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Date:   Thu, 13 May 2021 10:50:18 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Chuck Lever III <chuck.lever@oracle.com>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        David Wysochanski <dwysocha@redhat.com>,
        Bruce Fields <bfields@fieldses.org>
Subject: Re: [PATCH v2 01/25] NFSD: Fix TP_printk() format specifier in
 trace_nfsd_dirent()
Message-ID: <20210513105018.7539996a@gandalf.local.home>
In-Reply-To: <238C0E2D-C2A4-4578-ADD2-C565B3B99842@oracle.com>
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On Wed, 12 May 2021 16:52:05 +0000
Chuck Lever III <chuck.lever@oracle.com> wrote:

> The underlying need is to support non-NUL-terminated C strings.
> 
> I assumed that since the commentary around 9a6944fee68e claims
> the proper way to trace C strings is to use __string and friends,
> and those do not support non-NUL-terminated strings, that such
> strings are really not first-class citizens. Thus I concluded
> that my use of '%.*s' was incorrect.
> 
> Having some __string-style helpers that can deal with such
> strings would be valuable.

I guess the best I can do is a strncpy version, that will add the '\0' in
the ring buffer. That way we don't need to save the length as well (length
would need to be at least 4 bytes, where as '\0' is one).

Something like this?

I added "__string_len()" and "__assign_str_len()". You use them just like
__string() and __assign_str() but add a max length that you want to use
(although, it will always allocate "len" regardless if the string is
smaller). Then use __get_str() just like you use __string().

Would something like that work?

-- Steve

diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 8268bf747d6f..7ab23535a0c8 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -102,6 +102,9 @@ TRACE_MAKE_SYSTEM_STR();
 #undef __string
 #define __string(item, src) __dynamic_array(char, item, -1)
 
+#undef __string_len
+#define __string_len(item, src, len) __dynamic_array(char, item, -1)
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(char, item, -1)
 
@@ -197,6 +200,9 @@ TRACE_MAKE_SYSTEM_STR();
 #undef __string
 #define __string(item, src) __dynamic_array(char, item, -1)
 
+#undef __string_len
+#define __string_len(item, src, len) __dynamic_array(char, item, -1)
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)
 
@@ -444,6 +450,9 @@ static struct trace_event_functions trace_event_type_funcs_##call = {	\
 #undef __string
 #define __string(item, src) __dynamic_array(char, item, -1)
 
+#undef __string_len
+#define __string_len(item, src, len) __dynamic_array(char, item, -1)
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)
 
@@ -492,6 +501,9 @@ static struct trace_event_fields trace_event_fields_##call[] = {	\
 #define __string(item, src) __dynamic_array(char, item,			\
 		    strlen((src) ? (const char *)(src) : "(null)") + 1)
 
+#undef __string_len
+#define __string_len(item, src, len) __dynamic_array(char, item, (len) + 1)
+
 /*
  * __bitmask_size_in_bytes_raw is the number of bytes needed to hold
  * num_possible_cpus().
@@ -655,10 +667,18 @@ static inline notrace int trace_event_get_offsets_##call(		\
 #undef __string
 #define __string(item, src) __dynamic_array(char, item, -1)
 
+#undef __string_len
+#define __string_len(item, src, len) __dynamic_array(char, item, -1)
+
 #undef __assign_str
 #define __assign_str(dst, src)						\
 	strcpy(__get_str(dst), (src) ? (const char *)(src) : "(null)");
 
+#undef __assign_str_len
+#define __assign_str_len(dst, src, len)						\
+	strncpy(__get_str(dst), (src) ? (const char *)(src) : "(null)", len);	\
+	__get_str(dst)[len] = '\0';
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)