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

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

[Florian Weimer]

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 | 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
+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

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

[Andreas Schwab]

On Sep 23 2020, Florian Weimer via Libc-alpha wrote:

> +.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/interface/ ?

Andreas.

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

[Florian Weimer]

* Andreas Schwab:

> On Sep 23 2020, Florian Weimer via Libc-alpha wrote:
>
>> +.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/interface/ ?

Thanks.  I meant to write “implementation”.  Fixed locally.

Do you agree that this is how the handshake should work?

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

[Andreas Schwab]

On Sep 23 2020, Florian Weimer wrote:

> Do you agree that this is how the handshake should work?

I have never seen the audit interface being used, so I have no opinion.

Andreas.

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

[Carlos O'Donell]

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.

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

[Florian Weimer]

* Carlos O'Donell:

> May I additionally suggest something like this to the example?

Ah, I had missed the example at the end.  Yes, I'm going to send a v2.

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

[Michael Kerrisk (man-pages)]

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


-- 
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Linux/UNIX System Programming Training: http://man7.org/training/

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

[Carlos O'Donell]

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 *
>>
> 
> 


-- 
Cheers,
Carlos.

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

[Michael Kerrisk (man-pages)]

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 *
> 


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

[Florian Weimer]

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(-)

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 *

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