Test if selfmem can write to RX page

Also open only one `/proc/self/mem`.
There are apparently two instances of `RTDyldMemoryManagerJL` created.

Fix #22082

Author: yuyichao

src/cgmemmgr.cpp

@@ -264,9 +264,9 @@ static void *alloc_shared_page(size_t size, size_t *id, bool exec)
 #ifdef _OS_LINUX_
 // Using `/proc/self/mem`, A.K.A. Keno's remote memory manager.
 
-static int self_mem_fd = -1;
-
-static int init_self_mem()
+// Do not call this directly.
+// Use `get_self_mem_fd` which has a guard to call this only once.
+static int _init_self_mem()
 {
     struct utsname kernel;
     uname(&kernel);
@@ -288,22 +288,34 @@ static int init_self_mem()
         return -1;
     fcntl(fd, F_SETFD, FD_CLOEXEC);
 #endif
-    // buffer to check if write works;
-    volatile uint64_t buff = 0;
-    uint64_t v = 0x12345678;
-    int ret = pwrite(fd, (void*)&v, sizeof(uint64_t), (uintptr_t)&buff);
-    if (ret != sizeof(uint64_t) || buff != 0x12345678) {
+
+    // Check if we can write to a RX page
+    void *test_pg = mmap(nullptr, jl_page_size, PROT_READ | PROT_EXEC,
+                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    // We can ignore this though failure to allocate executable memory would be a bigger problem.
+    assert(test_pg != MAP_FAILED && "Cannot allocate executable memory");
+
+    const uint64_t v = 0xffff000012345678u;
+    int ret = pwrite(fd, (const void*)&v, sizeof(uint64_t), (uintptr_t)test_pg);
+    if (ret != sizeof(uint64_t) || *(volatile uint64_t*)test_pg != v) {
+        munmap(test_pg, jl_page_size);
         close(fd);
         return -1;
     }
-    self_mem_fd = fd;
+    munmap(test_pg, jl_page_size);
+    return fd;
+}
+
+static int get_self_mem_fd()
+{
+    static int fd = _init_self_mem();
     return fd;
 }
 
 static void write_self_mem(void *dest, void *ptr, size_t size)
 {
     while (size > 0) {
-        ssize_t ret = pwrite(self_mem_fd, ptr, size, (uintptr_t)dest);
+        ssize_t ret = pwrite(get_self_mem_fd(), ptr, size, (uintptr_t)dest);
         if ((size_t)ret == size)
             return;
         if (ret == -1 && (errno == EAGAIN || errno == EINTR))
@@ -657,7 +669,7 @@ class SelfMemAllocator : public ROAllocator<exec> {
         : ROAllocator<exec>(),
           temp_buff()
     {
-        assert(self_mem_fd != -1);
+        assert(get_self_mem_fd() != -1);
     }
     void finalize() override
     {
@@ -717,7 +729,7 @@ class RTDyldMemoryManagerJL : public SectionMemoryManager {
           code_allocated(false)
     {
 #ifdef _OS_LINUX_
-        if (!ro_alloc && init_self_mem() != -1) {
+        if (!ro_alloc && get_self_mem_fd() != -1) {
             ro_alloc.reset(new SelfMemAllocator<false>());
             exe_alloc.reset(new SelfMemAllocator<true>());
         }