From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 21A8725D8E1; Mon, 24 Mar 2025 10:19:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742811543; cv=none; b=m8LZH1Ww7qCxboRFF0gM4u/Kz2rn20C2YcLI5JhWM6PKrlcdbIC+exF0OaXLHc/1hAYR1RaPtqbeBA/env6gARTUg04tXjc/vuIkwGcaK0oYi0mgXkBAewvmUsvDSYdNy1dDU42WiMl1EV+3GlkkIbsqlRvZGwCtQ42UANXWIVU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742811543; c=relaxed/simple; bh=9rq2nJDVTpNtebFu0O0NSHqfeNp/dCiYbreZxeitcxY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=SfadF9lcijCKSBu28d1tMmhXkd+EvZGvgjvyBOSn3BDkUpZCVpD9V+Mv6pUGA7Js0Fm59zqHJjArz+uD2vrjFAnpDEG8NYCXvLFNRFb5mAYhDpSjJ7LUukUi9L+d2fTZ2Nf+3ecg9Gz09wxSUWANLlvnyYs61cWDChckJNBDePA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6E15F1A2D; Mon, 24 Mar 2025 03:19:00 -0700 (PDT) Received: from [10.1.38.52] (e132430.arm.com [10.1.38.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EBB273F63F; Mon, 24 Mar 2025 03:18:51 -0700 (PDT) Message-ID: Date: Mon, 24 Mar 2025 10:18:50 +0000 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] tracing: Fix synth event printk format for str fields To: "Masami Hiramatsu (Google)" , Steven Rostedt Cc: Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org References: <20250318180939.227696-1-douglas.raillard@arm.com> <20250319211858.6d8166e3fb202e6e5a658557@kernel.org> <9d234136-05b9-46b6-849f-901df83bf54c@arm.com> <20250324144551.fc290f9cf9f57b3d2f205354@kernel.org> Content-Language: en-US From: Douglas Raillard In-Reply-To: <20250324144551.fc290f9cf9f57b3d2f205354@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 24-03-2025 05:45, Masami Hiramatsu (Google) wrote: > On Wed, 19 Mar 2025 14:34:19 +0000 > Douglas Raillard wrote: > >> Hi, >> >> On 19-03-2025 12:18, Masami Hiramatsu (Google) wrote: >>> Hi, >>> >>> On Tue, 18 Mar 2025 18:09:38 +0000 >>> Douglas RAILLARD wrote: >>> >>>> From: Douglas Raillard >>>> >>>> The printk format for synth event uses "%.*s" to print string fields, >>>> but then only passes the pointer part as var arg. >>>> >>>> Add the missing precision var arg. >>> >>> I'm not sure what you want to. Would you mean showing the string length too? >>> But I think actual output(*) shows only string, right? >>> >>> (*) the output data which can be read from `trace` file. >> >> The "%.*s" printf/printk format specifier has 2 components: >> 1. the "*" part that in printf documentation called the "precision" >> 2. the "s" part that is called the "specifier" >> >> For a "s" specifier, the precision is interpreted as the number of chars to display from that string. >> In any case, using "*" as precision means the value is passed dynamically, as a parameter to printf/printk, >> before the actual char *. Both those args are consumed by printk to display "%.*s", the precision is never >> displayed on its own. >> >> In the current state, synthetic event use "%.*s" for string fields, but then only a single `__get_str(field)` >> is passed to printk, so it's missing the precision arg. Both trace-cmd and our parser report an error because of that. >> >> The alternative would be to just use "%s" instead of "%.*s", but I assume whoever wrote the code initially had a reason >> to use the precision (maybe to print non-null-terminated strings ?), so I simply fixed the arguments. > > Ah, got it. It seems that the first precision parameter is for limiting > the length of the string according to the commit; > > https://lore.kernel.org/all/b6bdb34e70d970e8026daa3503db6b8e5cdad524.1601848695.git.zanussi@kernel.org/T/#u > > So, I think it should always print the STR_VAR_LEN_MAX value. That makes sense. It's tempting to keep the actual length value though as native Rust strings are not null-terminated, so it could make it nicer to emit events from Rust code. From a cursory look, the in-tree Rust code seems to be using both &str and &CStr (the latter being null-terminated for FFI needs) so I'm not sure what's the plan around those and what's the established convention if any. > Steve, can you check it? > > Thank you, > >> >>> >>> Thank you, >>> >>>> >>>> Signed-off-by: Douglas Raillard >>>> --- >>>> kernel/trace/trace_events_synth.c | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c >>>> index e3f7d09e5512..274b9b1d9f7d 100644 >>>> --- a/kernel/trace/trace_events_synth.c >>>> +++ b/kernel/trace/trace_events_synth.c >>>> @@ -620,7 +620,8 @@ static int __set_synth_event_print_fmt(struct synth_event *event, >>>> if (event->fields[i]->is_string && >>>> event->fields[i]->is_dynamic) >>>> pos += snprintf(buf + pos, LEN_OR_ZERO, >>>> - ", __get_str(%s)", event->fields[i]->name); >>>> + ", (int)__get_dynamic_array_len(%s), __get_str(%s)", >>>> + event->fields[i]->name, event->fields[i]->name); >>>> else if (event->fields[i]->is_stack) >>>> pos += snprintf(buf + pos, LEN_OR_ZERO, >>>> ", __get_stacktrace(%s)", event->fields[i]->name); >>>> -- >>>> 2.43.0 >>>> >> Thank you, Douglas