Re: [Qemu-devel] [PATCH] tcg: Fix mmap lock assert on translation failure

[Richard Henderson]

On 6/17/19 9:18 AM, Alex Bennée wrote:
> 
> Richard Henderson <address@hidden> writes:
> 
>> Check page flags before letting an invalid pc cause a SIGSEGV.
>>
>> Prepare for eventially validating PROT_EXEC.  The current wrinkle being
>> that we have a problem with our implementation of signals.  We should
>> be using a vdso like the kernel, but we instead put the trampoline on
>> the stack.  In the meantime, let PROT_READ match PROT_EXEC.
> 
> We can come up with a test case for this right? Would it be triggered by
> having:
> 
> __attribute__((aligned(PAGE_SIZE)))
> void some_func(void) {
>      /* does something */
> }
> 
> __attribute__((aligned(PAGE_SIZE)))
> ... rest of code ...
> 
> main () {
>      mmap(&some_func, PAGE_SIZE, PROT_READ, MAP_ANONYMOUS, 0, 0);
>      some_func()
>      /* causes SEGV */
>      mmap(&some_func, PAGE_SIZE, PROT_READ|PROT_EXEC, MAP_ANONYMOUS, 0, 0);
>      some_func()
>      /* works */
> }
> 
> Or is it trickier to mess with your own mapped memory?

It's trickier than that, but I do have a simple test case.

  https://bugs.launchpad.net/qemu/+bug/1832916

But fixing that, as I mention above, makes signal trampolines fail.

Or did you mean for Christophe's failure?  That's easier -- just make a NULL
function call.


r~