Re: [PATCH V7 04/10] arm64: exception: handle Synchronous External Abort

From: Baicar, Tyler
Date: Tue Jan 24 2017 - 13:41:52 EST

On 1/23/2017 3:01 AM, James Morse wrote:
Hi Tyler,

On 20/01/17 20:35, Baicar, Tyler wrote:
On 1/19/2017 10:55 AM, James Morse wrote:
On 18/01/17 23:26, Baicar, Tyler wrote:
On 1/17/2017 3:31 AM, James Morse wrote:
On 12/01/17 18:15, Tyler Baicar wrote:
+ info.si_addr = (void __user *)addr;
addr here was read from FAR_EL1, but for some of the classes of exception you
have listed below this register isn't updated with the faulting address.

The ARM-ARM version 'k' in D1.10.5 "Summary of registers on faults taken to an
Exception level that is using Aarch64" has:
The architecture permits that the FAR_ELx is UNKNOWN for Synchronous External
Aborts other than Synchronous External Aborts on Translation Table Walks. In
this case, the ISS.FnV bit returned in ESR_ELx indicates whether FAR_ELx is
This is a problem if we get 'synchronous external abort' or 'synchronous parity
error' while a user space process was running.
It looks like this would just cause an incorrect address to be printed in the
above pr_err.
Unless I'm missing something, I don't see arm64_notify_die or anything that gets
called from
there using the info.si_addr variable.
I may be misreading something here...

This patch has:
info.si_addr = (void __user *)addr;
arm64_notify_die("", regs, &info, esr);
From arch/arm64/kernel/traps.c:arm64_notify_die():
if (user_mode(regs)) {
current->thread.fault_address = 0;
current->thread.fault_code = err;
force_sig_info(info->si_signo, info, current);
So if the SEA interrupted userspace, we put maybe-unknown addr into
force_sig_info() to deliver a signal to user space. User-space then gets a copy
of the info struct containing the maybe-unknown addr.

I think this is an existing bug, but if we are separating the synchronous
external aborts from the generic do_bad handler, we should probably check the
FnV bit. (I think we should still print it out)

What do you suggest I do here? The firmware should be reporting the physical and
address information if it is available in the HEST entry that the kernel will
Its not just firmware that may trigger this, other SoCs may use it for parity or
ECC errors, and they may not always have a valid address in FAR_EL1.

I think we should check the FnV bit in the esr variable and set info.si_addr to
0 if the addr we have isn't valid:
'For some implementations, the value of si_addr may be inaccurate.' [0]
Okay, that makes sense, we don't want userspace to be notified with an incorrect
I will add the check to verify it's valid. Which bit in the ESR is the FnV bit?
I'm not finding
the bit breakdown of the ISS that shows it.
The bits in ISS vary depending on the EC, so a little digging is required.
"D7.2.27 ESR_ELx, Exception Syndrome Register (ELx)" lists the EC values, from
there 'Instruction Abort' and 'Data Abort' both list FnV as bit 10. Version 'k'
of the ARM-ARM has this on pages D7-1953 to D7-1956.
Got it! I'll add the check for this in my next patchset.


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