Re: runtime check for omap-aes bus access permission (was: Re: 3.13-rc3 (commit 7ce93f3) breaks Nokia N900 DT boot)

["Pali Rohár" <pali.rohar@gmail.com>]

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On Wednesday 11 February 2015 16:22:51 Matthijs van Duin wrote:
> On 11 February 2015 at 13:39, Pali Rohár <pali.rohar@gmail.com> 
wrote:
> >> Anyhow, since checking the firewalls/APs to see if you have
> >> permission will probably only get you yet another fault if
> >> things are walled off, the robust way of dealing with this
> >> sort of situation is by probing the device with a read
> >> while trapping bus faults. This also handles modules that
> >> are unreachable for other reasons, e.g. being disabled by
> >> eFuse.
> > 
> > It is possible to patch kernel code to mask or ignore that
> > fault? Can you help me with something like that?
> 
> As I mentioned, I'm still learning my way around the kernel,
> so I don't feel very comfortable suggesting a concrete patch
> just yet. I've been browsing arch/arm/mm/ however and my
> impression is that all that would be required is editing
> fault.c by making a copy of do_bad but containing
>     return user_mode(regs) || !fixup_exception(regs);
> and hook it onto the appropriate fault codes.  However, this
> really needs the opinion of someone more familiar with this
> code.
> 
> I do have an observation to make on the issue of fault
> decoding: the list in fsr-2level.c may be "standard ARMv3 and
> ARMv4 aborts" but they are quite wrong for ARMv7 which has:
> 
> [ 0] -
> [ 1] alignment fault
> [ 2] debug event
> [ 3] section access flag fault
> [ 4] instruction cache maintainance fault (reported via data
> abort) [ 5] section translation fault
> [ 6] page access flag fault
> [ 7] page translation fault
> [ 8] bus error on access
> [ 9] section domain fault
> [10] -
> [11] page domain fault
> [12] bus error on section table walk
> [13] section permission fault
> [14] bus error on page table walk
> [15] page permission fault
> [16] (TLB conflict abort)
> [17] -
> [18] -
> [19] -
> [20] (lockdown abort)
> [21] -
> [22] async bus error (reported via data abort)
> [23] -
> [24] async parity/ECC error (reported via data abort)
> [25] parity/ECC error on access
> [26] (coprocessor abort)
> [27] -
> [28] parity/ECC error on section table walk
> [29] -
> [30] parity/ECC error on page table walk
> [31] -
> 
> Some entries are patched up near the bottom of fault.c but
> many bogus messages remain, for example the "on linefetch" vs
> "on non-linefetch" is misleading since no such thing can be
> inferred from the fault status on v7.  Also, the i-cache
> maintenance fault handling looks wrong to me: it should fetch
> the actual fault status from IFSR (even though the address
> still comes from DFSR) and dispatch based on that.
> 
> Async external aborts (async bus error and async parity/ECC
> error) give you basically no info. DFAR will contain garbage
> hence displaying it will confuse rather than enlighten, a
> traceback is pointless since the instruction that caused the
> access is long retired, likewise user_mode() doesn't matter
> since a transition to kernel space may have happened after
> the access that cause the abort. Basically they should be
> treated more as an IRQ than as a fault (note they can also be
> masked just like irqs). In case of a bus error, it may be
> appropriate to just warn about it, or perhaps send a signal
> to the current process, although in the latter case it should
> have some means to distinguish it from a synchronous bus
> error.
> 
> At least on the cortex-a8, a parity/ECC error (whether async
> or not) is to be regarded as absolutely fatal.  Quoth the
> TRM: "No recovery is possible. The abort handler must disable
> the caches, communicate the fail directly with the external
> system, request a reboot."
> 
> Bit 10 no longer indicates an asynchronous (let alone
> imprecise) fault.  Apart from the debug events and async
> aborts (and possibly some implementation-defined aborts), all
> aborts listed are synchronous, and DFAR/IFAR is valid.
> There's no technical obstruction to make these trappable via
> the kernel exception handling mechanism. (Though at least in
> case of parity/ECC errors one shouldn't.)

Tony, Nishanth, or somebody else... can you help with memory 
management? Or do you know some expert for arch/arm/mm/ code?

-- 
Pali Rohár
pali.rohar@gmail.com

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