malloc: Simplify tst-free-errno munmap failure test

The Linux specific test-case in tst-free-errno was backing up malloc
metadata for a large mmap'd block, overwriting the block with its own
mmap, then restoring malloc metadata and calling free to force an munmap
failure.  However, the backed up pages containing metadata can
occasionally be overlapped by the overwriting mmap, leading to a
metadata corruption.

This commit replaces this Linux specific test case with a simpler,
generic, three block allocation, expecting the kernel to coalesce the
VMAs, then cause a fragmentation to trigger the same failure.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

malloc/tst-free-errno.c

@@ -23,11 +23,11 @@
 #include <fcntl.h>
 #include <stdint.h>
 #include <string.h>
+#include <stdio.h>
 #include <sys/mman.h>
 #include <support/check.h>
 #include <support/support.h>
 #include <support/temp_file.h>
-#include <support/xunistd.h>
 
 /* The __attribute__ ((weak)) prevents a GCC optimization.  Without
    it, GCC would "know" that errno is unchanged by calling free (ptr),
@@ -67,63 +67,51 @@ do_test (void)
        - it has to unmap the middle part of a VMA, and
        - the number of VMAs of a process is limited and the limit is
          already reached.
-     The latter condition is fulfilled on Linux, when the file
-     /proc/sys/vm/max_map_count exists.  For all known Linux versions
-     the default limit is at most 65536.
-   */
-  #if defined __linux__
-  if (xopen ("/proc/sys/vm/max_map_count", O_RDONLY, 0) >= 0)
-    {
-      /* Preparations.  */
-      size_t pagesize = getpagesize ();
-      void *firstpage_backup = xmalloc (pagesize);
-      void *lastpage_backup = xmalloc (pagesize);
-      /* Allocate a large memory area, as a bumper, so that the MAP_FIXED
-         allocation later will not overwrite parts of the memory areas
-         allocated to ld.so or libc.so.  */
-      xmmap (NULL, 0x1000000, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, -1);
-      /* A file descriptor pointing to a regular file.  */
-      int fd = create_temp_file ("tst-free-errno", NULL);
-      if (fd < 0)
-	FAIL_EXIT1 ("cannot create temporary file");
+     On Linux, the default VMA count limit is 65536 although many systems
+     might override this.  For this test, irrespective of the system, we
+     try to create up to 65536 mappings in order to attempt to hit the
+     limit.  */
+  {
+    /* We expect the kernel to coalesce the VMAs for these large mallocs
+       (which will be mmap'd by malloc due to their size).  */
+    size_t big_size = 0x3000000;
+    void * volatile block1 = xmalloc (big_size - 100);
+    void * volatile block2 = xmalloc (big_size - 100);
+    void * volatile block3 = xmalloc (big_size - 100);
 
-      /* Do a large memory allocation.  */
-      size_t big_size = 0x3000000;
-      void * volatile ptr = xmalloc (big_size - 0x100);
-      char *ptr_aligned = (char *) ((uintptr_t) ptr & ~(pagesize - 1));
-      /* This large memory allocation allocated a memory area
-	 from ptr_aligned to ptr_aligned + big_size.
-	 Enlarge this memory area by adding a page before and a page
-	 after it.  */
-      memcpy (firstpage_backup, ptr_aligned, pagesize);
-      memcpy (lastpage_backup, ptr_aligned + big_size - pagesize,
-	      pagesize);
-      xmmap (ptr_aligned - pagesize, pagesize + big_size + pagesize,
-	     PROT_READ | PROT_WRITE,
-	     MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1);
-      memcpy (ptr_aligned, firstpage_backup, pagesize);
-      memcpy (ptr_aligned + big_size - pagesize, lastpage_backup,
-	      pagesize);
+    /* If block2 lands between block1 and block3, we can continue the test
+       since it depends on being able to free block2 to cause an munmap
+       failure.  */
+    if (((block2 > block1) && (block2 > block3))
+        || ((block2 < block1) && (block2 < block3)))
+      printf
+        ("warning: block2 was not allocated between block1 and block3\n");
 
-      /* Now add as many mappings as we can.
-	 Stop at 65536, in order not to crash the machine (in case the
-	 limit has been increased by the system administrator).  */
-      for (int i = 0; i < 65536; i++)
-	if (mmap (NULL, pagesize, PROT_READ, MAP_FILE | MAP_PRIVATE, fd, 0)
-	    == MAP_FAILED)
-	  break;
-      /* Now the number of VMAs of this process has hopefully attained
-	 its limit.  */
+    /* We will map this fd repeatedly to consume VMA mappings.  */
+    int fd = create_temp_file ("tst-free-errno", NULL);
+    if (fd < 0)
+      FAIL_EXIT1 ("cannot create temporary file for mmap'ing");
 
-      errno = 1789;
-      /* This call to free() is supposed to call
-	   munmap (ptr_aligned, big_size);
-	 which increases the number of VMAs by 1, which is supposed
-	 to fail.  */
-      free (ptr);
-      TEST_VERIFY (get_errno () == 1789);
-    }
-  #endif
+    /* Now add as many mappings as we can.
+       Stop at 65536, in order not to crash the machine (in case the
+       limit has been increased by the system administrator).  */
+    size_t pagesize = getpagesize ();
+    for (int i = 0; i < 65536; i++)
+      if (mmap (NULL, pagesize, PROT_READ, MAP_FILE | MAP_PRIVATE,
+                fd, 0)
+          == MAP_FAILED)
+        break;
+    /* Now the number of VMAs of this process has hopefully attained
+       its limit.  */
+
+    errno = 1789;
+    /* This call to free() is supposed to call munmap, which should
+       fail because the fragmentation of a bigger coalesced VMA will
+       lead to an increase in the number of VMAs which we already
+       maxed out.  */
+    free (block2);
+    TEST_VERIFY (get_errno () == 1789);
+  }
 
   return 0;
 }