[Qemu-devel] patch to swap SIGRTMIN + 1 and SIGRTMAX - 1

[Marlies Ruck <marlies.ruck@gmail.com>]

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Hi,

Attached is a patch to let guest programs use SIGRTMIN + 1 by swapping with
SIGRTMAX - 1. Since QEMU links against glibc, it reserves the signal for
itself and returns EINVAL (as noted in the commit message). This means
various applications that use SIGRTMIN + 1 cannot run on QEMU, including
G-WAN web server and Open TFTP.

Thanks,
Marli

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From 096c244668345f80a4582799d0b860b7c20a9b59 Mon Sep 17 00:00:00 2001
From: marlies ruck <marlies@forallsecure.com>
Date: Fri, 21 Jun 2019 14:42:42 -0700
Subject: [PATCH] Swap SIGRTMIN + 1 and SIGRTMAX - 1

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.

Credit: Peter Maydell https://patches.linaro.org/patch/63313/
Signed-off-by: marlies ruck <marlies@forallsecure.com>
---
 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 5cd237834d..137a449140 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];
-- 
2.20.1 (Apple Git-117)