nlattr: add value processing support for xlat/flags nlattr decoders

Needed for upcoming decoder of ethernet proto which is stored in the
host order and has to be converted to the network order before printing
as an xval constant.  This change also adds ability to provide
prefix/suffix in order to enclose the printed value in something that
describes the performed conversion.

* nlattr.h (struct decode_nla_xlat_opts): Add prefix, suffix, and
process_fn fields.
* nlattr.c (decode_nla_xval, decode_nla_flags): Handle process_fn,
prefix, snd uffix parameters.

2 files changed, 19 insertions, 0 deletions

diff --git a/nlattr.c b/nlattr.c
index 835cf25e5..b11f68210 100644
--- a/nlattr.c
+++ b/nlattr.c
@@ -273,8 +273,16 @@ decode_nla_xval(struct tcb *const tcp,
 	if (len > sizeof(data))
 		return false;
 	else if (!umoven_or_printaddr(tcp, addr, len, data.bytes + bytes_offs))
+	{
+		if (opts->process_fn)
+			data.val = opts->process_fn(data.val);
+		if (opts->prefix)
+			tprints(opts->prefix);
 		printxval_dispatch_ex(opts->xlat, opts->xlat_size, data.val,
 				      opts->dflt, opts->xt, opts->style);
+		if (opts->suffix)
+			tprints(opts->suffix);
+	}
 
 	return true;
 }
@@ -315,8 +323,16 @@ decode_nla_flags(struct tcb *const tcp,
 	if (len > sizeof(data))
 		return false;
 	else if (!umoven_or_printaddr(tcp, addr, len, data.bytes + bytes_offs))
+	{
+		if (opts->process_fn)
+			data.flags = opts->process_fn(data.flags);
+		if (opts->prefix)
+			tprints(opts->prefix);
 		printflags_ex(data.flags, opts->dflt, opts->style, opts->xlat,
 			      NULL);
+		if (opts->suffix)
+			tprints(opts->suffix);
+	}
 
 	return true;
 }
diff --git a/nlattr.h b/nlattr.h
index fc7a67d92..478b8f118 100644
--- a/nlattr.h
+++ b/nlattr.h
@@ -38,6 +38,9 @@ struct decode_nla_xlat_opts {
 	const char *dflt;
 	enum xlat_type xt;
 	enum xlat_style style;
+	const char *prefix;
+	const char *suffix;
+	uint64_t (*process_fn)(uint64_t val);
 };
 
 typedef bool (*nla_decoder_t)(struct tcb *, kernel_ulong_t addr,