make /dev/pci really readable

[Scott Long]

John-Mark Gurney wrote:
> Scott Long wrote this message on Mon, Jun 16, 2003 at 14:40 -0600:
> 
>>You should not always assume that reading PCI registers has no
>>side-effects.  It is certainly legal and possible for a PCI device to
>>detect the read request and alter the contents of the register (or some
>>other register) as a side effect, or change an internal state machine.
>>'Fixing' the various bits to allow unpriviledged access to 'pciconf -r'
>>is dangerous since you would have to teach the system about every pci
>>device in existance and how to trap on registers that have side-effects.
> 
> 
> hmmm.  are you sure about this?  wouldn't it mean that by simply probing
> for a device you could end up locking up the system? 
> 

The first 64 bytes in the space is likely safe, from bytes 65-255 it is
entirely vendor specific.

> 
>>I see little reason why unpriviledged users should be given
>>register-level access to anything.  We don't let them read /dev/mem, do
>>we?  Fixing 'pciconf -l' is fine, but it really doesn't need to extend
>>beyond that.  I would consider 'pciconf -r' to be a security risk and
>>would treat it as such when it comes time for a release.
> 
> 
> My only idea was for developers working on pci drivers.  It was
> invaluable to be able to read the registers when debuging the sparc64
> pci stuff and writing my zoran driver, but I didn't want to have to
> become root every time I wanted to look at this.  The only problem is
> that this requires three levels of permission, list, read, and write..
> changing it to support three is too much against (like overriding write
> to mean read, etc) POLA, so I abandonded it.
> 

I'll not argue your development practices.  However, I don't see it as
unreasonable to ask that driver writers who are going to need root
access to do their work anyways (modifying files, compiling kernels
and/or loading modules) also use root to access the pci registers from
userland.

I seem to remember Linx having a similar feature a few years ago and
naive sysadmins getting into serious trouble by pointing their tape
backups at the /proc/pci directory.

Scott