[glibc/release/2.36/master] elf: parse /proc/self/maps as the last resort to find the gap for tst-link-map-contiguous-ldso

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4561d6af9f324b1a877525c57ff4669d26159643

commit 4561d6af9f324b1a877525c57ff4669d26159643
Author: Xi Ruoyao <[email protected]>
Date:   Tue Feb 3 16:20:12 2026 +0800

    elf: parse /proc/self/maps as the last resort to find the gap for tst-link-map-contiguous-ldso
    
    The initialization process of libc.so calls mmap() several times and the
    kernel may lay the maps into the gap.  If all pages in the gap are
    occupied, the test would not be able to find the gap with mmap() and the
    test would fail.
    
    The failure reproduces most frequently on LoongArch because with the
    commonly used page size (16 KiB) the gap only contains 4 pages and the
    probability they are all occupied is not near to zero.
    
    With the changes in the patch, a test run may output:
    
        info: ld.so link map is not contiguous
        info: object "/dev/zero" found at 0x7ffff1fe0000 - 0x7ffff1fe4000
        info: anonymous mapping found at 0x7ffff1fe4000 - 0x7ffff1fec000
    
    Also take the chance to fix a mistake in the "object found at" message
    which has puzzled me during the initial debug session.
    
    Signed-off-by: Xi Ruoyao <[email protected]>
    Reviewed-by: Adhemerval Zanella  <[email protected]>
    (cherry picked from commit aed8390a6a22e5751fc12704c0c5f2a8271fc286)

Diff:
---
 elf/tst-link-map-contiguous-ldso.c | 64 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 62 insertions(+), 2 deletions(-)

diff --git a/elf/tst-link-map-contiguous-ldso.c b/elf/tst-link-map-contiguous-ldso.c
index 04de808bb2..f0e26682f2 100644
--- a/elf/tst-link-map-contiguous-ldso.c
+++ b/elf/tst-link-map-contiguous-ldso.c
@@ -18,15 +18,73 @@
 
 #include <dlfcn.h>
 #include <gnu/lib-names.h>
+#include <inttypes.h>
 #include <link.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <support/check.h>
+#include <support/support.h>
 #include <support/xdlfcn.h>
 #include <support/xunistd.h>
+#include <support/xstdio.h>
 #include <sys/mman.h>
 #include <unistd.h>
 
+/* Slow path in case we cannot find a gap with mmap (when the runtime has
+   mapped all the pages in the gap for some reason).  */
+static bool
+find_gap_with_proc_self_map (const struct link_map *l)
+{
+  int pagesize = getpagesize ();
+
+  support_need_proc ("Reads /proc/self/maps to find gap in ld.so mapping");
+
+  /* Parse /proc/self/maps and find all the mappings in the ld.so range
+     but not from ld.so.  */
+  FILE *f = xfopen ("/proc/self/maps", "r");
+  char *line = NULL, *path_ldso = NULL;
+  size_t len;
+  bool found = false;
+  while (xgetline (&line, &len, f))
+    {
+      uintptr_t from, to;
+      char *path = NULL;
+      int r = sscanf (line, "%" SCNxPTR "-%" SCNxPTR "%*s%*s%*s%*s%ms",
+                      &from, &to, &path);
+
+      TEST_VERIFY (r == 2 || r == 3);
+      TEST_COMPARE (from % pagesize, 0);
+      TEST_COMPARE (to % pagesize, 0);
+
+      if (path_ldso == NULL && l->l_map_start == from)
+        {
+          TEST_COMPARE (r, 3);
+          path_ldso = path;
+          continue;
+        }
+
+      if (from > l->l_map_start && to < l->l_map_end
+          && (r == 2 || (path_ldso != NULL && strcmp (path, path_ldso))))
+        {
+          if (r == 2)
+            printf ("info: anonymous mapping found at 0x%" PRIxPTR " - 0x%"
+                    PRIxPTR "\n", from, to);
+          else
+            printf ("info: object \"%s\" found at 0x%" PRIxPTR " - 0x%"
+                    PRIxPTR "\n", path, from, to);
+
+          found = true;
+        }
+
+      free (path);
+    }
+
+  free (path_ldso);
+  free (line);
+  xfclose (f);
+  return found;
+}
+
 static int
 do_test (void)
 {
@@ -64,16 +122,18 @@ do_test (void)
               if ((void *) dlfo.dlfo_link_map != (void *) l)
                 {
                   printf ("info: object \"%s\" found at %p\n",
-                          dlfo.dlfo_link_map->l_name, ptr);
+                          dlfo.dlfo_link_map->l_name, expected);
                   gap_found = true;
                 }
             }
           else
             TEST_COMPARE (dlfo_ret, -1);
+
           xmunmap (ptr, 1);
           addr += pagesize;
         }
-      if (!gap_found)
+
+      if (!gap_found && !find_gap_with_proc_self_map (l))
         FAIL ("no ld.so gap found");
     }
   else