hosted/pcm/alsa: Use alternate signal stack for the async callback.

Signals are by default executed on the user stack, i.e. the stack of
the currently active thread. This has two problems:
1) The stack size of the current stack is likely insufficient (unless
using sigaltstack threads) because our stack sizes are normally
below MINSIGSTKSIZE which is needed to deliver a signal.
2) Some of our asm code does nasty tricks with the stack pointer. When a
signal comes in during this bad things can happen, e.g. random memory
being overwritten or simply a crash.

Using a well defined stack fixes this. This is comparable with the
separate irq stack on native targets.

1 files changed, 31 insertions, 0 deletions

diff --git a/firmware/target/hosted/pcm-alsa.c b/firmware/target/hosted/pcm-alsa.c
index 7daf485f18..02cb34ea5b 100644
--- a/firmware/target/hosted/pcm-alsa.c
+++ b/firmware/target/hosted/pcm-alsa.c
@@ -27,6 +27,11 @@
  * This driver uses the so-called unsafe async callback method and hardcoded device
  * names. It fails when the audio device is busy by other apps.
  *
+ * To make the async callback safer, an alternative stack is installed, since
+ * it's run from a signal hanlder (which otherwise uses the user stack). If
+ * tick tasks are run from a signal handler too, please install
+ * an alternative stack for it too.
+ *
  * TODO: Rewrite this to do it properly with multithreading
  *
  * Alternatively, a version using polling in a tick task is provided. While
@@ -76,6 +81,7 @@ static size_t       pcm_size = 0;
 #ifdef USE_ASYNC_CALLBACK
 static snd_async_handler_t *ahandler;
 static pthread_mutex_t pcm_mtx;
+static char signal_stack[SIGSTKSZ];
 #else
 static int recursion;
 #endif
@@ -271,12 +277,37 @@ static int async_rw(snd_pcm_t *handle)
     short *samples;
 
 #ifdef USE_ASYNC_CALLBACK
+    /* assign alternative stack for the signal handlers */
+    stack_t ss = {
+        .ss_sp = signal_stack,
+        .ss_size = sizeof(signal_stack),
+        .ss_flags = 0
+    };
+    struct sigaction sa;
+
+    err = sigaltstack(&ss, NULL);
+    if (err < 0)
+    {
+        DEBUGF("Unable to install alternative signal stack: %s", strerror(err));
+        return err;
+    }
+    
     err = snd_async_add_pcm_handler(&ahandler, handle, async_callback, NULL);
     if (err < 0)
     {
         DEBUGF("Unable to register async handler: %s\n", snd_strerror(err));
         return err;
     }
+
+    /* only modify the stack the handler runs on */
+    sigaction(SIGIO, NULL, &sa);
+    sa.sa_flags |= SA_ONSTACK;
+    err = sigaction(SIGIO, &sa, NULL);
+    if (err < 0)
+    {
+        DEBUGF("Unable to install alternative signal stack: %s", strerror(err));
+        return err;
+    }
 #endif
 
     /* fill buffer with silence to initiate playback without noisy click */