linux-kernel - Re: [PATCH] stat: don't fail if the major number is >= 256

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Date:   Mon, 11 Apr 2022 13:13:03 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] stat: don't fail if the major number is >= 256

On Mon, 11 Apr 2022, Linus Torvalds wrote:

> On Mon, Apr 11, 2022 at 4:43 AM Mikulas Patocka <mpatocka@...hat.com> wrote:
> >
> > If you run a program compiled with OpenWatcom for Linux on a filesystem on
> > NVMe, all "stat" syscalls fail with -EOVERFLOW. The reason is that the
> > NVMe driver allocates a device with the major number 259 and it doesn't
> > pass the "old_valid_dev" test.
> 
> OpenWatcom? Really?

Yes. I use OpenWatcom to verify that my programs are clean ANSI C without 
any gccisms.

Other than that, it is not much useful - it has it's own libc, it's own 
module format, and programs compiled with OpenWatcom cannot be linked with 
existing *.a or *.so libraries.

> > This patch removes the tests - it's better to wrap around than to return
> > an error. (note that cp_old_stat also doesn't report an error and wraps
> > the number around)
> 
> Hmm. We've used majors over 256 for a long time, but some of them are
> admittedly very rare (SCSI OSD?)
> 
> Unfortunate. And in this case 259 aliases to 3, which is the old
> HD/IDE0 major number. That's not great - there would be other numbers
> that didn't have that problem (ie 4-6 are all currently only character
> device majors, I think).

Should we perhaps hash the number, take 16 bits of the hash and hope 
than the collision won't happen?

> Anyway, I think that check is just bogus. The cp_new_stat() thing uses
> 'struct stat' and it has
> 
>         unsigned long   st_dev;         /* Device.  */
>         unsigned long   st_rdev;        /* Device number, if device.  */
> 
> so there's no reason to limit things to the old 8-bit behavior.
> 
> Yes, it does that
> 
>   #define valid_dev(x)  choose_32_64(old_valid_dev(x),true)
>   #define encode_dev(x) choose_32_64(old_encode_dev,new_encode_dev)(x)
> 
>   static __always_inline u16 old_encode_dev(dev_t dev)
>   {
>         return (MAJOR(dev) << 8) | MINOR(dev);
>   }
> 
> which currently drops bits, but we should just *fix* that. We can put
> the high bits in the upper bits, not limit it to 16 bits when we have
> more space than that.

Yes - we can return values larger than 16-bit here. But there's a risk 
that the userspace code will extract the values using macros like this and 
lose the upper bits:

#define major(device)           ((int)(((device) >> 8) & 0xFF))
#define minor(device)           ((int)((device) & 0xff))

> Even the *really* old 'struct old_stat' doesn't really have a 16-bit
> st_dev/rdev.
> 
>            Linus

For me, the failure happens in cp_compat_stat (I have a 64-bit kernel). In 
struct compat_stat in arch/x86/include/asm/compat.h, st_dev and st_rdev 
are compat_dev_t which is 16-bit. But they are followed by 16-bit 
paddings, so they could be extended.

If you have a native 32-bit kernel, it uses 'struct stat' defined at the 
beginning of arch/x86/include/uapi/asm/stat.h that has 32-bit st_dev and 
st_rdev. If you use a 64-bit kernel with 32-bit compat, it uses 'struct 
compat_stat' defined in arch/x86/include/asm/compat.h that has 16-bit 
st_dev and st_rdev. That's an inconsistency that should be resolved.

What did glibc do? Did it use 16-bit dev_t with following padding or 
32-bit dev_t? (the current glibc just uses stat64 and 64-bit dev_t always)

Mikulas