Re: [Qemu-devel] [PATCH for-4.1] linux-user: Make sigaltstack stacks per-thread

[Laurent Vivier]

Le 25/07/2019 à 15:16, Peter Maydell a écrit :
> The alternate signal stack set up by the sigaltstack syscall is
> supposed to be per-thread.  We were incorrectly implementing it as
> process-wide.  This causes problems for guest binaries that rely on
> this.  Notably the Go runtime does, and so we were seeing crashes
> caused by races where two guest threads might incorrectly both
> execute on the same stack simultaneously.
> 
> Replace the global target_sigaltstack_used with a field
> sigaltstack_used in the TaskState, and make all the references to the
> old global instead get a pointer to the TaskState and use the field.
> 
> Fixes: https://bugs.launchpad.net/qemu/+bug/1696773
> Signed-off-by: Peter Maydell <address@hidden>
> ---
> I've marked this as "for-4.1" but it is quite late in the release
> cycle and I think this could use more testing than I have given it...
> 
> Thanks are due to:
>  * the original bug reporter, for providing a nice simple test case
>  * rr, for allowing me to capture and forensically examine a single
>    example of the failure
>  * the Go project for having a good clear HACKING.md that explained
>    their stack usage and mentioned specifically that signal stacks
>    are per-thread (per-M, in their terms)
>  * a colleague, for prodding me into actually spending the necessary
>    two days grovelling through gdb sessions and logs to figure out
>    what was actually going wrong
> ---
>  linux-user/qemu.h          |  2 ++
>  linux-user/signal-common.h |  1 -
>  linux-user/hppa/signal.c   |  3 ++-
>  linux-user/main.c          |  5 +++++
>  linux-user/signal.c        | 35 +++++++++++++++++++----------------
>  5 files changed, 28 insertions(+), 18 deletions(-)

Tested-by: Laurent Vivier <address@hidden>
Reviewed-by: Laurent Vivier <address@hidden>

This patch seems also to fix failure of LTP test waitpid02.

Thanks,
Laurent