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Date:   Thu, 10 Aug 2023 11:47:15 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Jijie Shao <shaojijie@huawei.com>,
        Leon Romanovsky <leon@kernel.org>, yisen.zhuang@huawei.com,
        salil.mehta@huawei.com, davem@davemloft.net, edumazet@google.com,
        pabeni@redhat.com, shenjian15@huawei.com, wangjie125@huawei.com,
        liuyonglong@huawei.com, chenhao418@huawei.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH net] net: hns3: fix strscpy causing content truncation
 issue
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In-Reply-To: <202308101103.D0827667B@keescook>
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        <7c44c161-9c86-8c60-f031-6d77d6c28c20@huawei.com>
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On Thu, 10 Aug 2023 11:23:46 -0700 Kees Cook wrote:
> tldr: use memcpy() instead of strscpy().
> 
> 
> Okay, I went to go read up on the history here. For my own notes, here's
> the original code, prior to 1cf3d5567f27 ("net: hns3: fix strncpy()
> not using dest-buf length as length issue"):
> 
> static void hns3_dbg_fill_content(char *content, u16 len,
> 				  const struct hns3_dbg_item *items,
> 				  const char **result, u16 size)
> {
> 	char *pos = content;
> 	u16 i;
> 
> 	memset(content, ' ', len);
> 	for (i = 0; i < size; i++) {
> 		if (result)
> 			strncpy(pos, result[i], strlen(result[i]));
> 		else
> 			strncpy(pos, items[i].name, strlen(items[i].name));
> 
> 		pos += strlen(items[i].name) + items[i].interval;
> 	}
> 
> 	*pos++ = '\n';
> 	*pos++ = '\0';
> }
> 
> The warning to be fixed was:
> 
> hclge_debugfs.c:90:25: warning: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
> 
> There are a few extra checks added in 1cf3d5567f27, but I'm more curious
> about this original code's intent. It seems very confusing to me.
> 
> Firstly, why is "pos" updated based on "strlen(items[i].name)" even when
> "result[i]" is used? Secondly, why is "interval" used? (These concerns
> are mostly addressed in 1cf3d5567f27.)
> 
> I guess I'd just like to take a step back and ask, "What is this
> function trying to do?" It seems to be building a series of strings in a
> " "-padding buffer, and it intends that the buffer be newline and %NUL
> terminated.
> 
> It looks very much like it wants to _avoid_ adding %NUL termination when
> doing copies, which is why it's using strncpy with a length argument of
> the source string length: it's _forcing_ the copy to not be terminated.
> This is just memcpy.
> 
> strtomem() is designed for buffer sizes that can be known at compile
> time, so it's not useful here (as was found), since a string is being
> built up and uses a moving pointer.
> 
> I think the correct fix is to use memcpy() instead of strscpy(). No
> %NUL-truncation is desired, the sizes are already determined and bounds
> checked. (And the latter is what likely silenced the compiler warning.)

Got it, thanks!