Make sure altstack_size_ is a multiple of the system page size (#1883)

Summary: This PR modifies the logic that sets the value of
`altstack_size_`. When deploying Pixie on a 7th gen AMI running bottle
rocket, the value of `altstack_size_` was set to `MINSIGSTKSZ`. This
value was 39616 and was not a multiple of the system page size.

In this [line of
code](https://github.com/pixie-io/pixie/blob/0888c38df2dc414a36cc84198c7b159c39eea0e0/src/common/signal/signal_action.cc#L149)
`mprotect` was then applied on a non page aligned region since its
address was set as `altstack_ + guard_size_ + altstack_size_` leading to
an error. This region in `altstack_` was also not the tailing
`guard_size_` area as
[mmap](https://github.com/pixie-io/pixie/blob/0888c38df2dc414a36cc84198c7b159c39eea0e0/src/common/signal/signal_action.cc#L145)
would have added extra bytes to `altstack_` in order to page align it.

As per the docs, the address passed to `mprotect` needs to be aligned to
a page boundary. The function in this PR makes sure that the value of
`altstack_size_` is a multiple of the page size so that the address
calculated/passed to `mprotect` is aligned to a page boundary and that
it protects the tailing `guard_size_` region of `altstack_`

Relevant Issues: Fixes #1882

Type of change: /kind bug

Test Plan: Skaffolded pixie on EKS clusters with m7i.large, r7i.large
instances and on a GKE cluster with e2-standard-4 nodes and saw the PEM
start up

---------

Signed-off-by: Kartik Pattaswamy <kpattaswamy@pixielabs.ai>

src/common/signal/signal_action.h

@@ -35,7 +35,7 @@ class SignalAction : public NotCopyable {
  public:
   SignalAction()
       : guard_size_(px::system::Config::GetInstance().PageSizeBytes()),
-        altstack_size_(std::max(guard_size_ * 4, static_cast<size_t>(MINSIGSTKSZ))) {
+        altstack_size_(DetermineAltStackSize()) {
     MapAndProtectStackMemory();
     InstallSigHandlers();
   }
@@ -71,6 +71,18 @@ class SignalAction : public NotCopyable {
    * Additionally, two guard pages will be allocated to bookend the usable area.
    */
   const size_t altstack_size_;
+  /**
+   * Determine the number of bytes to allocate to altstack_size_.
+   */
+  size_t DetermineAltStackSize() const {
+    // The size of altstack_size_ should be at least 4 * guard size or size MINSIGSTKSZ if it is
+    // greater. This size needs to be a multiple of the system page size.
+    const size_t min_altstack_size = 4 * guard_size_;
+    const size_t sig_page_count = IntRoundUpDivide(static_cast<size_t>(MINSIGSTKSZ), guard_size_);
+    const size_t sig_stack_size = sig_page_count * guard_size_;
+
+    return std::max(min_altstack_size, sig_stack_size);
+  }
   /**
    * Signal handlers will be installed for these signals which have a fatal outcome.
    */