Ports/gdb: Use mmap instead of malloc for sigaltstack()

Stack regions can't be made volatile, which makes it impossible for
malloc to manage memory that's used for `sigaltstack()`. Let's use mmap
instead.

Co-authored-by: Idan Horowitz <idan.horowitz@gmail.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/Ports/gdb/patches/alt-stack-no-malloc.patch b/Ports/gdb/patches/alt-stack-no-malloc.patch
new file mode 100644
index 0000000000..974d1db061
--- /dev/null
+++ b/Ports/gdb/patches/alt-stack-no-malloc.patch
@@ -0,0 +1,70 @@
+diff --git a/gdbsupport/alt-stack.h b/gdbsupport/alt-stack.h
+index 056ea41..b638533 100644
+--- a/gdbsupport/alt-stack.h
++++ b/gdbsupport/alt-stack.h
+@@ -20,7 +20,9 @@
+ #ifndef GDBSUPPORT_ALT_STACK_H
+ #define GDBSUPPORT_ALT_STACK_H
+ 
++#include "common-defs.h"
+ #include <signal.h>
++#include <sys/mman.h>
+ 
+ namespace gdb
+ {
+@@ -36,31 +38,44 @@ class alternate_signal_stack
+ public:
+   alternate_signal_stack ()
+   {
++    // We can't use xmalloc here on Serenity, because stack regions
++    // do not play well with how malloc manages its memory.
+ #ifdef HAVE_SIGALTSTACK
+-    m_stack.reset ((char *) xmalloc (SIGSTKSZ));
+-
+-    stack_t stack;
+-    stack.ss_sp = m_stack.get ();
+-    stack.ss_size = SIGSTKSZ;
+-    stack.ss_flags = 0;
+-
+-    sigaltstack (&stack, &m_old_stack);
++    void *ptr = mmap (nullptr, SIGSTKSZ, PROT_READ | PROT_WRITE,
++                      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
++    if (ptr == MAP_FAILED)
++      {
++        warning ("could not mmap alternate signal stack");
++      }
++    else
++      {
++        m_stack = ptr;
++        stack_t stack;
++        stack.ss_sp = m_stack;
++        stack.ss_size = SIGSTKSZ;
++        stack.ss_flags = 0;
++
++        sigaltstack (&stack, &m_old_stack);
++      }
+ #endif
+   }
+ 
+   ~alternate_signal_stack ()
+   {
+ #ifdef HAVE_SIGALTSTACK
+-    sigaltstack (&m_old_stack, nullptr);
++    if (m_stack != nullptr)
++      {
++        sigaltstack (&m_old_stack, nullptr);
++        munmap (m_stack, SIGSTKSZ);
++      }
+ #endif
+   }
+ 
+   DISABLE_COPY_AND_ASSIGN (alternate_signal_stack);
+ 
+ private:
+-
+ #ifdef HAVE_SIGALTSTACK
+-  gdb::unique_xmalloc_ptr<char> m_stack;
++  void *m_stack{ nullptr };
+   stack_t m_old_stack;
+ #endif
+ };