On 10/15/2012 08:28 AM, Aldy Hernandez wrote:
I am having a bit of a problem coming up with a generic testcase.
Perhaps Andrew or others have an idea.
The attached testcase fails to trigger without the patch, because
AFAICT we have no way of testing an addition of zero to a memory
location:
cmpl $1, flag(%rip)
setb %al
addl %eax, dont_write(%rip)
In the simulate-thread harness I can test the environment before an
instruction, and after an instruction, but adding 0 to *dont_write
produces no measurable effects, particularly in a back-end independent
manner. Ideas?
Hum. isn't that clever. Well, the instruction is executed pretty much
atomically... so a write of the same value becomes very difficult to
detect, and impossible within the existing harness. And I dont think a
hardware watch point can catch that...
The only way I can think of is if you put 'dont_write' into a section
which will trap if it is written to... I don't know the details of
doing such a thing or how you monitor the trap within the harness...
Other than that I'm not sure we can detect this with our current set of
tools, for the longer term we'd need a write detector. I don't suppose
something like systemtap can detect writes like this?
Andrew
