[PATCH] rtld-audit.7: Clarify la_version handshake
Carlos O'Donell
carlos@redhat.com
Thu Sep 24 01:30:23 GMT 2020
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.
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