Re: [PATCH] sparc64: fix FP corruption in user copy functions

[Rob Gardner <rob.gardner@oracle.com>]

On 12/23/2015 09:21 AM, David Miller wrote:
> From: Rob Gardner <rob.gardner@oracle.com>
> Date: Tue, 22 Dec 2015 23:24:49 -0700
>
>> Short story: Exception handlers used by some copy_to_user() and
>> copy_from_user() functions do not diligently clean up floating point
>> register usage, and this can result in a user process seeing invalid
>> values in floating point registers. This sometimes makes the process
>> fail.
> Can you show me a specific example where the FPU register contents
> actually matter?
>
> When we are copying to or from userspace, we are in a most of the time
> system call, and for that specific case all FPU registers are volatile
> across the system call.
>
> I guess it might matter for the perf stack backtrace stuff.
>

It does matter for the perf stack backtrace case. Running "perf record 
-g ..." can cause random processes to experience FP register corruption. 
Most of the time this is not noticed, but once in a while it can cause a 
process to get incorrect results or corrupted data.

This bug seriously affects system stability when using perf, and was 
discovered while studying the plethora of perf problems.

Rob