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

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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.
This commit is contained in:
Thomas Martitz 2012-01-13 09:47:01 +01:00
parent 109084d5cb
commit 34b0311d0e
1 changed files with 31 additions and 0 deletions
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31
firmware/target/hosted/pcm-alsa.c
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@ -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 */