[Qemu-devel] [RFC] linux-user: Use SIGRTMAX-1 for guest SIGRTMIN+1 to avoid conflict with host libc

[Qemu-devel] [RFC] linux-user: Use SIGRTMAX-1 for guest SIGRTMIN+1 to avoid conflict with host libc

[Peter Maydell]

We already have a hack whereby we flip the guest's SIGRTMAX and SIGRTMIN
signals, to avoid a collision between guest use of SIGRTMIN and the host
libc use of it for SIGCANCEL. However newer glibc also uses SIGRTMIN+1
for internal purposes (as SIGSETXID). Reverse SIGRTMIN+1 and SIGRTMAX-1
so the guest can successfully use SIGRTMIN+1.

This didn't cause any immediately observed issues in guests because glibc
does not check the return value when it registers a SIGSETXID handler(!).
However it meant that if a guest program with more than one thread issued
a setuid() syscall it would hang.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
This is sent as an RFC because although I think it is the right thing
it is potentially enabling a bunch of glibc code that we weren't
even going to attempt to run before. Probably this just means a
different style of deadlock if you attempt setuid() calls in a
multithreaded process, though...

Does anybody actually have an idea about how the "manual signal
delivery multiplexed over a single host signal" would work?

 linux-user/signal.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/linux-user/signal.c b/linux-user/signal.c
index 962111c..487cc5f 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -75,8 +75,12 @@ static uint8_t host_to_target_signal_table[_NSIG] = {
     /* Nasty hack: Reverse SIGRTMIN and SIGRTMAX to avoid overlap with
        host libpthread signals.  This assumes no one actually uses SIGRTMAX :-/
        To fix this properly we need to do manual signal delivery multiplexed
-       over a single host signal.  */
+       over a single host signal.
+       Similarly we reverse SIGRTMIN + 1 and SIGRTMAX - 1, because
+       host glibc uses SIGRTMIN+1 for SIGSETXID. */
     [__SIGRTMIN] = __SIGRTMAX,
+    [__SIGRTMIN + 1] = __SIGRTMAX - 1,
+    [__SIGRTMAX - 1] = __SIGRTMIN + 1,
     [__SIGRTMAX] = __SIGRTMIN,
 };
 static uint8_t target_to_host_signal_table[_NSIG];
-- 
1.9.1