rtld-audit.7: Clarify la_version handshake

Returning its argument without further checks is almost always
wrong for la_version.

Signed-off-by: Florian Weimer <fweimer@redhat.com>

---
 man7/rtld-audit.7 | 36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

On 9/25/20 10:48 AM, Florian Weimer wrote:
> Returning its argument without further checks is almost always
> wrong for la_version.
> 
> Signed-off-by: Florian Weimer <fweimer@redhat.com>

Hello Florian,

I've applied this patch locally. I'll merge into master
in a few hours. Perhaps in the meantime there are acks/reviews
that come in.

Cheers,

Michael

> ---
>  man7/rtld-audit.7 | 36 +++++++++++++++++++++++++-----------
>  1 file changed, 25 insertions(+), 11 deletions(-)
> 
> diff --git a/man7/rtld-audit.7 b/man7/rtld-audit.7
> index b1b7dfebc..21cc339d0 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 implementation to match the
> +requirements 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
> @@ -508,9 +521,10 @@ This is reportedly fixed in glibc 2.10.
>  unsigned int
>  la_version(unsigned int version)
>  {
> -    printf("la_version(): %u\en", version);
> +    printf("la_version(): version = %u; LAV_CURRENT = %u\en",
> +            version, LAV_CURRENT);
>  
> -    return version;
> +    return LAV_CURRENT;
>  }
>  
>  char *
>

On 9/25/20 6:47 AM, Michael Kerrisk (man-pages) wrote:
> On 9/25/20 10:48 AM, Florian Weimer wrote:
>> Returning its argument without further checks is almost always
>> wrong for la_version.
>>
>> Signed-off-by: Florian Weimer <fweimer@redhat.com>
> 
> Hello Florian,
> 
> I've applied this patch locally. I'll merge into master
> in a few hours. Perhaps in the meantime there are acks/reviews
> that come in.

Just for the record. This version looks good to me and addresses
my previous comments.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
 
> Cheers,
> 
> Michael
> 
>> ---
>>  man7/rtld-audit.7 | 36 +++++++++++++++++++++++++-----------
>>  1 file changed, 25 insertions(+), 11 deletions(-)
>>
>> diff --git a/man7/rtld-audit.7 b/man7/rtld-audit.7
>> index b1b7dfebc..21cc339d0 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 implementation to match the
>> +requirements 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
>> @@ -508,9 +521,10 @@ This is reportedly fixed in glibc 2.10.
>>  unsigned int
>>  la_version(unsigned int version)
>>  {
>> -    printf("la_version(): %u\en", version);
>> +    printf("la_version(): version = %u; LAV_CURRENT = %u\en",
>> +            version, LAV_CURRENT);
>>  
>> -    return version;
>> +    return LAV_CURRENT;
>>  }
>>  
>>  char *
>>
> 
>

On Fri, 25 Sep 2020 at 18:14, Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 9/25/20 6:47 AM, Michael Kerrisk (man-pages) wrote:
> > On 9/25/20 10:48 AM, Florian Weimer wrote:
> >> Returning its argument without further checks is almost always
> >> wrong for la_version.
> >>
> >> Signed-off-by: Florian Weimer <fweimer@redhat.com>
> >
> > Hello Florian,
> >
> > I've applied this patch locally. I'll merge into master
> > in a few hours. Perhaps in the meantime there are acks/reviews
> > that come in.
>
> Just for the record. This version looks good to me and addresses
> my previous comments.
>
> Reviewed-by: Carlos O'Donell <carlos@redhat.com>

Thanks, Carlos!

Cheers,

Michael