Re: [PATCH 0/4] linux-user: fix use of SIGRTMIN

[Josh Kunz]

On Sat, Feb 1, 2020 at 4:27 AM Laurent Vivier <address@hidden> wrote:
> This has been tested with Go (golang 1.10.1 linux/arm64, bionic) on x86_64
> fedora 31. We can avoid the failure in this case allowing the unsupported
> signals when we don't provide the "act" parameters to sigaction, only the
> "oldact" one. I have also run the LTP suite with several target and debian
> based distros.

This breaks with go1.13+ (I also verified at hash 753d56d364)[1]. I
tested using an aarch64 guest on an x86 system, but this should
manifest on any architecture when the guest/host have the same number
of signals (and glibc reserves some host signals). From the traceback,
you can see it dies in `initsig` which is called at startup. Any Go
program should fail.

Since go does not use a libc, it assumes that all signals from
[1.._NSIG) are available[2], and will panic if it cannot do an
rt_sigaction for all of them. Go already has some special handling for
QEMU where it silently discards failing rt_sigaction calls to signals
32, 33, and 64 [3]. Since this patch reserves an extra signal for
__SIGRTMIN+1 as well, it blocks out guest signal 63 and Go fails to
initialize.

While I personally support this patch series (current handling of
guest glibc signals is broken), it *will* break Go binaries. I don't
know of a way to avoid this while supporting guest __SIGRTMIN+1,
without either doing true signal multiplexing, or patching Go.

[1] https://gist.github.com/joshkunz/b6c80724072cc1dce79a6253d40b016f
[2] 
https://github.com/golang/go/blob/67f0f83216930e053441500e2b28c3fa2b667581/src/runtime/signal_unix.go#L123
[3] https://github.com/golang/go/blob/master/src/runtime/os_linux.go#L466-L473

>
> Laurent Vivier (4):
>   linux-user: add missing TARGET_SIGRTMIN for hppa
>   linux-user: cleanup signal.c
>   linux-user: fix TARGET_NSIG and _NSIG uses
>   linux-user: fix use of SIGRTMIN
>
>  linux-user/hppa/target_signal.h |   1 +
>  linux-user/signal.c             | 110 +++++++++++++++++++++++---------
>  linux-user/trace-events         |   3 +
>  3 files changed, 85 insertions(+), 29 deletions(-)
>
> --
> 2.24.1
>