Re: [PATCH] dlsym: Make RTLD_NEXT prefer default version definition [#BZ #14932]

[Florian Weimer <fweimer@redhat.com>]

I've been looking at this for a while.

We currently have this code in elf/dl-lookup.c:

      /* No specific version is selected.  There are two ways we
	 can got here:

	 - a binary which does not include versioning information
	 is loaded

	 - dlsym() instead of dlvsym() is used to get a symbol which
	 might exist in more than one form

	 If the library does not provide symbol version information
	 there is no problem at all: we simply use the symbol if it
	 is defined.

	 These two lookups need to be handled differently if the
	 library defines versions.  In the case of the old
	 unversioned application the oldest (default) version
	 should be used.  In case of a dlsym() call the latest and
	 public interface should be returned.  */
      if (verstab != NULL)
	{
	  if ((verstab[symidx] & 0x7fff)
	      >= ((flags & DL_LOOKUP_RETURN_NEWEST) ? 2 : 3))
	    {
	      /* Don't accept hidden symbols.  */
	      if ((verstab[symidx] & 0x8000) == 0
		  && (*num_versions)++ == 0)
		/* No version so far.  */
		*versioned_sym = sym;

	      return NULL;
	    }
	}

The numbers 2 and 3 look suspicious.  The condition involving
num_versions ensures that we only store the first matching symbol in
*versioned_sym.  But the skipped version indices are not special.
Indices are not specific to individual symbols, so it is no clear that
index 2 is special and contains the oldest possible version for that
particular symbol, and the next index will have the latest version.

Aligning dlvsym with dlsym still makes sense, so I suggest to proceed
with this patch.  But I think there are more bugs in this area.

> diff --git a/elf/nextmod3.c b/elf/nextmod3.c
> new file mode 100644
> index 0000000000..96608a65c0
> --- /dev/null
> +++ b/elf/nextmod3.c
> @@ -0,0 +1,19 @@
> +int
> +foo_v1 (int a)
> +{
> +  return 1;
> +}
> +asm (".symver foo_v1, foo@v1");
> +
> +int
> +foo_v2 (int a)
> +{
> +  return 2;
> +}
> +asm (".symver foo_v2, foo@v2");
> +
> +int
> +foo (int a)
> +{
> +  return 3;
> +}

Please set foo@@v3 explicitly.

> diff --git a/elf/nextmod3.map b/elf/nextmod3.map
> new file mode 100644
> index 0000000000..0a8e4e4ee3
> --- /dev/null
> +++ b/elf/nextmod3.map
> @@ -0,0 +1,3 @@
> +v1 { };
> +v2 { };
> +v3 { foo; };

These versions are not ordered.  Maybe use this instead?

v1 { };
v2 { } v1;
v3 { foo; } v2;

Thanks,
Florian