Re: [PATCH] rtld-audit.7: Clarify la_version handshake

[Carlos O'Donell <carlos@redhat.com>]

On 9/23/20 7:38 AM, Florian Weimer via Libc-alpha wrote:
> Returning its argument without further checks is almost always
> wrong for la_version.

What you write here is the way I understand the interface to work.

As Andreas noted there is one typo.

I suggest changing the example to match, otherwise people might
keep cribbing that code ;-)
 
> Signed-off-by: Florian Weimer <fweimer@redhat.com>
> 

I agree that la_version should return LAV_CURRENT, that is the
expected way to implement the function, and returning less than
LAV_CURRENT is required to implement older interfaces and be
more compatible with older releases.

As an example I encouraged Ludovic to solve a Guix path problem
with LD_AUDIT and you can see it here:
https://issues.guix.gnu.org/41189

+unsigned int
+la_version (unsigned int v)
+{
+  if (v != LAV_CURRENT)
+    error (1, 0, "cannot handle interface version %u", v);
+
...
+  return v;
+}

While this is OK, in that if it's not an exact match the audit
module errors out, or if it is a match returns v which is
LAV_CURRENT.

It could have been more relaxed:

if (v < LAV_CURRENT)
  error (...)
...
return LAV_CURRENT;

Your changes here int he man page would have clarified that to
some extent.

> ---
>  man7/rtld-audit.7 | 31 ++++++++++++++++++++++---------
>  1 file changed, 22 insertions(+), 9 deletions(-)
> 
> diff --git a/man7/rtld-audit.7 b/man7/rtld-audit.7
> index b1b7dfebc..ca8afa752 100644
> --- a/man7/rtld-audit.7
> +++ b/man7/rtld-audit.7
> @@ -70,17 +70,30 @@ the auditing library.
>  When invoking this function, the dynamic linker passes, in
>  .IR version ,
>  the highest version of the auditing interface that the linker supports.
> -If necessary, the auditing library can check that this version
> -is sufficient for its requirements.
>  .PP
> -As its function result,
> -this function should return the version of the auditing interface
> -that this auditing library expects to use (returning
> +A typical implementation of this function simply returns the constant
> +.BR LAV_CURRENT ,
> +which indicates the version of
> +.I <link.h>
> +that was used to build the audit module.  If the dynamic linker does
> +not support this version of the audit interface, it will refuse to
> +activate this audit module.  If the function returns zero, the dynamic
> +linker also does not activate this audit module.
> +.PP
> +In order to enable backwards compatibility with older dynamic linkers,
> +an audit module can examine the
> +.I version
> +argument and return an earlier version than
> +.BR LAV_CURRENT ,
> +assuming the module can adjust its implement to match the requirements

s/implement/implementation/g (Andreas caught this in his review)

> +of the previous version of the audit interface.  The
> +.B la_version
> +function should not return the value of
>  .I version
> -is acceptable).
> -If the returned value is 0,
> -or a version that is greater than that supported by the dynamic linker,
> -then the audit library is ignored.
> +without further checks because it could correspond to an interface
> +that does not match the
> +.I <link.h>
> +definitions used to build the audit module.
>  .SS la_objsearch()
>  \&
>  .nf
> 

May I additionally suggest something like this to the example?

diff --git a/man7/rtld-audit.7 b/man7/rtld-audit.7
index b1b7dfebc..8a4008b2a 100644
--- a/man7/rtld-audit.7
+++ b/man7/rtld-audit.7
@@ -510,7 +510,11 @@ la_version(unsigned int version)
 {
     printf("la_version(): %u\en", version);
 
-    return version;
+    if (version > LAV_CURRENT)
+      printf ("la_version(): rtld supports newer interfaces.\n");
+
+    /* Request the version we knew about at compile time.  */
+    return LAV_CURRENT;
 }
 
 char *
---

Post a v2?

-- 
Cheers,
Carlos.