[Bug 218339] kernel goes unresponsive if single-stepping over an instruction which writes to an address for which a hardware read/write watchpoint has been set

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=218339

Kishen Maloor (kishen.maloor@intel.com) changed:

           What    |Removed                     |Added
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                 CC|                            |kishen.maloor@intel.com

--- Comment #6 from Kishen Maloor (kishen.maloor@intel.com) ---
(In reply to Anthony L. Eden from comment #0)
> In a debian QEMU/KVM virtual machine, run `gdb` on any executable (e.g.
> `/usr/bin/ls`). Run the program by typing `starti`. Proceed to `_dl_start`
> (i.e. `break _dl_start`, `continue`). When you get there disassemble the
> function (i.e. `disas`). Find an instruction that's going to be executed for
> which you can compute the address in memory it will write to. Run the
> program to that instruction (i.e. `break *0xINSN`, `continue`). When you're
> on that instruction, set a read/write watchpoint on the address it will
> write to, then single-step (i.e. `stepi`) and the kernel will go
> unresponsive.
> 
> 
> >(gdb) x/1i $pc
> >=> 0x7ffff7fe6510 <_dl_start+48>:      mov    %rdi,-0x88(%rbp)
> >(gdb) x/1wx $rbp-0x88
> >0x7fffffffec28:        0x00000000
> >(gdb) awatch *0x7fffffffec28
> >Hardware access (read/write) watchpoint 2: *0x7fffffffec28
> >(gdb) stepi
> 

I can reproduce the behavior you describe. But it seems that you're not
invoking KVM at all, because when I add '-accel kvm' or '-enable-kvm' to your
qemu command line the problem goes away.

There may be an issue specifically in the handling of hardware watchpoints
on the qemu emulation. If I disable hardware watchpoints in gdb using 'set
can-use-hw-watchpoints 0' and then use 'watch *<ADDR>', that works.

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