Re: [PATCH net] net: hns3: fix strscpy causing content truncation issue

[Jakub Kicinski <kuba@kernel.org>]

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!