Re: Question about SEA handling process happened in user space

[James Morse <james.morse@arm.com>]

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/20200228174817.74278-1-james.morse@arm.com/

(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.)


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"                    },

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