guile/gc-platform-gnu-linux.c

// For pthread_getattr_np.
#define _GNU_SOURCE
#include <errno.h>
#include <link.h>
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

#define GC_IMPL 1

#include "debug.h"
#include "gc-assert.h"
#include "gc-inline.h"
#include "gc-platform.h"

void gc_platform_init(void) {
  // Nothing to do.
}

static uintptr_t fallback_current_thread_stack_base(void) GC_NEVER_INLINE;
static uintptr_t fallback_current_thread_stack_base(void) {
  // Sloppily assume that there are very few frames between us and the
  // thread entry or main function, and that therefore we haven't
  // consumed more than a page of stack; we can then just round up the
  // stack pointer to the page boundary.
  fprintf(stderr,
          "Using fallback strategy to capture stack base for thread %p.\n",
          (void*)pthread_self());
  int local;
  uintptr_t hot = (uintptr_t)&local;
  size_t page_size = getpagesize();
  return (hot + page_size) & ~(page_size - 1);
}

uintptr_t gc_platform_current_thread_stack_base(void) {
  pthread_t me = pthread_self();
  pthread_attr_t attr;
  int err = pthread_getattr_np(me, &attr);
  if (err) {
    errno = err;
    // This case can occur for the main thread when running in a
    // filesystem without /proc/stat.
    perror("Failed to capture stack base via pthread_getattr_np");
    return fallback_current_thread_stack_base();
  }

  void *stack_low_addr;
  size_t stack_size;
  err = pthread_attr_getstack(&attr, &stack_low_addr, &stack_size);
  pthread_attr_destroy(&attr);
  if (err) {
    // Should never occur.
    errno = err;
    perror("pthread_attr_getstack");
    return fallback_current_thread_stack_base();
  }

  return (uintptr_t)stack_low_addr + stack_size;
}

struct visit_data {
  void (*f)(uintptr_t start, uintptr_t end, struct gc_heap *heap, void *data);
  struct gc_heap *heap;
  void *data;
};

static int visit_roots(struct dl_phdr_info *info, size_t size, void *data) {
  struct visit_data *visit_data = data;
  uintptr_t object_addr = info->dlpi_addr;
  const char *object_name = info->dlpi_name;
  const ElfW(Phdr) *program_headers = info->dlpi_phdr;
  size_t program_headers_count = info->dlpi_phnum;

  // From the loader's perspective, an ELF image is broken up into
  // "segments", each of which is described by a "program header".
  // Treat all writable data segments as potential edges into the
  // GC-managed heap.
  //
  // Note that there are some RELRO segments which are initially
  // writable but then remapped read-only.  BDW-GC will exclude these,
  // but we just punt for the time being and treat them as roots
  for (size_t i = 0; i < program_headers_count; i++) {
    const ElfW(Phdr) *p = &program_headers[i];
    if (p->p_type == PT_LOAD && (p->p_flags & PF_W)) {
      uintptr_t start = p->p_vaddr + object_addr;
      uintptr_t end = start + p->p_memsz;
      DEBUG("found roots for '%s': [%p,%p)\n", object_name,
            (void*)start, (void*)end);
      visit_data->f(start, end, visit_data->heap, visit_data->data);
    }
  }

  return 0;
}

void gc_platform_visit_global_conservative_roots(void (*f)(uintptr_t start,
                                                           uintptr_t end,
                                                           struct gc_heap*,
                                                           void *data),
                                                 struct gc_heap *heap,
                                                 void *data) {
  struct visit_data visit_data = { f, heap, data };
  dl_iterate_phdr(visit_roots, &visit_data);
}