linux-kernel - Re: [PATCH v2 1/2] soc: qcom: cmd-db: replace strncpy() with strscpy

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Date:   Mon, 27 Jun 2022 16:06:49 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH v2 1/2] soc: qcom: cmd-db: replace strncpy() with strscpy_pad()

Hi,

On Thu, May 19, 2022 at 12:33 AM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> The use of strncpy() is considered deprecated for NUL-terminated
> strings[1].  Replace strncpy() with strscpy_pad(), to keep existing
> pad-behavior of strncpy.  This fixes W=1 warning:
>
>   drivers/soc/qcom/cmd-db.c: In function ‘cmd_db_get_header.part.0’:
>   drivers/soc/qcom/cmd-db.c:151:9: warning: ‘strncpy’ specified bound 8 equals destination size [-Wstringop-truncation]
>     151 |         strncpy(query, id, sizeof(query));
>
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> Reviewed-by: Stephen Boyd <sboyd@...nel.org>
>
> ---
>
> Changes since v1:
> 1. Split series per subsystem.
> 2. Add tag.
> ---
>  drivers/soc/qcom/cmd-db.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> index dd872017f345..c5137c25d819 100644
> --- a/drivers/soc/qcom/cmd-db.c
> +++ b/drivers/soc/qcom/cmd-db.c
> @@ -148,7 +148,7 @@ static int cmd_db_get_header(const char *id, const struct entry_header **eh,
>                 return ret;
>
>         /* Pad out query string to same length as in DB */
> -       strncpy(query, id, sizeof(query));
> +       strscpy_pad(query, id, sizeof(query));

Sorry to report that this breaks booting on
sc7280-herobrine-herobrine-r1. I believe that the function was
actually _relying_ on the "unsafe" behavior of strncpy(). Specifically
I think:
* The source string (id) was a '\0' terminated string.
* The destination (query) was a fixed 8-byte string and doesn't need
to be '\0' terminated.

So before your change we could actually support 8-byte strings. Now we
can't because you'll only copy 7 bytes to the destination to leave
room for the '\0' termination...

Looking at printouts, I see, for instance at least one ID that looks
like "lnbclka2".

Given the requirements of this function(), the old strncpy() is
actually _exactly_ what we want. Is there any way to disable the
warning? If not, I guess we could make "query" be 9 bytes bit, or
"sizeof(ent->id) + 1" bytes big... Happy to post a patch, but it's
basically a bikeshed for how you want it fixed (there are dozens of
ways) and I'd rather you just tell me instead of me guessing.

-Doug