linux-kernel - Re: [PATCH v2] maccess: fix strncpy_from_user

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Message-ID: <ccfc1eeb-8bd1-41f9-9146-c8752723ccad@gmail.com>
Date: Wed, 23 Apr 2025 15:48:52 +0100
From: Mykyta Yatsenko <mykyta.yatsenko5@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 mhiramat@...nel.org, andrii@...nel.org, kernel-team@...a.com,
 linux-kernel@...r.kernel.org, Mykyta Yatsenko <yatsenko@...a.com>,
 Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] maccess: fix strncpy_from_user_nofault empty string
 handling

On 4/23/25 14:59, Steven Rostedt wrote:
> On Wed, 23 Apr 2025 12:37:46 +0100
> Mykyta Yatsenko <mykyta.yatsenko5@...il.com> wrote:
>
>>> Does this fix any known runtime issue?  If so, please fully describe this?
>> Not that I'm aware of. The issue could be found when trying to copy empty
>> user space string in BPF program (and relying on return value).There are
>> some usage of
>> `strncpy_from_user_nofault` in tracing subsystem, but I'm not sure how to
>> hit those code paths.
>>>   
> Although your patch found a bug in the tracing subsystem, this wasn't the
> cause. It only cared if the read faulted or not. It was incorrectly
> checking for zero as non fault when in reality, it needed to check >= 0.
>
> With that fixed, it should work the same with or without this patch.
>
> -- Steve
Sure, I had in mind usages from trace_probe_kernel.h,
namely fetch_store_string_user, fetch_store_string, having a second look,
it appears these only used in trace_events_synth.c, and we are good there.