ` Roman Zippel
[not found] <20030421215009$2052@gated-at.bofh.it>
[not found] ` <20030421231010$7ee3@gated-at.bofh.it>
[not found] ` <20030422000016$17e3@gated-at.bofh.it>
[not found] ` <20030422083014$0fe2@gated-at.bofh.it>
2003-04-22 20:02 ` Arnd Bergmann
-- strict thread matches above, loose matches on Subject: below --
2003-04-22 1:02 Andries.Brouwer
2003-04-22 1:32 ` H. Peter Anvin
2003-04-22 2:01 ` Jamie Lokier
2003-04-22 2:52 ` H. Peter Anvin
2003-04-22 6:00 ` jw schultz
2003-04-22 17:17 ` H. Peter Anvin
2003-04-21 21:48 Andries.Brouwer
2003-04-21 22:07 ` Linus Torvalds
2003-04-21 21:43 Andries.Brouwer
2003-04-21 23:02 ` H. Peter Anvin
2003-04-21 23:50 ` Jamie Lokier
2003-04-22 8:24 ` Shachar Shemesh
2003-04-20 22:12 Andries.Brouwer
2003-04-21 6:31 ` H. Peter Anvin
2003-04-20 20:34 Andries.Brouwer
2003-04-20 21:12 ` David S. Miller
2003-04-20 18:39 Andries.Brouwer
2003-04-20 18:52 ` Christoph Hellwig
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