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References: <20231020-strncpy-drivers-power-supply-surface_battery-c-v1-1-cabaea50e667@google.com>
In-Reply-To: <20231020-strncpy-drivers-power-supply-surface_battery-c-v1-1-cabaea50e667@google.com>
From:   Justin Stitt <justinstitt@google.com>
Date:   Fri, 20 Oct 2023 12:39:51 -0700
Message-ID: <CAFhGd8pZYrQNCLA1LHckHQYFtAffSAwpf6uKx6cbf_rVPS_+jg@mail.gmail.com>
Subject: Re: [PATCH] platform/surface: aggregator: replace deprecated strncpy
 with strscpy
To:     Maximilian Luz <luzmaximilian@gmail.com>,
        Sebastian Reichel <sre@kernel.org>
Cc:     linux-pm@vger.kernel.org, platform-driver-x86@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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Fixed the subject line in [v2].

On Fri, Oct 20, 2023 at 12:35=E2=80=AFPM Justin Stitt <justinstitt@google.c=
om> wrote:
>
> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
>
> We expect bat->name to be NUL-terminated based on its usage with
> strcmp():
>
> power_supply_core.c:
> 445: return strcmp(psy->desc->name, name) =3D=3D 0;
>
> ... and also by the manual `... - 1` for the length argument of the
> original strncpy() invocation.
>
> Furthermore, no NUL-padding is needed as bat is zero-allocated before
> calling spwr_battery_init():
> 826: bat =3D devm_kzalloc(&sdev->dev, sizeof(*bat), GFP_KERNEL);
> 827: if (!bat)
> 828:   return -ENOMEM;
> 829:
> 830: spwr_battery_init(bat, sdev, p->registry, p->name);
>
> ... this means any further NUL-byte assignments (like the ones that
> strncpy() does) are redundant.
>
> Considering the above, a suitable replacement is `strscpy` [2] due to
> the fact that it guarantees NUL-termination on the destination buffer
> without unnecessarily NUL-padding.
>
> Let's also opt to use the more idiomatic strscpy() usage of:
> (dest, src, sizeof(dest)).
>
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strn=
cpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.h=
tml [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> Note: build-tested only.
>
> Found with: $ rg "strncpy\("
> ---
>  drivers/power/supply/surface_battery.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/power/supply/surface_battery.c b/drivers/power/suppl=
y/surface_battery.c
> index 19d2f8834e56..196d290dc596 100644
> --- a/drivers/power/supply/surface_battery.c
> +++ b/drivers/power/supply/surface_battery.c
> @@ -722,7 +722,7 @@ static void spwr_battery_init(struct spwr_battery_dev=
ice *bat, struct ssam_devic
>                               struct ssam_event_registry registry, const =
char *name)
>  {
>         mutex_init(&bat->lock);
> -       strncpy(bat->name, name, ARRAY_SIZE(bat->name) - 1);
> +       strscpy(bat->name, name, sizeof(bat->name));
>
>         bat->sdev =3D sdev;
>
>
> ---
> base-commit: bb55d7f7f7445abcc8db50e6a65d4315e79f75c7
> change-id: 20231020-strncpy-drivers-power-supply-surface_battery-c-b0c84b=
05ac28
>
> Best regards,
> --
> Justin Stitt <justinstitt@google.com>
>

[v2]: https://lore.kernel.org/r/20231020-strncpy-drivers-power-supply-surfa=
ce_battery-c-v2-1-29ed16b2caf1@google.com

Thanks
Justin