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


From: Peter Maydell
Subject: [Qemu-devel] [PATCH for-4.1] linux-user: Make sigaltstack stacks per-thread
Date: Thu, 25 Jul 2019 14:16:45 +0100

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(-)

diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 4258e4162d2..aac03346270 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -151,6 +151,8 @@ typedef struct TaskState {
      */
     int signal_pending;
 
+    /* This thread's sigaltstack, if it has one */
+    struct target_sigaltstack sigaltstack_used;
 } __attribute__((aligned(16))) TaskState;
 
 extern char *exec_path;
diff --git a/linux-user/signal-common.h b/linux-user/signal-common.h
index 51030a93069..1df1068552f 100644
--- a/linux-user/signal-common.h
+++ b/linux-user/signal-common.h
@@ -19,7 +19,6 @@
 
 #ifndef SIGNAL_COMMON_H
 #define SIGNAL_COMMON_H
-extern struct target_sigaltstack target_sigaltstack_used;
 
 int on_sig_stack(unsigned long sp);
 int sas_ss_flags(unsigned long sp);
diff --git a/linux-user/hppa/signal.c b/linux-user/hppa/signal.c
index b6927ee6735..d1a58feeb36 100644
--- a/linux-user/hppa/signal.c
+++ b/linux-user/hppa/signal.c
@@ -111,10 +111,11 @@ void setup_rt_frame(int sig, struct target_sigaction *ka,
     abi_ulong frame_addr, sp, haddr;
     struct target_rt_sigframe *frame;
     int i;
+    TaskState *ts = (TaskState *)thread_cpu->opaque;
 
     sp = get_sp_from_cpustate(env);
     if ((ka->sa_flags & TARGET_SA_ONSTACK) && !sas_ss_flags(sp)) {
-        sp = (target_sigaltstack_used.ss_sp + 0x7f) & ~0x3f;
+        sp = (ts->sigaltstack_used.ss_sp + 0x7f) & ~0x3f;
     }
     frame_addr = QEMU_ALIGN_UP(sp, 64);
     sp = frame_addr + PARISC_RT_SIGFRAME_SIZE32;
diff --git a/linux-user/main.c b/linux-user/main.c
index a59ae9439de..8ffc5251955 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -180,6 +180,11 @@ void stop_all_tasks(void)
 void init_task_state(TaskState *ts)
 {
     ts->used = 1;
+    ts->sigaltstack_used = (struct target_sigaltstack) {
+        .ss_sp = 0,
+        .ss_size = 0,
+        .ss_flags = TARGET_SS_DISABLE,
+    };
 }
 
 CPUArchState *cpu_copy(CPUArchState *env)
diff --git a/linux-user/signal.c b/linux-user/signal.c
index 5cd237834d9..5ca6d62b15d 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -25,12 +25,6 @@
 #include "trace.h"
 #include "signal-common.h"
 
-struct target_sigaltstack target_sigaltstack_used = {
-    .ss_sp = 0,
-    .ss_size = 0,
-    .ss_flags = TARGET_SS_DISABLE,
-};
-
 static struct target_sigaction sigact_table[TARGET_NSIG];
 
 static void host_signal_handler(int host_signum, siginfo_t *info,
@@ -251,13 +245,17 @@ void set_sigmask(const sigset_t *set)
 
 int on_sig_stack(unsigned long sp)
 {
-    return (sp - target_sigaltstack_used.ss_sp
-            < target_sigaltstack_used.ss_size);
+    TaskState *ts = (TaskState *)thread_cpu->opaque;
+
+    return (sp - ts->sigaltstack_used.ss_sp
+            < ts->sigaltstack_used.ss_size);
 }
 
 int sas_ss_flags(unsigned long sp)
 {
-    return (target_sigaltstack_used.ss_size == 0 ? SS_DISABLE
+    TaskState *ts = (TaskState *)thread_cpu->opaque;
+
+    return (ts->sigaltstack_used.ss_size == 0 ? SS_DISABLE
             : on_sig_stack(sp) ? SS_ONSTACK : 0);
 }
 
@@ -266,17 +264,21 @@ abi_ulong target_sigsp(abi_ulong sp, struct 
target_sigaction *ka)
     /*
      * This is the X/Open sanctioned signal stack switching.
      */
+    TaskState *ts = (TaskState *)thread_cpu->opaque;
+
     if ((ka->sa_flags & TARGET_SA_ONSTACK) && !sas_ss_flags(sp)) {
-        return target_sigaltstack_used.ss_sp + target_sigaltstack_used.ss_size;
+        return ts->sigaltstack_used.ss_sp + ts->sigaltstack_used.ss_size;
     }
     return sp;
 }
 
 void target_save_altstack(target_stack_t *uss, CPUArchState *env)
 {
-    __put_user(target_sigaltstack_used.ss_sp, &uss->ss_sp);
+    TaskState *ts = (TaskState *)thread_cpu->opaque;
+
+    __put_user(ts->sigaltstack_used.ss_sp, &uss->ss_sp);
     __put_user(sas_ss_flags(get_sp_from_cpustate(env)), &uss->ss_flags);
-    __put_user(target_sigaltstack_used.ss_size, &uss->ss_size);
+    __put_user(ts->sigaltstack_used.ss_size, &uss->ss_size);
 }
 
 /* siginfo conversion */
@@ -708,12 +710,13 @@ abi_long do_sigaltstack(abi_ulong uss_addr, abi_ulong 
uoss_addr, abi_ulong sp)
 {
     int ret;
     struct target_sigaltstack oss;
+    TaskState *ts = (TaskState *)thread_cpu->opaque;
 
     /* XXX: test errors */
     if(uoss_addr)
     {
-        __put_user(target_sigaltstack_used.ss_sp, &oss.ss_sp);
-        __put_user(target_sigaltstack_used.ss_size, &oss.ss_size);
+        __put_user(ts->sigaltstack_used.ss_sp, &oss.ss_sp);
+        __put_user(ts->sigaltstack_used.ss_size, &oss.ss_size);
         __put_user(sas_ss_flags(sp), &oss.ss_flags);
     }
 
@@ -760,8 +763,8 @@ abi_long do_sigaltstack(abi_ulong uss_addr, abi_ulong 
uoss_addr, abi_ulong sp)
             }
         }
 
-        target_sigaltstack_used.ss_sp = ss.ss_sp;
-        target_sigaltstack_used.ss_size = ss.ss_size;
+        ts->sigaltstack_used.ss_sp = ss.ss_sp;
+        ts->sigaltstack_used.ss_size = ss.ss_size;
     }
 
     if (uoss_addr) {
-- 
2.20.1




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