Re: [Valgrind-developers] [valgrind] coregrind/m_debuginfo: don't try to examine zero sized mmapped files

[Mark W. <ma...@kl...>]

Hi Chiaki,

On Mon, Mar 10, 2025 at 03:32:16AM +0900, ISHIKAWA,chiaki wrote:
> Does this help reduce the occurences of presumably non-critical
> error messages like the following?

No. These are indeed related messages, but caused by using memfd
allocated memory.

> I get this when I try to run mozilla's thunderbird mail client under
> valgrind.
> They appear many times.
> 
>   23:44.69 GECKO(2640779) --2640783-- WARNING: Serious error when
> reading debug info
>   23:44.69 GECKO(2640779) --2640783-- When reading debug info from
> /memfd:mozilla-ipc (deleted):
>   23:44.69 GECKO(2640779) --2640783-- failed to stat64/stat this file
>   23:44.69 GECKO(2640779) --2640783-- WARNING: Serious error when
> reading debug info
>   23:44.69 GECKO(2640779) --2640783-- When reading debug info from
> /memfd:mozilla-ipc (deleted):
>   23:44.69 GECKO(2640779) --2640783-- failed to stat64/stat this file
> 
> I have no idea why valgrind wants try to to read debug info from
> memory region.

I presume because valgrind thinks it might be a real memory mapped
file. The code triggering this warning is:

   /* Don't let the stat call fail silently.  Filter out some known
      sources of noise before complaining, though. */
   if (sr_isError(statres)) {
      DebugInfo fake_di;
      Bool quiet = VG_(strstr)(filename, "/var/run/nscd/") != NULL
                   || VG_(strstr)(filename, "/dev/shm/") != NULL;
      if (!quiet && VG_(clo_verbosity) > 1) { 
         VG_(memset)(&fake_di, 0, sizeof(fake_di));
         fake_di.fsm.filename = ML_(dinfo_strdup)("di.debuginfo.nmm", filename);
         ML_(symerr)(&fake_di, True, "failed to stat64/stat this file");
      }
      return 0;
   }

So we probably should add "/memfd:" to the patterns for which not to
warn.

https://www.man7.org/linux/man-pages/man2/memfd_create.2.html says:

       int memfd_create(const char *name, unsigned int flags);

    The name supplied in name is used as a filename and will be
    displayed as the target of the corresponding symbolic link in the
    directory /proc/self/fd/.  The displayed name is always prefixed
    with memfd: and serves only for debugging purposes.

So I would propose something like the following (untested) patch:

diff --git a/coregrind/m_debuginfo/debuginfo.c b/coregrind/m_debuginfo/debuginfo.c
index 15686fc5c46f..6c4a6926560b 100644
--- a/coregrind/m_debuginfo/debuginfo.c
+++ b/coregrind/m_debuginfo/debuginfo.c
@@ -1196,7 +1196,9 @@ ULong VG_(di_notify_mmap)( Addr a, Bool allow_SkFileV, Int use_fd )
    if (sr_isError(statres)) {
       DebugInfo fake_di;
       Bool quiet = VG_(strstr)(filename, "/var/run/nscd/") != NULL
-                   || VG_(strstr)(filename, "/dev/shm/") != NULL;
+                   || VG_(strstr)(filename, "/dev/shm/") != NULL
+                   || VG_(strncmp)("/memfd:", filename,
+                                   VG_(strlen)("/memfd:")) == 0;
       if (!quiet && VG_(clo_verbosity) > 1) {
          VG_(memset)(&fake_di, 0, sizeof(fake_di));
          fake_di.fsm.filename = ML_(dinfo_strdup)("di.debuginfo.nmm", filename);

Does that work for you?

Thanks,

Mark