Subject: Re: Question about SEA handling process happened in user space

From: Xiaofei Tan <[email-masked]>
Date: Thu, 2 Apr 2020 14:35:15 +0800

Hi James,

On 2020/3/31 0:49, James Morse wrote:
> Hi Xiaofei,
> 
> On 3/30/20 2:10 PM, Xiaofei Tan wrote:
>> I'm a little confused about the handling process of SEA happened in user space.
> 
>> Following the description of FnV bit of register ESR_ELx in ARMv8.4 SPEC,FAR is
>> valid only for synchronous External abort on a translation table walk.
> 
>> But for this FAR valid scenario(attached code from line 684 to 687),
>> we send signal SIGKILL to kill process. For some other scenario, such as line 680,
>> FAR is not valid, but we send SIGBUS and transfer error address to process to try to do some recovery.
> 
> 'FAR is not valid': its optional. The ESR_EL1.FnV bit can be set for the 'catch
> all' external abort fault-status-code. This lets the CPU tell us that it doesn't
> know what the faulting virtual address is.
> 
> do_sea() checks for this:
> |	if (esr & ESR_ELx_FnV)
> |		siaddr = NULL;
> |	else
> |		siaddr  = (void __user *)addr;
> 
> If we can't know the address, there isn't much we can do.
> (This check is really making sure we don't pass junk to user-space when FnV is set)
> 
> 
>> should it be an problem ?
> 
> I'm not quite sure what your question is.
> 
> If the CPU doesn't tell us the address, we can't tell user-space what it is. The
> alternative is to upgrade to SIGKILL in that case.
> 
> 
> If you see this instead of the address provided via firmware-first, there is a
> series to improve that here:
> https://lore.kernel.org/linux-acpi/[email-masked]/
> 
> (We skip this signal code of APEI promises it did all the work. This lets you
> take the signal from memory_failure() instead, which may have better information.)
> 

There may be an competition issue.
APEI run memory_failure() in an bottom half for memory errors. Then it may be not finished
before here SEA handling end, and application process may back to run.

> 
> If its the SIGKILL entries: these are for the translation table walk.
> There is no point telling user-space about corruption in its page tables as it
> can't do anything about it. The kernel's handling of this is to kill the
> process. (page tables make up a very small amount of memory, so this should be
> rarer than the regular 'external abort' case)
> 
> 
> Thanks,
> 
> James
> 
> 
> 
>> 680         { do_sea,               SIGBUS,  BUS_OBJERR,    "synchronous external abort"    },
>> 684         { do_sea,               SIGKILL, SI_KERNEL,     "level 0 (translation table walk)"      },
>> 685         { do_sea,               SIGKILL, SI_KERNEL,     "level 1 (translation table walk)"      },
>> 686         { do_sea,               SIGKILL, SI_KERNEL,     "level 2 (translation table walk)"      },
>> 687         { do_sea,               SIGKILL, SI_KERNEL,     "level 3 (translation table walk)"      },
>> 688         { do_sea,               SIGBUS,  BUS_OBJERR,    "synchronous parity or ECC error" },    // Reserved when RAS is implemented
>> 692         { do_sea,               SIGKILL, SI_KERNEL,     "level 0 synchronous parity error (translation table walk)"     },      // Reserved when RAS is implemented
>> 693         { do_sea,               SIGKILL, SI_KERNEL,     "level 1 synchronous parity error (translation table walk)"     },      // Reserved when RAS is implemented
>> 694         { do_sea,               SIGKILL, SI_KERNEL,     "level 2 synchronous parity error (translation table walk)"     },      // Reserved when RAS is implemented
>> 695         { do_sea,               SIGKILL, SI_KERNEL,     "level 3 synchronous parity error (translation table walk)"     },      // Reserved when RAS is implemented
>> 696         { do_bad,               SIGKILL, SI_KERNEL,     "unknown 32"                    },
> 
> .
> 

-- 
 thanks
tanxiaofei

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