Messages in this thread | | | From | Matt Chapman <> | Date | Sat, 18 Oct 2003 11:31:37 +1000 | Subject | Re: [RFC] prevent "dd if=/dev/mem" crash |
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On Fri, Oct 17, 2003 at 05:49:55PM -0700, Andrew Morton wrote: > David Mosberger <davidm@napali.hpl.hp.com> wrote: > > > > One philosophy states that if > > your kernel touches random addresses, it's better to signal a visible > > error (machine-check) than to risk silent data corruption. > > An access to an illegal address should generate a fault, period. This puts > the processing into the hands of software. If software chooses to silently > ignore the fault (ie: "silent data corruption") then it is poorly designed.
It *does* signal a fault, in the form of a machine check. On other architectures I'm familiar with this is usually implemented as an interrupt, but the idea is similar - when the system bus controller detects a bad address on the bus, it returns all 1s (for a read) and signals an interrupt. Usually you can turn this interrupt off (and most likely you can on Itanium chipsets too) but that is not a good idea.
The problem is that this interrupt is not synchronous with respect to the instruction stream, and this makes it difficult for software to recover from, particularly in a monolithic system like Linux where you can't just terminate the faulting driver. The best you can usually do is to print the details and hope that it's a once-off. It is not something that you can sensibly use to abort copy_*_user.
In any case touching random addresses is just plain bad. What if there's a device mapped there which happens to have read side effects like clearing the interrupt cause, so e.g. every time you read /dev/mem you cause a timeout on your SCSI bus :)
Matt
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