linux-kernel - Re: [PATCH] tracing/uprobes: Fix output for multiple string arguments

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Message-ID: <20190116084021.34b0beee@gandalf.local.home>
Date:   Wed, 16 Jan 2019 08:40:21 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andreas Ziegler <andreas.ziegler@....de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/uprobes: Fix output for multiple string
 arguments

On Wed, 16 Jan 2019 10:41:12 +0100
Andreas Ziegler <andreas.ziegler@....de> wrote:

> When printing multiple uprobe arguments as strings the output for the
> earlier arguments would also include all later string arguments.
> 
> This is best explained in an example:
> 
> Consider adding a uprobe to a function receiving two strings as
> parameters which is at offset 0xa0 in strlib.so and we want to print
> both parameters when the uprobe is hit (on x86_64):
> 
> $ echo 'p:func /lib/strlib.so:0xa0 +0(%di):string +0(%si):string' > \
>     /sys/kernel/debug/tracing/uprobe_events
> 
> When the function is called as func("foo", "bar") and we hit the probe,
> the trace file shows a line like the following:
> 
>   [...] func: (0x7f7e683706a0) arg1="foobar" arg2="bar"
> 
> Note the extra "bar" printed as part of arg1. This behaviour stacks up
> for additional string arguments.
> 
> The strings are stored in a dynamically growing part of the uprobe
> buffer by fetch_store_string() after copying them from userspace via
> strncpy_from_user(). The return value of strncpy_from_user() is then
> directly used as the required size for the string. However, this does
> not take the terminating null byte into account as the documentation
> for strncpy_from_user() cleary states that it "[...] returns the
> length of the string (not including the trailing NUL)" even though the
> null byte will be copied to the destination.
> 
> Therefore, subsequent calls to fetch_store_string() will overwrite
> the terminating null byte of the most recently fetched string with
> the first character of the current string, leading to the
> "accumulation" of strings in earlier arguments in the output.
> 
> Fix this by incrementing the return value of strncpy_from_user() by
> one if we did not hit the maximum buffer size.
> 
> Signed-off-by: Andreas Ziegler <andreas.ziegler@....de>
> ---
>  kernel/trace/trace_uprobe.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> index e335576b9411..dfb9bbc7fd82 100644
> --- a/kernel/trace/trace_uprobe.c
> +++ b/kernel/trace/trace_uprobe.c
> @@ -160,6 +160,13 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
>  	if (ret >= 0) {
>  		if (ret == maxlen)
>  			dst[ret - 1] = '\0';
> +		else if (ret > 0)

Do we need the ret > 0 check? What if the value is ""?

Doesn't that cause the same issue?

-- Steve

> +			/*
> +			 * Include the terminating null byte. In this case it
> +			 * was copied by strncpy_from_user but not accounted
> +			 * for in ret.
> +			 */
> +			ret++;
>  		*(u32 *)dest = make_data_loc(ret, (void *)dst - base);
>  	}
>