[PATCH v5] elf: parse /proc/self/maps as the last resort to find the gap for tst-link-map-contiguous-ldso

[Adhemerval Zanella Netto]

On 03/02/26 05:20, Xi Ruoyao wrote:
> 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 <xry111@xry111.site>

I think this version addresses the missing point raised by Florian.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
> 
> ChangeLog:
> 
> v1 -> v2: parse mmap to avoid false postive
> v2 -> v3: use %ms to simplify the code and get the full path of ld.so by
> matching l_map_start in the maps file
> v3 -> v4: move the definition of "path" into the loop body
> v4 -> v5: fix memory and fd leak, fix format string macro for printf,
> and make pointer nullity check more explicit
> 
>  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 9f97b8b95a..4d04d37581 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