Re: [iptables PATCH 4/4] nft: Fix EPERM handling for extensions without rev 0

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, May 04, 2022 at 12:34:16PM +0200, Phil Sutter wrote:
> Treating revision 0 as compatible in EPERM case works fine as long as
> there is a revision 0 of that extension defined in DSO. Fix the code for
> others: Extend the EPERM handling to all revisions and keep the existing
> warning for revision 0.
> 
> Fixes: 17534cb18ed0a ("Improve error messages for unsupported extensions")
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
>  iptables/nft.c                                     | 14 ++++++++++----
>  .../shell/testcases/iptables/0008-unprivileged_0   |  6 ++++++
>  2 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/iptables/nft.c b/iptables/nft.c
> index 33813ce1b9202..95e6c222682c0 100644
> --- a/iptables/nft.c
> +++ b/iptables/nft.c
> @@ -3510,15 +3510,21 @@ int nft_compatible_revision(const char *name, uint8_t rev, int opt)
>  err:
>  	mnl_socket_close(nl);
>  
> -	/* pretend revision 0 is valid -
> +	/* ignore EPERM and errors for revision 0 -
>  	 * this is required for printing extension help texts as user, also
>  	 * helps error messaging on unavailable kernel extension */
> -	if (ret < 0 && rev == 0) {
> -		if (errno != EPERM)
> +	if (ret < 0) {
> +		if (errno == EPERM) {
> +			fprintf(stderr,
> +				"%s: Could not determine whether revision %u is supported, assuming it is.\n",

I'm not sure the user can do much about this error message, to me the
revisions concept are developer-only, I don't think we expose this
implementation detail in the documentation.

Why warn users in this case?

> +				name, rev);
> +			return 1;
> +		} else if (rev == 0) {
>  			fprintf(stderr,
>  				"Warning: Extension %s revision 0 not supported, missing kernel module?\n",
>  				name);
> -		return 1;
> +			return 1;
> +		}
>  	}
>  
>  	return ret < 0 ? 0 : 1;
> diff --git a/iptables/tests/shell/testcases/iptables/0008-unprivileged_0 b/iptables/tests/shell/testcases/iptables/0008-unprivileged_0
> index 43e3bc8721dbd..983531fef4720 100755
> --- a/iptables/tests/shell/testcases/iptables/0008-unprivileged_0
> +++ b/iptables/tests/shell/testcases/iptables/0008-unprivileged_0
> @@ -35,6 +35,12 @@ let "rc+=$?"
>  grep_or_rc "DNAT target options:" <<< "$out"
>  let "rc+=$?"
>  
> +# TEE has no revision 0
> +out=$(run $XT_MULTI iptables -j TEE --help)
> +let "rc+=$?"
> +grep_or_rc "TEE target options:" <<< "$out"
> +let "rc+=$?"
> +
>  out=$(run $XT_MULTI iptables -p tcp -j DNAT --help)
>  let "rc+=$?"
>  grep_or_rc "tcp match options:" <<< "$out"
> -- 
> 2.34.1
>